Acidic Amino Acids -

Except for glycine, $\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}$, the $\alpha$ carbons of amino acids are chirality centers. Two enantiomers of each are therefore possible, but nature uses only one to build proteins. In Fischer projections, naturally occurring amino acids are represented by placing the $-\mathrm{CO}_{2}{ }^{-}$group at the top and pointing the side chain downwards, as if drawing a carbohydrate (Section 25.2) and then placing the $-\mathrm{NH}_{3}{ }^{+}$group on the left. Because of their stereochemical similarity to L sugars (Section 25.3), the naturally occurring $\alpha$-amino acids are often referred to as L amino acids. The nonnaturally occurring enantiomers are called D amino acids.
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L-Serine
(S)-Serine
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L-Cysteine
(R)-Cysteine
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L-Alanine
(S)-Alanine
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Acidic Amino Acids -

D-Alanine
( $R$ )-Alanine


Acidic Amino Acids -

The 20 common amino acids can be further classified as neutral, acidic, or basic, depending on the structure of their side chains. Fifteen of the twenty have neutral side chains, two (aspartic acid and glutamic acid) have an extra carboxylic acid function in their side chains, and three (lysine, arginine, and histidine) have basic amino groups in their side chains. Note that both cysteine (a thiol) and tyrosine (a phenol), although usually classified as neutral amino acids, nevertheless have weakly acidic side chains that can be deprotonated in a sufficiently basic solution.


Acidic Amino Acids -

At the physiological pH of 7.3 , the side-chain carboxyl groups of aspartic acid and glutamic acid are deprotonated and the basic side-chain nitrogens of lysine and arginine are protonated. Histidine, however, which contains a heterocyclic imidazole ring in its side chain, is not quite basic enough to be protonated at pH 7.3. Note that only the pyridine-like, doubly bonded nitrogen in histidine is basic. The pyrrole-like singly bonded nitrogen is nonbasic because its lone pair of electrons is part of the six- $\pi$-electron aromatic imidazole ring (Section 24.9).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0942.jpg?height=367&width=1045&top_left_y=2055&top_left_x=532)


Acidic Amino Acids -

Histidine
Humans are able to biosynthesize only 11 of the 20 protein amino acids, called nonessential amino acids.


Acidic Amino Acids -

The other 9, called essential amino acids, are biosynthesized only in plants and microorganisms and must be obtained in our diet. The division between essential and nonessential amino acids is not clear-cut, however. Tyrosine, for instance, is sometimes considered nonessential because humans can produce it from phenylalanine, but phenylalanine itself is essential and must be obtained in the diet. Arginine can be synthesized by humans, but much of the arginine we need also comes from our diet.


Acidic Amino Acids -

PROBLEM How many of the $\alpha$-amino acids shown in Table 26.1 contain aromatic rings? How many contain 26-1 sulfur? How many contain alcohol groups? How many contain hydrocarbon side chains?


Acidic Amino Acids -

PROBLEM Of the 19 l amino acids, 18 have the $S$ configuration at the $\alpha$ carbon. Cysteine is the only lamino 26-2 acid that has an $R$ configuration. Explain.


Acidic Amino Acids -

PROBLEM The amino acid threonine, ( $2 S, 3 R$ )-2-amino-3-hydroxybutanoic acid, has two chirality centers. 26-3
(a) Draw threonine, using normal, wedged, and dashed lines to show dimensionality.
(b) Draw a diastereomer of threonine, and label its chirality centers as $R$ or $S$.


Acidic Amino Acids - 26.2 Amino Acids and the Henderson-Hasselbalch Equation: Isoelectric Points

According to the Henderson-Hasselbalch equation (Section 20.3 and Section 24.5), if we know both the pH of a solution and the $\mathrm{p} K_{\mathrm{a}}$ of an acid HA, we can calculate the ratio of [ $\mathrm{A}^{-}$] to $[\mathrm{HA}]$ in the solution. Furthermore, when $\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}$, the two forms $\mathrm{A}^{-}$and HA are present in equal amounts because $\log 1=0$.


$$
\mathrm{pH}=\mathrm{p} K_{\mathrm{a}}+\log \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]} \quad \text { or } \quad \log \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]}=\mathrm{pH}-\mathrm{p} K_{\mathrm{a}}
$$


To apply the Henderson-Hasselbalch equation to an amino acid, let's find out what species are present in a 1.00 M solution of alanine at $\mathrm{pH}=9.00$. According to TABLE 26.1, protonated alanine $\left[{ }^{+} \mathrm{H}_{3} \mathrm{NCH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2} \mathrm{H}\right]$ has $\mathrm{p} K_{\mathrm{a} 1}$ $=2.34$ and neutral zwitterionic alanine $\left[{ }^{+} \mathrm{H}_{3} \mathrm{NCH}\left(\mathrm{CH}_{3}\right) \mathrm{CO}_{2}{ }^{-}\right]$has $\mathrm{p} K_{\mathrm{a} 2}=9.69$ :
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0943.jpg?height=345&width=1012&top_left_y=1394&top_left_x=546)


Because the pH of the solution is much closer to $\mathrm{p} K_{\mathrm{a} 2}$ than to $\mathrm{p} K_{\mathrm{a} 1}$, we need to use $\mathrm{p} K_{\mathrm{a} 2}$ for the calculation. From the Henderson-Hasselbalch equation, we have:


$$
\log \frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]}=\mathrm{pH}-\mathrm{p} K_{\mathrm{a}}=9.00-9.69=-0.69
$$


So


$$
\frac{\left[\mathrm{A}^{-}\right]}{[\mathrm{HA}]}=\operatorname{antilog}(-0.69)=0.20 \text { and }\left[\mathrm{A}^{-}\right]=0.20[\mathrm{HA}]
$$


In addition, we know that


$$
\left[\mathrm{A}^{-}\right]+[\mathrm{HA}]=1.00 \mathrm{M}
$$


Solving the two simultaneous equations gives $[\mathrm{HA}]=0.83$ and $\left[\mathrm{A}^{-}\right]=0.17$. In other words, at $\mathrm{pH}=9.00,83 \%$ of alanine molecules in a 1.00 M solution are neutral (zwitterionic) and $17 \%$ are deprotonated. Similar calculations can be done at other pH values and the results plotted to give the titration curve shown in FIGURE 26.2.

Each leg of the titration curve is calculated separately. The first leg, from pH 1 to 6 , corresponds to the dissociation of protonated alanine, $\mathrm{H}_{2} \mathrm{~A}^{+}$. The second leg, from pH 6 to 11, corresponds to the dissociation of zwitterionic alanine, HA. It's as if we started with $\mathrm{H}_{2} \mathrm{~A}^{+}$at low pH and then titrated with NaOH . When 0.5
equivalent of NaOH is added, the deprotonation of $\mathrm{H}_{2} \mathrm{~A}^{+}$is $50 \%$ complete; when 1.0 equivalent of NaOH is added, the deprotonation of $\mathrm{H}_{2} \mathrm{~A}^{+}$is finished and HA predominates; when 1.5 equivalents of NaOH is added, the deprotonation of HA is $50 \%$ complete; and when 2.0 equivalents of NaOH is added, the deprotonation of HA is finished.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0944.jpg?height=789&width=1595&top_left_y=386&top_left_x=265)

FIGURE 26.2 A titration curve for alanine, plotted using the Henderson-Hasselbalch equation. Each of the two legs is plotted separately. At $\mathrm{pH}<1$, alanine is entirely protonated; at $\mathrm{pH}=2.34$, alanine is a $50: 50 \mathrm{mix}$ of protonated and neutral forms; at $\mathrm{pH}=6.01$, alanine is entirely neutral; at $\mathrm{pH}=9.69$, alanine is a $50: 50 \mathrm{mix}$ of neutral and deprotonated forms; at $\mathrm{pH}>11.5$, alanine is entirely deprotonated.

Look carefully at the titration curve in FIGURE 26.2. In acid solution, the amino acid is protonated and exists primarily as a cation. In basic solution, the amino acid is deprotonated and exists primarily as an anion. In between the two is an intermediate pH at which the amino acid is exactly balanced between anionic and cationic forms, existing primarily as the neutral, dipolar zwitterion. This pH is called the amino acid's isoelectric point (pI) and has a value of 6.01 for alanine.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0944.jpg?height=307&width=839&top_left_y=1538&top_left_x=643)

The isoelectric point of an amino acid depends on its structure, with values for the 20 common amino acids given previously in TABLE 26.1. The 15 neutral amino acids have isoelectric points near neutrality, in the pH range 5.0 to 6.5 . The two acidic amino acids have isoelectric points at lower pH so that deprotonation of the side-chain $-\mathrm{CO}_{2} \mathrm{H}$ does not occur at their pI, and the three basic amino acids have isoelectric points at higher pH so that protonation of the side-chain amino group does not occur at their pI.


More specifically, the $\mathrm{p} I$ of any amino acid is the average of the two acid-dissociation constants that involve the neutral zwitterion. For the 13 amino acids with a neutral side chain, $\mathrm{p} I$ is the average of $\mathrm{p} K_{\mathrm{a} 1}$ and $\mathrm{p} K_{\mathrm{a} 2}$. For the four amino acids with either a strongly or weakly acidic side chain, $\mathrm{p} I$ is the average of the two lowest $\mathrm{p} K_{\mathrm{a}}$ values. For the three amino acids with a basic side chain, $\mathrm{p} I$ is the average of the two highest $\mathrm{p} K_{\mathrm{a}}$ values.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0945.jpg?height=492&width=454&top_left_y=193&top_left_x=288)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0945.jpg?height=481&width=410&top_left_y=193&top_left_x=803)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0945.jpg?height=486&width=576&top_left_y=191&top_left_x=1254)


Just as individual amino acids have isoelectric points, proteins have an overall $\mathrm{p} I$ due to the cumulative effect of all the acidic or basic amino acids they may contain. The enzyme lysozyme, for instance, has a preponderance of basic amino acids and thus has a high isoelectric point ( $\mathrm{p} I=11.0$ ). Pepsin, however, has a preponderance of acidic amino acids and a low isoelectric point ( $\mathrm{p} I \sim 1.0$ ). Not surprisingly, the solubilities and properties of proteins with different prs are strongly affected by the pH of the medium. Solubility in water is usually lowest at the isoelectric point, where the protein has no net charge, and is higher both above and below the pI, where the protein is charged.


We can take advantage of the differences in isoelectric points to separate a mixture of proteins into its pure components. Using a technique known as electrophoresis, a mixture of proteins is placed near the center of a strip of paper or gel. The paper or gel is moistened with an aqueous buffer of a given pH , and electrodes are connected to the ends of the strip. When an electric potential is applied, the proteins with negative charges (those that are deprotonated because the pH of the buffer is above their isoelectric point) migrate slowly toward the positive electrode. At the same time, those amino acids with positive charges (those that are protonated because the pH of the buffer is below their isoelectric point) migrate toward the negative electrode.

Different proteins migrate at different rates, depending on their isoelectric points and on the pH of the aqueous buffer, thereby effecting a separation of the mixture into its components. FIGURE 26.3 illustrates this process for a mixture containing basic, neutral, and acidic components.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0945.jpg?height=264&width=919&top_left_y=1494&top_left_x=603)

FIGURE 26.3 Separation of a protein mixture by electrophoresis. At $\mathrm{pH}=6.00$, a neutral protein does not migrate, a basic protein is protonated and migrates toward the negative electrode, and an acidic protein is deprotonated and migrates toward the positive electrode.


PROBLEM Hemoglobin has $\mathrm{p} I=6.8$. Does hemoglobin have a net negative charge or net positive charge at pH 26-4 $=5.3$ ? At $\mathrm{pH}=7.3$ ?


Acidic Amino Acids - 26.3 Synthesis of Amino Acids

$\alpha$-Amino acids can be synthesized in the laboratory using some of the reactions discussed in previous chapters. One of the oldest methods of $\alpha$-amino acid synthesis begins with $\alpha$ bromination of a carboxylic acid by treatment with $\mathrm{Br}_{2}$ and $\mathrm{PBr}_{3}$ (the Hell-Volhard-Zelinskii reaction; Section 22.4). $\mathrm{S}_{\mathrm{N}} 2$ substitution of the $\alpha$-bromo acid with ammonia then yields an $\alpha$-amino acid.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0945.jpg?height=126&width=464&top_left_y=2214&top_left_x=478)


4-Methylpentanoic acid
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0945.jpg?height=150&width=261&top_left_y=2218&top_left_x=967)


2-Bromo-4-methylpentanoic acid


PROBLEM Show how you could prepare the following $\alpha$-amino acids from the appropriate carboxylic acids:
26-5 (a) Phenylalanine (b) Valine


The Amidomalonate Synthesis -

A more general method for preparation of $\alpha$-amino acids is the amidomalonate synthesis, a straightforward extension of the malonic ester synthesis (Section 22.7). The reaction begins with the conversion of diethyl acetamidomalonate into an enolate ion by treatment with base, followed by $\mathrm{S}_{\mathrm{N}} 2$ alkylation with a primary alkyl halide. Hydrolysis of both the amide protecting group and the esters occurs when the alkylated product is warmed with aqueous acid, and decarboxylation then takes place to yield an $\alpha$-amino acid. For example, aspartic acid can be prepared from ethyl bromoacetate, $\mathrm{BrCH}_{2} \mathrm{CO}_{2} \mathrm{Et}$ :
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0946.jpg?height=339&width=1270&top_left_y=649&top_left_x=430)


The Amidomalonate Synthesis -

PROBLEM What alkyl halides would you use to prepare the following $\alpha$-amino acids by the amidomalonate 26-6 method?
(a) Leucine
(b) Histidine
(c) Tryptophan
(d) Methionine


Reductive Amination of $\boldsymbol{\alpha}$-Keto Acids -

Yet another method for the synthesis of $\alpha$-amino acids is by reductive amination of an $\alpha$-keto acid with ammonia and a reducing agent. Alanine, for instance, is prepared by treatment of pyruvic acid with ammonia in the presence of $\mathrm{NaBH}_{4}$. As described in Section 24.6, the reaction proceeds through formation of an intermediate imine which is then reduced.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0946.jpg?height=236&width=966&top_left_y=1427&top_left_x=577)


Enantioselective Synthesis -

The synthesis of an $\alpha$-amino acid from an achiral precursor by any of the methods just described yields a racemic mixture, with equal amounts of $S$ and $R$ enantiomers. To use an amino acid in the laboratory synthesis of a naturally occurring protein, however, the pure $S$ enantiomer must be obtained.

Two methods are used in practice to obtain enantiomerically pure amino acids. One way requires resolving the racemic mixture into its pure enantiomers (Section 5.8). A more direct approach, however, is to use an enantioselective synthesis to prepare only the desired $S$ enantiomer directly. As discussed in the Chapter 19 Chemistry Matters, the idea behind enantioselective synthesis is to find a chiral reaction catalyst that will temporarily hold a substrate molecule in an unsymmetrical, chiral environment. While in that chiral environment, the substrate may be more open to reaction on one side than on another, leading to an excess of one enantiomeric product.


Enantioselective Synthesis -

William Knowles at the Monsanto Company discovered in 1968 that $\alpha$-amino acids can be prepared enantioselectively by hydrogenation of a $Z$ enamido acid with a chiral hydrogenation catalyst. (S)Phenylalanine, for instance, is prepared at $98.7 \%$ purity, contaminated by only $1.3 \%$ of the $(R)$ enantiomer, when using a chiral rhodium catalyst. For this discovery, Knowles shared the 2001 Nobel Prize in Chemistry.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0947.jpg?height=259&width=1124&top_left_y=177&top_left_x=495)


Enantioselective Synthesis -

A (Z) enamido acid
(S)-Phenylalanine


Enantioselective Synthesis -

The most effective catalysts for enantioselective amino acid synthesis are coordination complexes of rhodium(I) with 1,5-cyclooctadiene (COD) and a chiral diphosphine such as ( $R, R$ )-1,2-bis( $o$-anisylphenylphosphino)ethane, the so-called DiPAMP ligand. This complex owes its chirality to the presence of trisubstituted phosphorus atoms (Section 5.10).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0947.jpg?height=307&width=823&top_left_y=692&top_left_x=651)


Enantioselective Synthesis -

PROBLEM Show how you could prepare the following amino acid enantioselectively:
26-7
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0947.jpg?height=301&width=513&top_left_y=1080&top_left_x=416)


Enantioselective Synthesis - 26.4 Peptides and Proteins

Proteins and peptides are amino acid polymers in which the individual amino acids, called residues, are linked together by amide bonds, or peptide bonds. An amino group from one residue forms an amide bond with the carboxyl of a second residue, the amino group of the second forms an amide bond with the carboxyl of a third, and so on. For example, alanylserine is the dipeptide that results when an amide bond forms between the alanine carboxyl and the serine amino group.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0947.jpg?height=568&width=1361&top_left_y=1689&top_left_x=374)


Note that two dipeptides can result from reaction between alanine and serine, depending on which carboxyl group reacts with which amino group. If the alanine amino group reacts with the serine carboxyl, serylalanine results.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0948.jpg?height=565&width=1351&top_left_y=184&top_left_x=384)


The long, repetitive sequence of $-\mathrm{N}-\mathrm{CH}-\mathrm{CO}-$ atoms that makes up a continuous chain is called the protein's backbone. By convention, peptides are written with the $\mathbf{N}$-terminal amino acid (the one with the free $-\mathrm{NH}_{3}{ }^{+}$ group) on the left and the $\mathbf{C}$-terminal amino acid (the one with the free $-\mathrm{CO}_{2}{ }^{-}$group) on the right. The name of a peptide is denoted by the abbreviations listed in TABLE 26.1 for each amino acid. Thus, alanylserine is abbreviated Ala-Ser or A-S, and serylalanine is abbreviated Ser-Ala or S-A. The one-letter abbreviations are more convenient, though less immediately recognizable, than the three-letter abbreviations.


The amide bond that links amino acids together in peptides is no different from any other amide bond (Section 24.3). An amide nitrogen is nonbasic because its unshared electron pair is delocalized by resonance with the carbonyl group. This overlap of the nitrogen $p$ orbital with the $p$ orbitals of the carbonyl group imparts a certain amount of double-bond character to the $\mathrm{C}-\mathrm{N}$ bond and restricts rotation around it. The amide bond is therefore planar, and the $\mathrm{N}-\mathrm{H}$ is oriented $180^{\circ}$ to the $\mathrm{C}=\mathrm{O}$.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0948.jpg?height=570&width=1090&top_left_y=1257&top_left_x=512)


A second kind of covalent bonding in peptides occurs when a disulfide linkage, RS-SR, is formed between two cysteine residues. As we saw in Section 18.7, a disulfide is formed by mild oxidation of a thiol, RSH, and is cleaved by mild reduction.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0948.jpg?height=297&width=1321&top_left_y=1971&top_left_x=397)


A disulfide bond between cysteine residues in different peptide chains links the otherwise separate chains together, whereas a disulfide bond between cysteine residues in the same chain forms a loop. Insulin, for instance, is composed of two chains that total 51 amino acids and are linked by two cysteine disulfide bridges.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0949.jpg?height=543&width=1223&top_left_y=189&top_left_x=448)


PROBLEM Six isomeric tripeptides contain valine, tyrosine, and glycine. Name them using both three- and 26-8 one-letter abbreviations.


PROBLEM Draw the structure of M-P-V-G, and indicate its amide bonds. 26-9


Enantioselective Synthesis - 26.5 Amino Acid Analysis of Peptides

To determine the structure of a protein or peptide, we need to answer three questions: What amino acids are present? How much of each is present? In what sequence do the amino acids occur in the peptide chain? The answers to the first two questions are provided by an automated instrument called an amino acid analyzer.


An amino acid analyzer is based on techniques worked out in the 1950s by William Stein and Stanford Moore at the Rockefeller University, who shared the 1972 Nobel Prize in Chemistry for their work. In preparation for analysis, the peptide is broken into its constituent amino acids by reducing all disulfide bonds, capping the - SH groups of cysteine residues by $\mathrm{S}_{\mathrm{N}} 2$ reaction with iodoacetic acid, and hydrolyzing the amide bonds by heating with aqueous 6 M HCl at $110^{\circ} \mathrm{C}$ for 24 hours. The resultant amino acid mixture is then separated into its components by a technique called chromatography, either high-pressure liquid chromatography (HPLC) or ion-exchange chromatography.


In both HPLC and ion-exchange chromatography, the mixture to be separated is dissolved in a solvent, called the mobile phase, and passed through a metal tube or glass column that contains an adsorbent material, called the stationary phase. Because different compounds adsorb to the stationary phase to different extents, they migrate through the chromatography column at different rates and are separated as they emerge (elute) from the end.


In the ion-exchange technique, separated amino acids eluting from the chromatography column mix with a solution of a substance called ninhydrin and undergo a rapid reaction that produces an intense purple color. The color is measured by a spectrometer, and a plot of elution time versus spectrometer absorbance is obtained.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0949.jpg?height=337&width=1494&top_left_y=1905&top_left_x=310)

Because the time required for a given amino acid to elute from a standard column is reproducible, the identities of the amino acids in a peptide can be determined. The amount of each amino acid in the sample is determined by measuring the intensity of the purple color resulting from its reaction with ninhydrin. FIGURE 26.4 shows the results of amino acid analysis of a standard equimolar mixture of $17 \alpha$-amino acids. Typically, amino acid analysis requires about 100 picomoles $(2-3 \mu \mathrm{~g})$ of sample for a protein containing about 200 residues.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0950.jpg?height=720&width=1037&top_left_y=193&top_left_x=544)


FIGURE 26.4 Amino acid analysis of an equimolar mixture of 17 amino acids.
PROBLEM Show the structure of the product you would expect to obtain by $\mathrm{S}_{\mathrm{N}} 2$ reaction of a cysteine residue 26-10 with iodoacetic acid.


PROBLEM Show the structures of the products obtained on reaction of valine with ninhydrin.
26-11


Enantioselective Synthesis - 26.6 Peptide Sequencing: The Edman Degradation

With the identities and relative amounts of amino acids known, a peptide can then be sequenced to find out in what order the amino acids are linked. Much peptide sequencing is now done by mass spectrometry, using either electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) linked to a time-offlight (TOF) mass analyzer, as described in Section 12.4. Also in common use is a chemical method of peptide sequencing called the Edman degradation.


The general idea of peptide sequencing by Edman degradation is to cleave one amino acid at a time from an end of the peptide chain. That terminal amino acid is then separated and identified, and the cleavage reactions are repeated on the chain-shortened peptide until the entire peptide sequence is known. Automated protein sequencers are available that allow as many as 50 repetitive sequencing cycles to be carried out before a buildup of unwanted by-products interferes with the results. So efficient are these instruments that sequence information can be obtained from as little as 1 to 5 picomoles of sample-less than $0.1 \mu \mathrm{~g}$.


As shown in FIGURE 26.5, Edman degradation involves treatment of a peptide with phenyl isothiocyanate (PITC), $\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{N}=\mathrm{C}=\mathrm{S}$, followed by reaction with trifluoroacetic acid. The first step attaches the PITC to the $-\mathrm{NH}_{2}$ group of the N -terminal amino acid, and the second step splits the N -terminal residue from the peptide chain, yielding an anilinothiazolinone (ATZ) derivative plus the chain-shortened peptide. Further acidcatalyzed rearrangement of the ATZ derivative with aqueous acid converts it into a phenylthiohydantoin (PTH), which is identified by comparison of its elution time with the known elution times of PTH derivatives of the 20 common amino acids. The chain-shortened peptide is then automatically resubmitted for another round of Edman degradation.


FIGURE 26.5 MECHANISM -

Mechanism of the Edman degradation for $\mathbf{N}$-terminal analysis of peptides.
(1) Nucleophilic addition of the peptide terminal amino group to phenyl isothiocyanate (PITC) gives an N -phenylthiourea derivative.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0951.jpg?height=695&width=573&top_left_y=382&top_left_x=998)
(2) Acid-catalyzed cyclization of the phenylthiourea yields a tetrahedral intermediate...
(2) $\downarrow{ }_{\downarrow} \mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}$
(3)... which expels the chainshortened peptide and forms an anilinothiazolinone (ATZ) derivative.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0951.jpg?height=243&width=486&top_left_y=1212&top_left_x=1034)
(3) $\downarrow$
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0951.jpg?height=207&width=617&top_left_y=1599&top_left_x=1036)


FIGURE 26.5 MECHANISM -


FIGURE 26.5 MECHANISM -

Complete sequencing of large proteins by Edman degradation is impractical because of the buildup of unwanted by-products. To get around this problem, a large peptide chain is first cleaved by partial hydrolysis into a number of smaller fragments, the sequence of each fragment is determined, and the individual fragments are
fitted together by matching their overlapping ends. In this way, protein chains with more than 400 amino acids have been sequenced.


FIGURE 26.5 MECHANISM -

Partial hydrolysis of a peptide can be carried out either chemically with aqueous acid or enzymatically. Acid hydrolysis is unselective and gives a more-or-less random mixture of small fragments, but enzymatic hydrolysis is quite specific. The enzyme trypsin, for instance, catalyzes hydrolysis of peptides only at the carboxyl side of the basic amino acids arginine and lysine; chymotrypsin cleaves only at the carboxyl side of the aryl-substituted amino acids phenylalanine, tyrosine, and tryptophan.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0952.jpg?height=150&width=942&top_left_y=543&top_left_x=586)


FIGURE 26.5 MECHANISM -

PROBLEM The octapeptide angiotensin II has the sequence Asp-Arg-Val-Tyr-Ile-His-Pro-Phe. What fragments 26-12 would result if angiotensin II were cleaved with trypsin? With chymotrypsin?


FIGURE 26.5 MECHANISM -

PROBLEM What is the N -terminal residue on a peptide that gives the following PTH derivative upon Edman 26-13 degradation?
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0952.jpg?height=392&width=695&top_left_y=910&top_left_x=425)

PROBLEM Draw the structure of the PTH derivative that would be formed by Edman degradation of 26-14 angiotensin II (Problem 26-12).


FIGURE 26.5 MECHANISM -

PROBLEM Give the amino acid sequence of hexapeptides that produce the following sets of fragments upon
26-15 partial acid hydrolysis:
(a) Arg, Gly, Ile, Leu, Pro, Val gives Pro-Leu-Gly, Arg-Pro, Gly-Ile-Val
(b) $\mathrm{N}, \mathrm{L}, \mathrm{M}, \mathrm{W}, \mathrm{V}_{2}$ gives V-L, V-M-W, W-N-V


FIGURE 26.5 MECHANISM - 26.7 Peptide Synthesis

Once the structure of a peptide is known, its synthesis can be undertaken-perhaps to obtain a larger amount for biological evaluation. A simple amide might be formed by treating an amine and a carboxylic acid with a carbodiimide (either DCC or EDC; Section 21.3), but peptide synthesis is a more difficult problem because many different amide bonds must be formed in a specific order, rather than at random.


The solution to the specificity problem is protection (Section 17.8). If we want to couple alanine with leucine to synthesize Ala-Leu, for instance, we could protect the $-\mathrm{NH}_{2}$ group of alanine and the $-\mathrm{CO}_{2} \mathrm{H}$ group of leucine to shield them from reacting, then form the desired Ala-Leu amide bond by reaction with EDC or DCC, and then remove the protecting groups.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0953.jpg?height=489&width=1527&top_left_y=189&top_left_x=296)


A number of different amino- and carboxyl-protecting groups have been devised, but only a few are used in peptide synthesis. Carboxyl groups are often protected simply by converting them into methyl or benzyl esters. Both groups are easily introduced by standard methods of ester formation (Section 21.6) and are easily removed by mild hydrolysis with aqueous NaOH . Benzyl esters can also be cleaved by catalytic hydrogenolysis of the weak benzylic $\mathrm{C}-\mathrm{O}$ bond $\left(\mathrm{RCO}_{2}-\mathrm{CH}_{2} \mathrm{Ph}+\mathrm{H}_{2} \rightarrow \mathrm{RCO}_{2} \mathrm{H}+\mathrm{PhCH}_{3}\right)$.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0953.jpg?height=606&width=1137&top_left_y=906&top_left_x=494)

Amino groups are sometimes protected as their tert-butyloxycarbonyl amide (Boc) or, more commonly, as their fluorenylmethyloxycarbonyl amide (Fmoc). The Boc protecting group is introduced by reaction of the amino acid with di-tert-butyl dicarbonate in a nucleophilic acyl substitution reaction and is removed by brief treatment with a strong acid such as trifluoroacetic acid, $\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}$. The Fmoc protecting group is introduced by reaction with fluorenylmethyloxycarbonyl chloride and is removed by treatment with a $20 \%$ solution of the amine piperidine in dimethylformamide as solvent.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0954.jpg?height=258&width=1117&top_left_y=188&top_left_x=477)


Boc-Ala
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0954.jpg?height=475&width=1169&top_left_y=519&top_left_x=475)


Fmoc-Ala
Thus, five steps are needed to synthesize a dipeptide such as Ala-Leu:


|  | Ala | <smiles>[14CH3]O14C[18O]</smiles> | Leu | $+\mathrm{CH}_{3} \mathrm{OH}$ |
| :---: | :---: | :---: | :---: | :---: |
| (1) The amino group of alanine is protected as the Boc derivative, and <br> (2) the carboxyl group of leucine is protected as the methyl ester. |  | (1) $\downarrow$ |  | (2) $\downarrow \begin{aligned} & \mathrm{H}^{+} \\ & \text {catalyst }\end{aligned}$ |
|  |  | Boc-Ala |  | Leu-OCH3 |
| (3) The two protected amino acids are coupled using DCC. | (3) $\downarrow$ CC |  |  |  |
|  | Boc-Ala-Leu-OCH3 |  |  |  |
| (4) The Boc protecting group is removed by acid treatment. | (4) $\downarrow \mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}$ |  |  |  |
|  | Ala-Leu- $\mathrm{OCH}_{3}$ |  |  |  |
| (5) The methyl ester is removed by basic hydrolysis. | 5 |  | $\downarrow{ }^{\mathrm{NaOH}} \mathrm{H}$ |  |
|  |  | Ala- | -Leu |  |


These steps can be repeated to add one amino acid at a time to the growing chain or to link two peptide chains together. Many remarkable achievements in peptide synthesis have been reported, including a complete synthesis of human insulin. Insulin is composed of two chains totaling 51 amino acids linked by two disulfide bridges. The three-dimensional structure of insulin, shown previously, was determined by Dorothy Crowfoot Hodgkin, a British chemist who received the 1964 Nobel Prize in Chemistry for her work on this and other complex biological molecules.


PROBLEM Show the mechanism for formation of a Boc derivative by reaction of an amino acid with 26-16 di-tert-butyl dicarbonate.


PROBLEM Write all five steps required for the synthesis of Leu-Ala from alanine and leucine.


FIGURE 26.5 MECHANISM - 26.8 Automated Peptide Synthesis: The Merrifield Solid-Phase Method

As you might imagine, the synthesis of a large peptide chain by sequential addition of one amino acid at a time is a long and arduous process. An immense simplification is possible, however, using methods introduced by R. Bruce Merrifield, who received the 1984 Nobel Prize in Chemistry. In the Merrifield solid-phase method, peptide synthesis is carried out with the growing amino acid chain covalently bonded to small beads of a polymer resin, rather than in solution.


In the original procedure, polystyrene resin was used, prepared so that one of every hundred or so benzene rings contained a chloromethyl $\left(-\mathrm{CH}_{2} \mathrm{Cl}\right)$ group. A Boc-protected C-terminal amino acid was then attached to the resin through an ester bond formed by $\mathrm{S}_{\mathrm{N}} 2$ reaction.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0955.jpg?height=941&width=1139&top_left_y=646&top_left_x=490)

With the first amino acid bonded to the resin, a repeating series of four steps is then carried out to build a peptide.
(1) A Boc-protected amino acid is covalently linked to the polystyrene polymer by formation of an ester bond $\left(S_{N} 2\right.$ reaction).
(2) The polymer-bonded amino acid is washed free of excess reagent and then treated with trifluoroacetic acid to remove the Boc group.
(3) A second Boc-protected amino acid is coupled to the first by reaction with DCC. Excess reagents are removed by washing them from the insoluble polymer.
(4) The cycle of deprotection, coupling, and washing is repeated as many times as desired to add amino acid units to the growing chain.
(5) After the desired peptide has been made, treatment with anhydrous HF removes the final Boc group and cleaves the ester bond to the polymer, yielding the free peptide.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0956.jpg?height=150&width=568&top_left_y=196&top_left_x=1066)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0956.jpg?height=655&width=690&top_left_y=361&top_left_x=956)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0956.jpg?height=272&width=673&top_left_y=1016&top_left_x=967)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0956.jpg?height=413&width=773&top_left_y=1295&top_left_x=958)


The steps in the solid-phase procedure have been improved substantially over the years, but the fundamental idea remains the same. A common procedure is use of the Wang resin, developed by Su-Sun Wang, with the Fmoc protecting group. Less common is using the PAM (phenylacetamidomethyl) resin with Boc as the N protecting group.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0957.jpg?height=701&width=1418&top_left_y=178&top_left_x=348)


Robotic peptide synthesizers are now used to automatically repeat the coupling, washing, and deprotection steps with different amino acids. Each step occurs in high yield, and mechanical losses are minimized because the peptide intermediates are never removed from the insoluble polymer until the final step. Peptides with 20 amino acids can be routinely prepared in a few hours.


FIGURE 26.5 MECHANISM - 26.9 Protein Structure

Proteins are usually classified as either fibrous or globular, according to their three-dimensional shape. Fibrous proteins, such as the collagen in tendons and connective tissue and the myosin in muscle tissue, consist of polypeptide chains arranged side by side in long filaments. Because these proteins are tough and insoluble in water, they are used in nature for structural materials. Globular proteins, by contrast, are usually coiled into compact, roughly spherical shapes. These proteins are generally soluble in water and are mobile within cells. Most of the more than 3000 or so enzymes that have been characterized are globular proteins, including the 1300 different enzymes in the human body.


Proteins are so large that the word structure takes on a broader meaning than with simpler organic compounds. In fact, chemists speak of four different levels of structure when describing proteins.


- The primary structure of a protein is simply the amino acid sequence.
- The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern.
- The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape.
- The quaternary structure describes how different protein molecules come together to yield large aggregate structures.


Primary structure is determined, as we've seen, by sequencing the protein. Secondary, tertiary, and quaternary structures are determined either by NMR or by X-ray crystallography (Chapter 12 Chemistry Matters).


The most common secondary structures are the $\alpha$ helix and the $\beta$-pleated sheet. An $\boldsymbol{\alpha}$ helix is a right-handed coil of the protein backbone, much like the coil of a spiral staircase (FIGURE 26.6a). Each turn of the helix contains 3.6 amino acid residues, with a distance between coils of 540 pm , or 5.4 A. The structure is stabilized by hydrogen bonds between amide $\mathrm{N}-\mathrm{H}$ groups and $\mathrm{C}=\mathrm{O}$ groups four residues away, with an $\mathrm{N}-\mathrm{H} \cdots \mathrm{O}$ distance of $2.8 \AA$. The $\alpha$ helix is an extremely common secondary structure, and nearly all globular proteins contain many helical segments. Myoglobin, a small globular protein containing 153 amino acid residues in a single chain, is one example (FIGURE 26.6b).


A $\boldsymbol{\beta}$-pleated sheet differs from an $\alpha$ helix in that the peptide chain is fully extended rather than coiled and the hydrogen bonds occur between residues in adjacent chains (FIGURE 26.7a). The neighboring chains can run either in the same direction (parallel) or in opposite directions (antiparallel), although the antiparallel arrangement is more common and somewhat more energetically favorable. Concanavalin A, for instance,
consists of two identical chains of 237 residues, with extensive regions of antiparallel $\beta$ sheets (FIGURE 26.7b).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0958.jpg?height=785&width=1705&top_left_y=258&top_left_x=248)


FIGURE 26.6 (a) The $\alpha$-helical secondary structure of proteins is stabilized by hydrogen bonds between the $\mathrm{N}-\mathrm{H}$ group of one residue and the $\mathrm{C}=\mathrm{O}$ group four residues away. (b) The structure of myoglobin, a globular protein with extensive helical regions that are shown as coiled ribbons in this representation.
(a)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0958.jpg?height=481&width=1443&top_left_y=1142&top_left_x=363)
(b)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0958.jpg?height=543&width=771&top_left_y=1688&top_left_x=357)


FIGURE 26.7 (a) The $\beta$-pleated sheet secondary structure of proteins is stabilized by hydrogen bonds between parallel or antiparallel chains. (b) The structure of concanavalin A, a protein with extensive regions of antiparallel $\beta$ sheets, shown as flat ribbons.


What about tertiary structure? Why does a protein adopt the shape it does? The forces that determine the tertiary structure of a protein are the same forces that act on all molecules, regardless of size, to provide maximum stability. Particularly important are the hydrophilic (water-loving; Section 2.12) interactions of the polar side chains on acidic or basic amino acids and the hydrophobic (water-fearing) interactions of nonpolar
side chains. These acidic or basic amino acids with charged side chains tend to congregate on the exterior of the protein, where they can be solvated by water. Amino acids with neutral, nonpolar side chains tend to congregate on the hydrocarbon-like interior of a protein molecule, away from the aqueous medium.


Also important for stabilizing a protein's tertiary structure are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the presence of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein.


Because the tertiary structure of a globular protein is delicately maintained by weak intramolecular attractions, a modest change in temperature or pH is often enough to disrupt that structure and cause the protein to become denatured. Denaturation occurs under such mild conditions that the primary structure remains intact, but the tertiary structure unfolds from a specific globular shape to a randomly looped chain (FIGURE 26.8).
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0959.jpg?height=446&width=1229&top_left_y=734&top_left_x=448)


FIGURE 26.8 A representation of protein denaturation. A globular protein loses its specific three-dimensional shape and becomes randomly looped.


Denaturation is accompanied by changes in both physical and biological properties. Solubility is drastically decreased, as occurs when egg white is cooked and the albumins unfold and coagulate. Most enzymes lose all catalytic activity when denatured, since a precisely defined tertiary structure is required for their action. Although most denaturation is irreversible, some cases are known where spontaneous renaturation of an unfolded protein to its stable tertiary structure occurs, accompanied by a full recovery of biological activity.


FIGURE 26.5 MECHANISM - 26.10 Enzymes and Coenzymes

An enzyme is a substance-usually a large protein-that acts as a catalyst for a biological reaction. Like all catalysts, an enzyme doesn't affect the equilibrium constant of a reaction and can't bring about a chemical change that is otherwise unfavorable. An enzyme acts only to lower the activation energy for a reaction, thereby making the reaction take place more rapidly than it otherwise would. Sometimes, in fact, the rate acceleration brought about by enzymes is extraordinary. Millionfold rate increases are common, and the glycosidase enzymes that hydrolyze polysaccharides increase the reaction rate by a factor of more than $10^{17}$, changing the time required for the reaction from millions of years to milliseconds!


Unlike many of the catalysts that chemists use in the laboratory, enzymes are usually specific in their action. Often, in fact, an enzyme will catalyze only a single reaction of a single compound, called the enzyme's substrate. For example, the enzyme amylase, found in the human digestive tract, catalyzes only the hydrolysis of starch to yield glucose; cellulose and other polysaccharides are untouched by amylase.


Different enzymes have different specificities. Some, such as amylase, are specific for a single substrate, but others operate on a range of substrates. Papain, for instance, a globular protein of 212 amino acids isolated from papaya fruit, catalyzes the hydrolysis of many kinds of peptide bonds. In fact, it's this ability to hydrolyze peptide bonds that makes papain useful as a cleaner for contact lenses.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0959.jpg?height=156&width=1218&top_left_y=2291&top_left_x=451)


Enzymes function through a pathway that involves initial formation of an enzyme-substrate complex (E $\cdot \mathrm{S}$ ), followed by a multistep chemical conversion of the enzyme-bound substrate into enzyme-bound product ( $\mathrm{E} \cdot \mathrm{P}$ )
and final release of product from the complex.


$$
\mathrm{E}+\mathrm{S} \rightleftarrows \mathrm{E} \cdot \mathrm{~S} \rightleftarrows \mathrm{E} \cdot \mathrm{P} \rightleftarrows \mathrm{E}+\mathrm{P}
$$


The overall rate constant for conversion of the $\mathrm{E} \cdot \mathrm{S}$ complex to products $\mathrm{E} \cdot \mathrm{P}$ is called the turnover number because it represents the number of substrate molecules a single enzyme molecule turns over into product per unit time. A value of about $10^{3}$ per second is typical, although carbonic anhydrase can reach a value of up to 600,000.


The extraordinary rate accelerations achieved by enzymes are due to a combination of several factors. One important factor is simple geometry: an enzyme will adjust its shape to hold the substrate, other reactants, and various catalytic sites on acidic or basic residues in the precise geometry needed for reaction. In addition, the wrapping of the enzyme around the substrate can create specialized microenvironments that protect the substrate from the aqueous medium and can dramatically change the behavior of acid-base catalytic residues in the active site. But perhaps most important is that the enzyme stabilizes and thus lowers the energy of the rate-limiting transition state for reaction. That is, it's not the ability of the enzyme to bind the substrate that matters but rather its ability to bind and stabilize the transition state. Often, in fact, the enzyme binds the transition structure as much as $10^{12}$ times more tightly than it binds the substrate or products. An energy diagram for an enzyme-catalyzed process might resemble that in FIGURE 26.9.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0960.jpg?height=652&width=939&top_left_y=975&top_left_x=590)


FIGURE 26.9 Energy diagrams for uncatalyzed and enzyme-catalyzed processes. The enzyme makes available an alternative, lowerenergy pathway. Rate enhancement is due to the ability of the enzyme to bind to the transition state for product formation, thereby lowering its energy.


Enzymes are classified into six categories depending on the kind of reaction they catalyze, as shown in TABLE 26.2. Oxidoreductases catalyze oxidations and reductions; transferases catalyze the transfer of a group from one substrate to another; hydrolases catalyze hydrolysis reactions of esters, amides, and related substrates; lyases catalyze the elimination or addition of a small molecule such as $\mathrm{H}_{2} \mathrm{O}$ from or to a substrate; isomerases catalyze isomerizations; and ligases catalyze the bonding of two molecules, often coupled with the hydrolysis of ATP. The systematic name of an enzyme has two parts, ending with -ase. The first part identifies the enzyme's substrate, and the second part identifies its class. Hexose kinase, for example, is a transferase that catalyzes the transfer of a phosphate group from ATP to a hexose sugar.


TABLE 26.2 Classification of Enzymes


| Class | Some subclasses | Function |
| :--- | :--- | :--- |
| Oxidoreductases | Dehydrogenases
 Oxidases
 Reductases | Introduction of double bond
 Oxidation
 Reduction |
| Transferases | Kinases
 Transaminases | Transfer of phosphate group
 Transfer of amino group |
| Hydrolases | Lipases
 Nucleases | Hydrolysis of ester
 Hydrolysis of phosphate
 Hydrolysis of amide |
| Lyases | Decarboxylases | Loss of $\mathrm{CO}_{2}$ |
| Isomerases | Epimerases | Isomerization of chirality center |
| Ligases | Carboxylases
 Synthetases | Addition of CO
 2 |


In addition to their protein part, most enzymes also contain a small non-protein part called a cofactor. A cofactor can be either an inorganic ion, such as $\mathrm{Zn}^{2+}$, or a small organic molecule, called a coenzyme. A coenzyme is not a catalyst but is a reactant that undergoes chemical change during the reaction and requires an additional step to return to its initial state.


Many coenzymes are derived from vitamins-substances that an organism requires in small amounts for growth but is unable to synthesize and must receive in its diet (Chapter 20 Chemistry Matters). Coenzyme A from pantothenate (vitamin $\mathrm{B}_{3}$ ), $\mathrm{NAD}^{+}$from niacin, FAD from riboflavin (vitamin $\mathrm{B}_{2}$ ), tetrahydrofolate from folic acid, pyridoxal phosphate from pyridoxine (vitamin $B_{6}$ ), and thiamin diphosphate from thiamin (vitamin $B_{1}$ ) are examples. TABLE 26.3 shows the structures of some common coenzymes.


PROBLEM To what classes do the following enzymes belong?
26-18 (a) Pyruvate decarboxylase
(b) Chymotrypsin (c) Alcohol dehydrogenase


FIGURE 26.5 MECHANISM - 26.11 How Do Enzymes Work? Citrate Synthase

As we saw in the previous section, enzymes work by bringing substrate and other reactant molecules together, holding them in the orientation necessary for reaction, providing any necessary acidic or basic sites to catalyze specific steps, and stabilizing the transition state for reaction. As an example, let's look at citrate synthase, an enzyme that catalyzes the aldol-like addition of acetyl CoA to oxaloacetate to give citrate. This reaction is the first step in the citric acid cycle, in which acetyl groups produced by degradation of food molecules are metabolized to yield $\mathrm{CO}_{2}$ and $\mathrm{H}_{2} \mathrm{O}$. We'll look at the details of the citric acid cycle in Section 29.7.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0961.jpg?height=193&width=1275&top_left_y=2343&top_left_x=428)


Citrate synthase is a globular protein of 433 amino acids with a deep cleft lined by an array of functional groups that can bind to the substrate, oxaloacetate. On binding oxaloacetate, the original cleft closes and another opens up nearby to bind acetyl CoA. This second cleft is also lined by appropriate functional groups, including a histidine at position 274 and an aspartic acid at position 375 . The two reactants are now held by the enzyme in close proximity and with a suitable orientation for reaction. FIGURE 26.10 shows the structure of citrate synthase as determined by X-ray crystallography, along with a close-up of the active site.


TABLE 26.3 Structures and Functions of Some Common Coenzymes


Adenosine triphosphate-ATP (phosphorylation) -


Coenzyme A (acyl transfer) -


Coenzyme A (acyl transfer) -

Nicotinamide adenine dinucleotide-NAD ${ }^{+}$(oxidation/reduction)
(NADP ${ }^{+}$)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0963.jpg?height=340&width=698&top_left_y=1256&top_left_x=513)


TABLE 26.3 Structures and Functions of Some Common Coenzymes -


Tetrahydrofolate (transfer of $\mathrm{C}_{1}$ units) -


S-Adenosylmethionine (methyl transfer) -


FIGURE 26.10 X-ray crystal structure of citrate synthase. Part (a) is a space-filling model and part (b) is a ribbon model, which emphasizes the $\alpha$-helical segments of the protein chain and indicates that the enzyme is dimeric; that is, it consists of two identical chains held together by hydrogen bonds and other intermolecular attractions. Part (c) is a close-up of the active site in which oxaloacetate and an unreactive acetyl CoA mimic are bound.


As shown in FIGURE 26.11, the first step in the aldol reaction of acetyl CoA and oxaloacetate is generation of the enol of acetyl CoA. The side-chain carboxyl of an aspartate residue acts as base to abstract an acidic $\alpha$ proton, while at the same time the side-chain imidazole ring of a histidine donates $\mathrm{H}^{+}$to the carbonyl oxygen. The enol thus produced then performs a nucleophilic addition to the ketone carbonyl group of oxaloacetate. The first histidine acts as a base to remove the -OH hydrogen from the enol, while a second histidine residue simultaneously donates a proton to the oxaloacetate carbonyl group, giving citryl CoA. Water then hydrolyzes the thiol ester group in citryl CoA in a nucleophilic acyl substitution reaction, releasing citrate and coenzyme A as the final products.


FIGURE 26.11 MECHANISM -

Mechanism of the addition of acetyl CoA to oxaloacetate to give (S)-citryl CoA, catalyzed by citrate synthase.
(1) The side-chain carboxylate group of an aspartic acid acts as a base and removes an acidic $\alpha$ proton from acetyl CoA, while the $\mathrm{N}-\mathrm{H}$ group on the side chain of a histidine acts as an acid and donates a proton to the carbonyl oxygen, giving an enol.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0966.jpg?height=397&width=635&top_left_y=430&top_left_x=992)


FIGURE 26.11 MECHANISM -

Acetyl CoA
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0966.jpg?height=472&width=722&top_left_y=867&top_left_x=1038)


FIGURE 26.11 MECHANISM -

A histidine deprotonates the acetyl-CoA enol, which adds to the ketone carbonyl group of oxaloacetate in an aldol-like reaction. Simultaneously, an acid N-H proton of another histidine protonates the carbonyl oxygen, producing (S)-citryl CoA.


FIGURE 26.11 MECHANISM -


FIGURE 26.11 MECHANISM -

Citrate


The Protein Data Bank -

Enzymes are so large, so structurally complex, and so numerous that the use of computer databases and molecular visualization programs has become an essential tool for studying biological chemistry. Of the various databases available online, the Kyoto Encyclopedia of Genes and Genomes (KEGG) database (http://www.genome.jp/kegg/pathway.html), maintained by the Kanehisa Laboratory of Kyoto University Bioinformatics Center, is useful for obtaining information on biosynthetic pathways of the sort we'll be describing in Chapter 29. For obtaining information on a specific enzyme, the BRENDA database (http://www.brenda-enzymes.org), maintained by the Institute of Biochemistry at the University of Cologne, Germany, is particularly valuable.


The Protein Data Bank -

Perhaps the most useful of all biological databases is the Protein Data Bank (PDB), operated by the Research Collaboratory for Structural Bioinformatics (RCSB). The PDB is a worldwide repository of X-ray and NMR structural data for biological macromolecules. In mid-2022, data for more than 192,888 structures were available, and more than 7000 new ones were being added yearly. To access the Protein Data Bank, go to www.rcsb.org and a home page like that shown in FIGURE 26.12 will appear. As with much that is available online, however, the PDB site is changing rapidly, so you may not see quite the same thing.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0967.jpg?height=529&width=899&top_left_y=1096&top_left_x=613)


The Protein Data Bank -

FIGURE 26.12 The Protein Data Bank home page. (credit: RCSB Protein Data Bank)
To learn how to use the PDB, begin by running the short tutorial listed under Learn on the left side column. After that introduction, start exploring. Let's say you want to view citrate synthase, the enzyme that catalyzes the addition of acetyl CoA to oxaloacetate to give citrate. Type "citrate synthase" (with quotation marks) into the small search box on the top line, click on "Search," and a list of 2,400 or so structures will appear. Scroll down near the end of the list until you find the entry with a PDB code of 5CTS and the title "Proposed Mechanism for the Condensation Reaction of Citrate Synthase: 1.9-Angstroms Structure of the Ternary Complex with Oxaloacetate and Carboxymethyl Coenzyme A." Alternatively, if you know the code of the enzyme you want, you can enter it directly into the search box. Click on the PDB code of entry 5CTS, and a new page containing information about the enzyme will open.


The Protein Data Bank -

If you choose, you can download the structure file to your computer and open it with any of numerous molecular graphics programs to see an image like that in FIGURE 26.13. The biologically active molecule is a dimer of two identical subunits consisting primarily of $\alpha$-helical regions displayed as coiled ribbons. For now, just select "View in Jmol" under the enzyme image on the right side of the screen to see some of the options for visualizing and further exploring the enzyme.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0968.jpg?height=554&width=746&top_left_y=243&top_left_x=684)


The Protein Data Bank -

FIGURE 26.13 An image of citrate synthase, downloaded from the Protein Data Bank. (credit: Protein Data Bank, 1AL6 B. PDB ID: 1AL6 B. Schwartz, K.W. Vogel, K.C. Usher, C. Narasimhan, H.M. Miziorko, S.J. Remington, D.G. Drueckhammer. Mechanisms of Enzyme-Catalyzed Deprotonation of Acetyl-Coenzyme A, CC BY 1.0)


Key Terms -

- $\alpha$-amino acids
- $\alpha$ helix
- backbone
- $\beta$-pleated sheet
- C-terminal amino acid
- coenzyme
- cofactor
- denatured
- Edman degradation
- enzyme
- fibrous proteins
- globular proteins
- isoelectric point (pI)
- N -terminal amino acid
- peptides
- primary structure
- proteins
- quaternary structure
- residues
- secondary structure
- side chain
- tertiary structure
- turnover number
- zwitterion


Key Terms -


Summary -

Proteins and peptides are large biomolecules made of $\boldsymbol{\alpha}$-amino acid residues linked together by amide, or peptide, bonds. Twenty amino acids are commonly found in proteins, and all except glycine have stereochemistry similar to that of L sugars. In neutral solution, amino acids exist as dipolar zwitterions.


Summary -

Amino acids can be synthesized in racemic form by several methods, including ammonolysis of an $\alpha$-bromo acid, alkylation of diethyl acetamidomalonate, and reductive amination of an $\alpha$-keto acid. Alternatively, an enantioselective synthesis of amino acids can be carried out using a chiral hydrogenation catalyst.


Summary -

Determining the structure of a peptide or protein begins with amino acid analysis. The peptide is hydrolyzed to its constituent $\alpha$-amino acids, which are separated and identified. Next, the peptide is sequenced. Edman degradation by treatment with phenyl isothiocyanate (PITC) cleaves one residue from the N terminus of the peptide and forms an easily identifiable phenylthiohydantoin (PTH) derivative of the $\mathbf{N}$-terminal amino acid. An automated series of Edman degradations can sequence peptide chains up to 50 residues in length.


Summary -

Peptide synthesis involves the use of protecting groups. An N-protected amino acid with a free $-\mathrm{CO}_{2} \mathrm{H}$ group is coupled using DCC or EDC to an O-protected amino acid with a free $-\mathrm{NH}_{2}$ group. Amide formation occurs, the protecting groups are removed, and the sequence is repeated. Amines are usually protected as their tert-butyloxycarbonyl (Boc) or fluorenylmethyloxycarbonyl (Fmoc) derivatives; acids are usually protected as esters. The synthesis is often carried out by the Merrifield solid-phase method, in which the peptide is bonded to insoluble polymer beads.


Summary -

Proteins have four levels of structure. Primary structure describes a protein's amino acid sequence; secondary structure describes how segments of the protein chain orient into regular patterns-either $\boldsymbol{\alpha}$ helix or $\boldsymbol{\beta}$-pleated sheet; tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape; and quaternary structure describes how individual protein molecules aggregate into larger structures.


Summary -

Proteins are classified as either globular or fibrous. Fibrous proteins such as $\alpha$-keratin are tough, rigid, and water-insoluble; globular proteins such as myoglobin are water-soluble and roughly spherical in shape. Many globular proteins are enzymes-substances that act as catalysts for biological reactions. Enzymes are grouped into six classes according to the kind of reaction they catalyze. In addition to their protein part, many enzymes contain cofactors, which can be either metal ions or small organic molecules called coenzymes.


Summary of Reactions -


Summary of Reactions -

An (S)-amino acid
2. Peptide sequencing by Edman degradation (Section 26.6)
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0969.jpg?height=240&width=1201&top_left_y=1688&top_left_x=489)
3. Peptide synthesis (Section 26.7)
a. Amine protection
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0969.jpg?height=218&width=1202&top_left_y=1997&top_left_x=513)


Summary of Reactions -

Boc-protected amino acid
b. Carboxyl protection
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0970.jpg?height=378&width=733&top_left_y=185&top_left_x=742)


Additional Problems -

Visualizing Chemistry
PROBLEM Identify the following amino acids:


26-19 (a) -


26-19 (a) -

PROBLEM Give the sequence of the following tetrapeptide (yellow = $S$ ):
26-20
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0970.jpg?height=562&width=903&top_left_y=1492&top_left_x=416)


26-19 (a) -

PROBLEM Isoleucine and threonine are the only two amino acids with two chirality centers. Assign $R$ or $S$
26-21 configuration to the methyl-bearing carbon atom of isoleucine.
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0970.jpg?height=345&width=453&top_left_y=2180&top_left_x=413)


26-19 (a) -

PROBLEM Is the following structure a D amino acid or an L amino acid? Identify it.
26-22
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0971.jpg?height=327&width=413&top_left_y=254&top_left_x=417)


26-19 (a) -

PROBLEM Give the sequence of the following tetrapeptide:
26-23
![](https://cdn.mathpix.com/cropped/2025_02_06_5556f7656cbdcaa0f9a7g-0971.jpg?height=499&width=982&top_left_y=653&top_left_x=409)