this equation: [ c y t c r e d ] [ c y t c o x ] = ( [ n a d h ] [ n a d ] + ) 1 2 ( [ a d p ] [ p i ] [ a t p ] ) k e q =\left(\right)^\left(\right)k_ }} thus, a high ratio of to or a high ratio of to imply a high amount of reduced cytochrome c and a high level of cytochrome c oxidase activity. an additional level of regulation is introduced by the transport rates of atp and nadh between the mitochondrial matrix and the cytoplasm. ketosis main article: ketone bodies ketone bodies can be used as fuels, yielding 22 atp and 2 gtp molecules per acetoacetate molecule when oxidized in the mitochondria. ketone bodies are transported from the liver to other tissues, where acetoacetate and beta-hydroxybutyrate can be reconverted to acetyl-coa to produce reducing equivalents (nadh and fadh2), via the citric acid cycle. ketone bodies cannot be used as fuel by the liver, because the liver lacks the enzyme β-ketoacyl-coa transferase, also called thiolase. acetoacetate in low concentrations is taken up by the liver and undergoes detoxification through the methylglyoxal pathway which ends with lactate. acetoacetate in high concentrations is absorbed by cells other than those in the liver and enters a different pathway via 1,2-propanediol. though the pathway follows a different series of steps requiring atp, 1,2-propanediol can be turned into pyruvate. production, anaerobic conditions fermentation is the metabolism of organic compounds in the absence of air. it involves substrate-level phosphorylation in the absence of a respiratory electron transport chain. the equation for the reaction of glucose to form lactic acid is: c6h12o6 + 2 adp + 2 pi → 2 ch3ch(oh)cooh + 2 atp + 2 h2o anaerobic respiration is respiration in the absence of o2. prokaryotes can utilize a variety of electron acceptors. these include nitrate, sulfate, and carbon dioxide. atp replenishment by nucleoside diphosphate kinases atp can also be synthesized through several so-called "replenishment" reactions catalyzed by the enzyme families of nucleoside diphosphate kinases (ndks), which use other nucleoside triphosphates as a high-energy phosphate donor, and the atp:guanido-phosphotransferase family. atp production during photosynthesis in plants, atp is synthesized in the thylakoid membrane of the chloroplast. the process is called photophosphorylation. the "machinery" is similar to that in mitochondria except that light energy is used to pump protons across a membrane to produce a proton-motive force. atp synthase then ensues exactly as in oxidative phosphorylation. some of the atp produced in the chloroplasts is consumed in the calvin cycle, which produces triose sugars. atp recycling the total quantity of atp in the human body is about 0.1 mol/l. the majority of atp is recycled from adp by the aforementioned processes. thus, at any given time, the total amount of atp + adp remains fairly constant. the energy used by human cells in an adult requires the hydrolysis of 100 to 150 mol/l of atp daily, which means a human will typically use their body weight worth of atp over the course of the day. each equivalent of atp is recycled 1000–1500 times during a single day (150 / 0.1 = 1500), at approximately 9×1020 molecules/s. an example of the rossmann fold, a structural domain of a decarboxylase enzyme from the bacterium staphylococcus epidermidis (pdb: 1g5q​) with a bound flavin mononucleotide cofactor. biochemical functions intracellular signaling atp is involved in signal transduction by serving as substrate for kinases, enzymes that transfer phosphate groups. kinases are the most common atp-binding proteins. they share a small number of common folds. phosphorylation of a protein by a kinase can activate a cascade such as the mitogen-activated protein kinase cascade. atp is also a substrate of adenylate cyclase, most commonly in g protein-coupled receptor signal transduction pathways and is transformed to second messenger, cyclic amp, which is involved in triggering calcium signals by the release of calcium from intracellular stores. this form of signal transduction is particularly important in brain function, although it is involved in the regulation of a multitude of other cellular processes. dna and rna synthesis atp is one of four monomers required in the synthesis of rna. the process is promoted by rna polymerases. a similar process occurs in the formation of dna, except that atp is first converted to the deoxyribonucleotide datp. like many condensation reactions in nature, dna replication and dna transcription also consume atp. amino acid activation in protein synthesis aminoacyl-trna synthetase enzymes consume atp in the attachment trna to amino acids, forming aminoacyl-trna complexes. aminoacyl transferase binds amp-amino acid to trna. the coupling reaction proceeds in two steps: aa + atp ⟶ aa-amp + ppi aa-amp + trna ⟶ aa-trna + amp the amino acid is coupled to the penultimate nucleotide at the 3′-end of the trna (the a in the sequence cca) via an ester bond (roll over in illustration). atp binding cassette transporter transporting chemicals out of a cell against a gradient is often associated with atp hydrolysis. transport is mediated by atp binding cassette transporters. the human genome encodes 48 abc transporters, that are used for exporting drugs, lipids, and other compounds. extracellular signalling and neurotransmission cells secrete atp to communicate with other cells in a process called purinergic signalling. atp serves as a neurotransmitter in many parts of the nervous system, modulates ciliary beating, affects vascular oxygen supply etc. atp is either secreted directly across the cell membrane through channel proteins or is pumped into vesicles which then fuse with the membrane. cells detect atp using the purinergic receptor proteins p2x and p2y. protein solubility atp has recently been proposed to act as a biological hydrotrope and has been shown to affect proteome-wide solubility. atp analogues biochemistry laboratories often use in vitro studies to explore atp-dependent molecular processes. atp analogs are also used in x-ray crystallography to determine a protein structure in complex with atp, often together with other substrates. enzyme inhibitors of atp-dependent enzymes such as kinases are needed to examine the binding sites and transition states involved in atp-dependent reactions. most useful atp analogs cannot be hydrolyzed as atp would be; instead, they trap the enzyme in a structure closely related to the atp-bound state. adenosine 5′-(γ-thiotriphosphate) is an extremely common atp analog in which one of the gamma-phosphate oxygens is replaced by a sulfur atom; this anion is hydrolyzed at a dramatically slower rate than atp itself and functions as an inhibitor of atp-dependent processes. in crystallographic studies, hydrolysis transition states are modeled by the bound vanadate ion. caution is warranted in interpreting the results of experiments using atp analogs, since some enzymes can hydrolyze them at appreciable rates at high concentration. medical use atp is used intravenously for some heart related conditions. history atp was discovered in 1929 by karl lohmann and jendrassik and, independently, by cyrus fiske and yellapragada subba rao of harvard medical school, both teams competing against each other to find an assay for phosphorus. it was proposed to be the intermediary between energy-yielding and energy-requiring reactions in cells by fritz albert lipmann in 1941. it was first synthesized in the laboratory by alexander todd in 1948, and he was awarded the nobel prize in chemistry in 1957 partly for this work. the 1978 nobel prize in chemistry was awarded to dr. peter dennis mitchell for the discovery of the chemiosmotic mechanism of atp synthesis. the 1997 nobel prize in chemistry was divided, one half jointly to paul d. boyer and john e. walker "for their elucidation of the enzymatic mechanism underlying the synthesis of adenosine triphosphate (atp)" and the other half to jens c. skou "for the first discovery of an ion-transporting enzyme, na+, k+ -atpase."