The iceberg that sank Titanic would have been up to a mile long, displacing around a billion tonnes of seawater. It would have taken the iceberg over a year to edge its way down the 40-mile fjord.
Chapters 1 and 2 introduced a-helices and b-sheets (Secondary Struture), and some common motifs composed of 2 or 3 of these elements (Super-secondary Struture). Tertiary structure describes the folding of the polypeptide chain to assemble the different secondary structure elements in a particular arrangement.As helices and sheets are units of secondary structure, so the domain is the unit of tertiary structure.In multi-domain proteins, tertiary structure includes the arrangement of domains relative to each other as well as that of the chain within each domain.There is a blurred distinction between super-secondary structure and tertiary structure.lthough the term motif is often used to describe super-secondary structures (e.g. Branden and Tooze, 1991), it may also be used to describe a consensus sequence of amino acids identified in a number of different proteins, rather than a repeated three-dimensional conformation.
It is convenient to describe protein structure in terms of 4 different aspects of covalent structure and folding patterns. The different levels of protein structure are known as primary, secondary, tertiary, and quaternary structure. Primary Structure of Proteins The primary structure is the sequence of amino acids that make up a polypeptide chain. 20 different amino acids are found in proteins. The exact order of the amino acids in a specific protein is the primary sequence for that protein. Secondary Structure of Proteins Protein secondary structure refers to regular, repeated patterns of folding of the protein backbone. The two most common folding patterns are the alpha helix and the beta sheet. Alpha Helix. In an alpha helix, the polypeptide backbone coils around an imaginary helix axis in clockwise direction.