Structure of proteins

Beyond simple shape classification (globular vs fibrous, section 14.3.3), the true diversity of protein shapes and functions -- from the load-bearing fibres of hair and muscle, to the catalytic pockets of enzymes -- is properly understood through a formal hierarchy of FOUR nested structural levels, each one built on top of the level before it. Primary structure is the flat, one-dimensional SEQUENCE of amino-acid residues along the chain (the level considered in section 14.3.4a). Secondary structure is the local, repeating three-dimensional folding pattern -- chiefly the alpha-helix and the beta-pleated sheet -- that short stretches of the primary sequence adopt, driven by hydrogen bonding along the peptide backbone itself (section 14.3.4b). Tertiary structure is the overall, non-repeating three-dimensional shape into which one ENTIRE polypeptide chain folds, combining its secondary-structure elements together with looser connecting regions, stabilised by a wider range of non-covalent (and sometimes covalent, disulfide) forces between side chains that may be far apart in the sequence but close together once folded (section 14.3.4c). Quaternary structure is the highest level, describing how two or more separately-folded (tertiary-structure) polypeptide chains associate together into one larger, functional multi-subunit protein complex (section 14.3.4d). Together, these four levels -- each covered in i …