1 The three-dimensional nature of proteins
Proteins are made up of one or more polypeptide chains, polymers of amino acid residues found in all proteins. The structure that a polypeptide adopts is determined by the component amino acid units – both their chemical properties and the order in which they occur in the polymer – and by the structure of the peptide bond that links them.
Protein structure is described in terms of four levels of organisation: primary, secondary, tertiary and quaternary. The linear sequence of amino acids in the polymer is its primary structure and the conformations that the polypeptide adopts are described as secondary structure. The arrangement of elements of secondary structure in a folded polypeptide defines its tertiary structure, and, finally, the quaternary structure describes the association of two or more folded polypeptides in multisubunit proteins. In this section, we will begin by considering the structure of the peptide bond, and then go on to look at each of the levels of protein structure in turn.