A complex biological macromolecule consisting of one or more * polypeptide chains. Two major classes of protein are recognized: globular proteins, most of which are enzymes; and fibrous proteins, which are usually structural or contractile in function. The structure of a protein can be divided into primary, secondary, tertiary, and quaternary components. Primary structure, the amino acid sequence of the polypeptide chain, determines the three-dimensional shape of the protein. The secondary structure is the coiling or pleating of the polypeptide chain. In globular proteins the coiled polypeptide chain is further folded into a three-dimensional shape, which is maintained by weak hydrophobic and polar interactions between amino acid residues. Quaternary structure is the combination of more than one polypeptide chain to form an oligomeric protein molecule. See also conjugated protein. |
