A chain

1. a polypeptide component of insulin containing 21 amino acyl residues, beginning with a glycyl residue (NH2-terminus); insulin is formed by the linkage of an A chain to a B chain by two disulfide bonds; the amino-acid composition of the A chain is a function of species; Syn: glycyl chain.

2. in general, one of the polypeptides in a multiprotein complex.