A comparison of the antioxidant requirements of proteins with those of synthetic polymers suggests an antioxidant function for clusters of aromatic and bivalent sulphur-containing amino acid residues.

Many proteins which function in extracellular environments potentially rich in oxygen-derived free radicals contain clustered tyrosine and cysteine residues which, by analogy with the chemistry of antioxidants used with synthetic polymers, may provide an appreciable antioxidant and redox stabilization activity. Such proteins may function as antioxidants, and as ligand binding sites for free radicals and other active molecules employed in normal biochemical processes.