Oxidation of methionine in proteins: Roles in antioxidant defense and cellular regulation

The roles of methionine residues in proteins have not been well defined, bu t a review of available studies leads to the conclusion that methionine, li ke cysteine, functions as an antioxidant and as a key component of a system for regulation of cellular metabolism. Methionine is readily oxidized to m ethionine sulfoxide by many reactive species. The oxidation of surface expo sed methionines thus serves to protect other functionally essential residue s from oxidative damage. Methionine sulfoxide reductases have the potential to reduce the residue back to methionine, increasing the scavenging effici ency of the system. Reversible covalent modification of amino acids in prot eins provides the mechanistic basis for most systems of cellular regulation . Interconversion of methionine and methionine sulfoxide can function to re gulate the biological activity of proteins, through alteration in catalytic efficiency and through modulation of the surface hydrophobicity of the pro tein.