ribosomal protein, L12. Proteins with great amount of methionine residues tend to exist within the lipid bilayer as methionine is one of the most hydrophobic amino acids. Those methionine residues that are exposed to the aqueous exterior thus are vulnerable to oxidation. The oxidized residues tend to be arrayed around the active site and may guard access to this site by reactive oxygen species. Once oxidized, the MetO residues are reduced back to methionine by the enzyme methionine sulfoxide reductase. Thus, an oxidation–reduction cycle occurs in which exposed methionine residues are oxidized (e.g., by H2O2) to methionine sulfoxide residues, which are subsequently reduced. Methionine(protein)+ H2O2→ Methionine Sulfoxide(protein)+ H2O Methionine Sulfoxide(protein)+ NADPH+H+→ Methionine(protein)+ NADP++H2O See also MSRA (gene) MSRB2 enzyme SEPX1 enzyme References External links Methionine oxidation chemistry Biochemistry Organic redox reactions Amino acid derivatives