|Jmol-3D images||Image 1|
|Molar mass||165.21 g mol−1|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
Oxidation of the sulfur of methionine results in methionine sulfoxide or methionine sulfone. The sulfur-containing amino acids methionine and cysteine are more easily oxidized than the other amino acids. Unlike oxidation of other amino acids, the oxidation of methionine can be reversed by enzymatic action, specifically by enzymes in the methionine sulfoxide reductase family of enzymes. The three known methionine sulfoxide reductases are MsrA, MsrB, and fRmsr. Oxidation of methionine results in a mixture of the two diastereomers methionine-S-sulfoxide and methionine-R-sulfoxide, which are reduced by MsrA and MsrB, respectively. MsrA can reduce both free and protein-based methionine-S-sulfoxide, whereas MsrB is specific for protein-based methionine-R-sulfoxide. fRmsr, however, catalyzes the reduction of free methionine-R-sulfoxide. Thioredoxin serves to recycle by reduction some of the methionine sulfoxide reductase family of enzymes, whereas others can be reduced by metallothionein.
Methionine sulfoxide increases with age in body tissues, which is believed to contribute to ageing. Transgenic Drosophila (fruit flies) that overexpress methionine sulfoxide reductase show extended lifespan. Methionine sulfoxide (Met(o)) is the oxidized form of amino acid methionine (Met). The oxidation of methionine serves as a switch that deactivates certain protein activities such as E.coli ribosomal protein, L12. Protein with great amount of methionine residues tend to exist within the lipid bilayer as methionine is one of the most hydrophobic amino acid. Those methionine residues that are exposed to the aqueous exterior thus are vulnerable to oxidation.2 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 met(o) residues are reduced back to methionine by the enzyme methionine sulfoxide reductase.3 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
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- Brot, N; Weissbach, L; Werth, J; Weissbach, H (April 1981). "Enzymatic reduction of protein-bound methionine sulfoxide.". Proceedings of the National Academy of Sciences of the United States of America 78 (4): 2155–8. doi:10.1073/pnas.78.4.2155. PMC 319302. PMID 7017726.
- Levine, RL; Mosoni, L; Berlett, BS; Stadtman, ER (Dec 24, 1996). "Methionine residues as endogenous antioxidants in proteins.". Proceedings of the National Academy of Sciences of the United States of America 93 (26): 15036–40. doi:10.1073/pnas.93.26.15036. PMC 26351. PMID 8986759.