The Enzyme Database

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Accepted name: arsenite methyltransferase
Reaction: (1) S-adenosyl-L-methionine + arsenic triglutathione + thioredoxin + 2 H2O = S-adenosyl-L-homocysteine + methylarsonous acid + 3 glutathione + thioredoxin disulfide
(2) 2 S-adenosyl-L-methionine + arsenic triglutathione + 2 thioredoxin + H2O = S-adenosyl-L-homocysteine + dimethylarsinous acid + 3 glutathione + 2 thioredoxin disulfide
(3) 3 S-adenosyl-L-methionine + arsenic triglutathione + 3 thioredoxin = S-adenosyl-L-homocysteine + trimethylarsane + 3 glutathione + 3 thioredoxin disulfide
For diagram of arsenate catabolism, click here
Other name(s): AS3MT (gene name); arsM (gene name); S-adenosyl-L-methionine:arsenic(III) methyltransferase; S-adenosyl-L-methionine:methylarsonite As-methyltransferase; methylarsonite methyltransferase
Systematic name: S-adenosyl-L-methionine:arsenous acid As-methyltransferase
Comments: An enzyme responsible for synthesis of trivalent methylarsenical antibiotics in microbes [11] or detoxification of inorganic arsenous acid in animals. The in vivo substrate is arsenic triglutathione or similar thiol (depending on the organism) [6], from which the arsenic is transferred to the enzyme forming bonds with the thiol groups of three cysteine residues [10] via a disulfide bond cascade pathway [7, 8]. Most of the substrates undergo two methylations and are converted to dimethylarsinous acid [9]. However, a small fraction are released earlier as methylarsonous acid, and a smaller amount proceeds via a third methylation, resulting in the volatile product trimethylarsane. Methylation involves temporary oxidation to arsenic(V) valency, followed by reduction back to arsenic(III) valency using electrons provided by thioredoxin or a similar reduction system. The arsenic(III) products are quickly oxidized in the presence of oxygen to the corresponding arsenic(V) species.
Links to other databases: BRENDA, EAWAG-BBD, EXPASY, KEGG, MetaCyc, PDB, CAS registry number: 167140-41-2
1.  Zakharyan, R.A., Wu, Y., Bogdan, G.M. and Aposhian, H.V. Enzymatic methylation of arsenic compounds: assay, partial purification, and properties of arsenite methyltransferase and monomethylarsonic acid methyltransferase of rabbit liver. Chem. Res. Toxicol. 8 (1995) 1029–1038. [PMID: 8605285]
2.  Zakharyan, R.A., Wildfang, E. and Aposhian, H.V. Enzymatic methylation of arsenic compounds. III. The marmoset and tamarin, but not the rhesus, monkeys are deficient in methyltransferases that methylate inorganic arsenic. Toxicol. Appl. Pharmacol. 140 (1996) 77–84. [DOI] [PMID: 8806872]
3.  Zakharyan, R.A. and Aposhian, H.V. Enzymatic reduction of arsenic compounds in mammalian systems: the rate-limiting enzyme of rabbit liver arsenic biotransformation is MMA(V) reductase. Chem. Res. Toxicol. 12 (1999) 1278–1283. [DOI] [PMID: 10604879]
4.  Zakharyan, R.A., Ayala-Fierro, F., Cullen, W.R., Carter, D.M. and Aposhian, H.V. Enzymatic methylation of arsenic compounds. VII. Monomethylarsonous acid (MMAIII) is the substrate for MMA methyltransferase of rabbit liver and human hepatocytes. Toxicol. Appl. Pharmacol. 158 (1999) 9–15. [DOI] [PMID: 10387927]
5.  Lin, S., Shi, Q., Nix, F.B., Styblo, M., Beck, M.A., Herbin-Davis, K.M., Hall, L.L., Simeonsson, J.B. and Thomas, D.J. A novel S-adenosyl-L-methionine:arsenic(III) methyltransferase from rat liver cytosol. J. Biol. Chem. 277 (2002) 10795–10803. [DOI] [PMID: 11790780]
6.  Hayakawa, T., Kobayashi, Y., Cui, X. and Hirano, S. A new metabolic pathway of arsenite: arsenic-glutathione complexes are substrates for human arsenic methyltransferase Cyt19. Arch Toxicol 79 (2005) 183–191. [DOI] [PMID: 15526190]
7.  Dheeman, D.S., Packianathan, C., Pillai, J.K. and Rosen, B.P. Pathway of human AS3MT arsenic methylation. Chem. Res. Toxicol. 27 (2014) 1979–1989. [DOI] [PMID: 25325836]
8.  Marapakala, K., Packianathan, C., Ajees, A.A., Dheeman, D.S., Sankaran, B., Kandavelu, P. and Rosen, B.P. A disulfide-bond cascade mechanism for arsenic(III) S-adenosylmethionine methyltransferase. Acta Crystallogr. D Biol. Crystallogr. 71 (2015) 505–515. [DOI] [PMID: 25760600]
9.  Yang, H.C. and Rosen, B.P. New mechanisms of bacterial arsenic resistance. Biomed J 39 (2016) 5–13. [DOI] [PMID: 27105594]
10.  Packianathan, C., Kandavelu, P. and Rosen, B.P. The structure of an As(III) S-adenosylmethionine methyltransferase with 3-coordinately bound As(III) depicts the first step in catalysis. Biochemistry 57 (2018) 4083–4092. [DOI] [PMID: 29894638]
11.  Chen, J., Yoshinaga, M. and Rosen, B.P. The antibiotic action of methylarsenite is an emergent property of microbial communities. Mol. Microbiol. 111 (2019) 487–494. [DOI] [PMID: 30520200]
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