Proofreading and the evolution of a methyl donor function. Cyclization of methionine to S-methyl homocysteine thiolactone by Escherichia coli methionyl-tRNA synthetase.
A cyclic sulfonium compound, S-methyl homocysteine thiolactone (SMHT), is formed from methionine during in vitro tRNA aminoacylation catalyzed by Escherichia coli methionyl-tRNA synthetase. The mechanism of SMHT formation involves enzymatic deacylation of Met-tRNA (k = 0.06 s-1) and, to a lesser extent, Met-AMP (k = 0.02 s-1). Cyclization of methionine, reminiscent of cyclization of homocysteine during editing, illustrates the limited ability of methionyl-tRNA synthetase to discriminate against the cognate methionine at the editing site designed for the noncognate homocysteine. In early stages of biotic evolution, SMHT, a sulfonium compound, may have fulfilled the present day methyl donor function of S-adenosylmethionine. Existing homologies between methionyl-tRNA synthetase and S-adenosylmethionine synthetase indicate evolutionary relatedness of the two proteins.
Jakubowski
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Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark 07103.
D000215:Acylation; D000595:Amino Acid Sequence; D000596:Amino Acids / Q000378:metabolism; D005075:Biological Evolution; D002855:Chromatography, Thin Layer; D003500:Cyclization; D004926:Escherichia coli / Q000201:enzymology*; D006710:Homocysteine / Q000031:analogs & derivatives* / Q000378:metabolism; D007700:Kinetics; D008715:Methionine / Q000737:chemistry / Q000378:metabolism*; D008718:Methionine-tRNA Ligase / Q000378:metabolism*; D008745:Methylation; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D012358:RNA, Transfer, Met / Q000378:metabolism; D012451:Safrole / Q000031:analogs & derivatives / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D000596:Amino Acids; D012358:RNA, Transfer, Met; D006710:Homocysteine; C080350:S-methyl homocysteine thiolactone; D008715:Methionine; D008718:Methionine-tRNA Ligase; D012451:Safrole; C005746:sulfoxide