Editing function of Escherichia coli cysteinyl-tRNA synthetase: cyclization of cysteine to cysteine thiolactone.
A cyclic sulfur compound, identified as cysteine thiolactone by several chemical and enzymatic tests, is formed from cysteine during in vitro tRNA(Cys) aminoacylation catalyzed by Escherichia coli cysteinyl-tRNA synthetase. The mechanism of cysteine thiolactone formation involves enzymatic deacylation of Cys-tRNA(Cys) (k = 0.017 s-1) in which nucleophilic sulfur of the side chain of cysteine in Cys-tRNA(Cys) attacks its carboxyl carbon to yield cysteine thiolactone. Nonenzymatic deacylation of Cys-tRNA(Cys) (k = 0.0006 s-1) yields cysteine, as expected. Inhibition of enzymatic deacylation of Cys-tRNA(Cys) by cysteine and Cys-AMP, but not by ATP, indicates that both synthesis of Cys-tRNA(Cys) and cyclization of cysteine to the thiolactone occur in a single active site of the enzyme. The cyclization of cysteine is mechanistically similar to the editing reactions of methionyl-tRNA synthetase. However, in contrast to methionyl-tRNA synthetase which needs the editing function to reject misactivated homocysteine, cysteinyl-tRNA synthetase is highly selective and is not faced with a problem in rejecting noncognate amino acids. Despite this, the present day cysteinyl-tRNA synthetase, like methionyl-tRNA synthetase, still retains an editing activity toward the cognate product, the charged tRNA. This function may be a remnant of a chemistry used by an ancestral cysteinyl-tRNA synthetase.
Jakubowski
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Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, Newark 07103.
D000604:Amino Acyl-tRNA Synthetases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002855:Chromatography, Thin Layer; D003545:Cysteine / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology*; D006710:Homocysteine / Q000031:analogs & derivatives* / Q000378:metabolism; D011755:Pyrophosphatases / Q000378:metabolism; D017393:RNA Editing
D006710:Homocysteine; C007957:homocysteine thiolactone; D011755:Pyrophosphatases; C019757:ATP pyrophosphohydrolase; D000604:Amino Acyl-tRNA Synthetases; C019975:cysteinyl-tRNA synthetase; D003545:Cysteine