Cysteinyl-tRNA synthetase from Escherichia coli does not need an editing mechanism to reject serine and alanine. High binding energy of small groups in specific molecular interactions.

The cysteinyl-tRNA synthetase from Escherichia coli only very slowly activates serine, alanine, and alpha-aminobutyrate, the possible competitors of cysteine. The upper limits on the values of kcat/KM for the amino acid dependent ATP/pyrophosphate exchange reactions, relative to that of cysteine, are less than 10(-8), 2 x 10(-7), and 3 x 10(-6), respectively. It is calculated from these data and the concentrations of the amino acids in vivo that the error rates for the misincorporation of serine and alanine for cysteine are less than 10(-9) and 5 x 10(-8), respectively. There is no need for an error correcting mechanism and no evidence has been found to implicate one: there is no detectable ATP/pyrophosp hatase activity of the enzyme in the presence of tRNACys and alanine; Ala-tRNACys has been synthesized by the reductive desulfurization of Cys-tRNACys and has been found to be relatively resistant to the enzyme-catalyzed deacylation. Part of the high selectivity of the enzyme for the -SH group of cysteine (approximately 5 kcal/mol) appears to be caused by dispersion forces: simple calculations suggest that the dispersion energy between sulfur and a methylene group is about 2.5 times greater than that between two methylene groups. This high "hydrophobicity" of sulfur is consistent with the relative binding energies of substrates of the methionyl-tRNA synthetase. The rest of the high binding energy of the-SH group may come from hydrogen bonding.