Probing the principles of amino acid selection using the alanyl-tRNA synthetase from Escherichia coli.

The alanyl-tRNA synthetase from Escherichia coli activates cysteine, alpha-aminobutyrate and other-noncognate amino acids that are larger than alanine so slowly that no editing mechanism is required for error correction. Serine, however, is activated sufficiently rapidly that an editing mechanism is required to remove the products of misactivation. The distinction between the nominally isosteric trio of cysteine, alpha-aminobutyrate and serine by the activation site of the enzyme is attributed to the effect of small differences in size on steric repulsion, the C-O bond length being somewhat shorter than either the C-C or C-S and the van der Waals' radius of -O- being smaller than that of -S- or -CH2-. The smaller amino acid glycine is also readily activated and its reaction products rapidly removed by hydrolytic editing.