Human tryptophan transfer ribonucleic acid synthetase. Composition, function of thiol groups, and structure of thiol peptides.

Human tryptophanyl-tRNA synthetase resembles its counterpart in Escherichia coli in quaternary structure (alpha2), but differs in molecular weight, amino acid composition, the number of thiol groups, and the relationship of the thiol groups to enzyme activity. Nevertheless, one of the thiol groups resides in a heptapeptide sequence homologous to a heptapeptide sequence containing a thiol group in the E. coli enzyme. Each subunit of the enzyme has 6 half-cystine residues, and four thiol groups are readily titrated with 5,5'-dithiobis(2-nitrobenzoic acid). Titration of these four thiol groups inactivates the enzyme, and the inactivation is partially reversible by reduction with dithiothreitol. One thiol group reacts rapidly unless L-tryptophan, ATP, and Mg2+ are present together.