The stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the alpha-protons of amino acids.

13C-NMR has been used to follow the tryptophan synthase (EC 4.2.1.20) catalysed hydrogen-deuterium exchange of the pro-2R and pro-2S protons of [2-13C]glycine at pH 7.8. 1H-NMR has also been used to follow the tryptophan-synthase-catalysed hydrogen-deuterium exchange of the alpha-protons of a range of L- and D-amino acids at pH 7.8. The pK(a) values of the alpha-protons of these amino acids have been estimated and we have determined whether or not their exchange rates can be predicted from their pK(a) values. With the exception of tryptophan and norleucine, the stereospecificities of the first-order alpha-proton exchange rates are independent of the size and electronegativity of the amino acid R-group. Similar results are obtained with the second-order alpha-proton exchange rates, except that both L-tryptophan and L-serine have much higher stereospecificities than all the other amino acids studied.