The stereospecificity of the serine hydroxymethyltransferase (EC 2.1.2.1)- and tryptophan synthase (EC 4.2.1.20)- catalysed exchange of the pro-2R and pro-2S alpha-protons of glycine was investigated by using 13C n.m.r. The exchange process is described in terms of a minimal four-step mechanism, and a method for analysing the exchange process by complete progress curves is presented. It is shown that serine hydroxymethyltransferase does not have absolute stereospecificity for the pro-2S-proton of glycine, but it catalyses the exchange of this proton 7400 times faster than the pro-2R proton of glycine. Tryptophan synthase is shown preferentially to catalyse the exchange of the pro-2R proton of glycine at a rate 380 times faster than the pro-2S proton of glycine. The exchange rates for the rapidly exchanged alpha-protons of glycine are similar for both enzymes. However, the exchange rates of the slowly exchanged alpha-protons differ by an order of magnitude. The structural features that may be responsible for the differences in the stereospecificity of the two enzymes are discussed.

Isotopically enriched amino acids have a wide range of uses in biochemistry, e.g. I3C enrichment can be utilized for 13C nuclear magnetic resonance (n.m.r.) studies of enzymes (Malthouse, 1986). Isotopically labelled serine could also be used as precursor for the synthesis of other amino acids. Most chemical o r enzymic procedures have focused on preparing deuterium-or tritium-enriched serine and either produce low yields (less than 20%) or poor stereospecificity (Slieker & Benkovic, 1982). Consequently, isotopically enriched serine is expensive, e.g. I3C-enriched L-serine is at *To whom correspondence should he addressed. A 3