A comparative study of the kinetics and stereochemistry of the serine hydroxymethyltransferase- and tryptophan synthase-catalysed exchange of the pro-2R and pro-2S protons of glycine.
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.
Malthouse
J P
JP
Department of Biochemistry, University College Dublin, Belfield, Ireland.
Milne
J J
JJ
Gariani
L S
LS
D000818:Animals; D002384:Catalysis; D002417:Cattle; D004926:Escherichia coli / Q000201:enzymology; D005998:Glycine / Q000737:chemistry*; D012696:Glycine Hydroxymethyltransferase / Q000737:chemistry*; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D009682:Magnetic Resonance Spectroscopy; D011522:Protons; D011732:Pyridoxal Phosphate / Q000737:chemistry; D012486:Salmonella typhimurium / Q000201:enzymology; D013237:Stereoisomerism; D013379:Substrate Specificity; D014367:Tryptophan Synthase / Q000737:chemistry*
D011522:Protons; D011732:Pyridoxal Phosphate; D012696:Glycine Hydroxymethyltransferase; D014367:Tryptophan Synthase; D005998:Glycine