A substrate‐induced change in the stereospecificity of the serine‐hydroxymethyltransferase‐catalysed exchange of the α‐protons of amino acids

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Abstract
NMR has been used to study the catalysis of the hydrogen−deuterium exchange of the α‐protons of amino acids by serine hydroxymethyltransferase ( EC2.1.2.1) from Escherichia coli.13C‐NMR was used to follow the exchange of the α‐protons of [2‐13C]glycine. The enzyme‐catalysed first‐order exchange rate of the pro‐2S proton of glycine was ≈7000 times more efficient than that of the pro‐2R proton of glycine at both pH 7.0 and 7.8. 1H‐NMR was used to follow the hydrogen−deuterium exchange rates of the α‐protons of L‐ and D‐2‐amino derivatives of butyric, pentanoic and hexanoic acids at pH 7.8. Increasing the size of the R‐group leads to a progressive change in the stereospecificity of the exchange reaction from the pro‐2S proton of glycine to the 2R proton of L‐amino acids. The stereospecificity for the α‐protons of L‐amino acids increased as the size of the R‐group increased. With glycine, removal of tetrahydrofolate led to a large decrease in the stereospecificity of the exchange reaction but did not affect the exchange rates of the α‐protons of any of the larger amino acids studied. We show that the Schiff base formed between L‐2‐aminohexanoic acid ( L‐norleucine) and pyridoxal 5′‐phosphate binds at a different site from the Schiff base between glycine and pyridoxal 5′‐phosphate. The molecular basis of these results is discussed.