1H-NMR has been used to follow the tryptophan synthase (EC 4.2.1.20) catalysed hydrogen–deuterium exchange of the α-protons of L- and D-alanine and -tryptophan. The first-order and second-order rate constants for exchange have been determined at pH 7.8 in the presence and absence of the allosteric effector, DL-α-glycerol 3-phosphate. In the presence of DL-α-glycerol 3-phosphate the stereospecificity of the tryptophan synthase-catalysed first-order exchange rates was in the order tryptophan > alanine > glycine. This increase in stereospecificity was largely due to the decrease in the magnitude of the first-order exchange rate of the slowly exchanged α-proton. A similar increase in the stereospecificity of the second-order exchange rates for alanine was also largely due to the decrease in the magnitude of the first-order exchange rate of the slowly exchanged α-proton of D-alanine. Adding DL-α-glycerol 3-phosphate produced an increase in the stereospecificity of the second-order exchange rate observed with alanine but no significant change in the stereospecificity of the first-order exchange rate with tryptophan. The α-subunits are shown to increase the exchange rates of the α-protons of L-alanine and L-tryptophan. We conclude that the contribution of the R-group of an amino acid to the stereospecificity of the exchange reactions of its α-proton can be similar to or larger than that of its α-carboxylate group. Possible mechanisms that could explain the stereospecificity of these exchange reactions are discussed.
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Research Article|
March 15 1996
The effect of different amino acid side chains on the stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the α-protons of amino acids
John J. MILNE;
John J. MILNE
1Department of Biochemistry, University College Dublin, Dublin 4, Ireland
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J. Paul G. MALTHOUSE
J. Paul G. MALTHOUSE
*
1Department of Biochemistry, University College Dublin, Dublin 4, Ireland
*To whom correspondence should be addressed.
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Publisher: Portland Press Ltd
Received:
August 25 1995
Revision Received:
October 31 1995
Accepted:
November 07 1995
Online ISSN: 1470-8728
Print ISSN: 0264-6021
The Biochemical Society, London © 1996
1996
Biochem J (1996) 314 (3): 787–791.
Article history
Received:
August 25 1995
Revision Received:
October 31 1995
Accepted:
November 07 1995
Citation
John J. MILNE, J. Paul G. MALTHOUSE; The effect of different amino acid side chains on the stereospecificity and catalytic efficiency of the tryptophan synthase-catalysed exchange of the α-protons of amino acids. Biochem J 15 March 1996; 314 (3): 787–791. doi: https://doi.org/10.1042/bj3140787
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