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Journal of Experimental Botany, Vol. 53, No. 368, pp. 439-445, March 1, 2002
© 2002 Oxford University Press

Original Papers

Modulation of cyanoalanine synthase and O-acetylserine (thiol) lyases A and B activity by ß-substituted alanyl and anion inhibitors

Andrew G. S. Warrilow and Malcolm J. Hawkesford1

IACR-Rothamsted, Agriculture and Environment Division, Harpenden, Hertfordshire AL5 2JQ, UK

The reaction mechanisms of three enzymes belonging to a single gene family are compared: a cyanoalanine synthase and two isoforms of O-acetylserine (thiol) lyase (O-ASTL) isolated from spinach (Spinacea oleracea L. cv. Medina). O-ASTL represents a major regulatory point in the S-assimilatory pathway, and the related cyanoalanine synthase, which is specific to the mitochondrial compartment, has evolved an independent function of cyanide detoxification. All three enzymes catalysed both the cysteine synthesis and cyanoalanine synthesis reactions although with different efficiencies, and which may be explained by a single amino acid substitution in the substrate-binding pocket of the enzyme. Substituted alanine and nucleophillic inhibitors caused predominantly non-competitive inhibition, indicating binding to both E- and F-forms of the enzyme in a bi–bi ping-pong kinetic model. Michaelis–Menten kinetics were observed when the alanyl substrate was varied in the presence and absence of inhibitors. The use of alanyl inhibitors has shown that the alanyl half-cycle of both the cysteine synthesis and cyanoalanine synthesis reactions of cyanoalanine synthase and O-acetylserine (thiol) lyases are similar. This is in contrast to the results observed with nucleophillic inhibitors, which have shown that the mechanisms of anion binding and processing differ between cyanoalanine synthase and O-ASTLs.

Key words: Cyanoalanine synthase, cysteine synthase, O-acetylserine(thiol)lyase, sulphur metabolism.


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