Kinetics of sulfhydryl group modification of thymidylate synthetase: a proposal for activation of catalytic cysteinly residues.

The pKa of ～8.0 for the catalytic cysteine residue of thymidylate synthetase was determined from the pH dependence of inactivation by the sulfhydryl reagents methyl methanethiolsulfonate and 5,5′ dithiobis (2-nitrobenzoic acid). At low pH (5.8–6.8) a rate of reaction significantly greater than can be accounted for by the concentration of thiolate anion was observed. The observed pKa and reactivity for thymidylate synthetase are comparable to those reported for papain (Little, G. L. and Brocklehurst, K. (1972) Biochem. J. $\text{128}$, 475–477) (1). On the basis of these observations we propose that the cysteines of thymidylate synthetase involved in covalent catalysis may be activated through interaction with a general base in the active site of the enzyme.