The role of cysteine 206 in allosteric inhibition of Escherichia coli citrate synthase. Studies by chemical modification, site-directed mutagenesis, and 19F NMR.

Escherichia coli citrate synthase is strongly and specifically inhibited by NADH, but this inhibition can be prevented by reacting the enzyme with Ellman's reagent. We have now labeled the single reactive cysteine covalently with monobromobimane and isolated and sequenced the bimane-containing cyanogen bromide peptide and identified the cysteine as Cys-206. Modeling studies suggest that this residue is on the subunit surface, 25-30 A from the active site. Mutation of Cys-206 to serine (C206S), or of Gly-207 to alanine (E207A), weakened NADH binding and inhibition; when these mutations were present together, NADH binding was weaker by 18-fold and inhibition by 250-fold. The mutations also had small effects on substrate binding at the active site. Cys-206 of wild type enzyme and of the mutant E207A was alkylated with 1,1,1-trifluorobromoacetone and the environment of the fluorine nuclei studied by 19F NMR. With wild type enzyme, the NMR spectrum consisted of two peaks of about equal intensity but different line widths, at -8.65 ppm (line width 11.2 +/- 0.5 Hz) and -7.6 ppm (line width 57 +/- 4 Hz). As the labeled wild type citrate synthase was titrated with KCl, the narrow peak converted to the broad one. The same range of KCl concentrations was needed for this conversion as for the allosteric activation of E. coli citrate synthase. The E207A mutant gave the broader NMR peak almost exclusively. We propose that the fluorine label in wild type citrate synthase exists in two conformational states with different mobilities, exchanging slowly on the NMR time scale, and that treatment with KCl, or truncation of the Glu-207 side chain by mutagenesis, stabilizes one of these states. Consistent with this explanation is the finding that Cys-206 reacts more quickly with Ellman's reagent in the presence of KCl, and that this rate is faster yet in the E207A mutant.