Figure 6.

Proposed model for differential modulation of catalytic cysteine nucleophilicity in basal-plant (left) and euphyllophyte (right) CHSs. In basal-plant CHSs (left), the serine (Ser-340 in SmCHS) interacts more strongly with the histidine of the catalytic triad, weakening the ionic interaction that stabilizes the thiolate form of the catalytic cysteine. This is depicted as a shift of the equilibrium toward a state in which the positive charge on the histidine (blue) is shifted away from the catalytic cysteine (Cys-159 in SmCHS), and the shared proton interacts more closely with cysteine. In euphyllophyte CHSs (right), this position mutated to a cysteine (Cya-347 in AtCHS), which interacts relatively loosely with the catalytic histidine, in turn strengthening the ionic interaction between the catalytic histidine and the activated thiolate of the catalytic cysteine. This is depicted as a shift of the equilibrium toward a state in which the positive charge on the histidine (blue) is shifted toward the catalytic cysteine (Cys-169 in AtCHS).