Acid-base catalysis in the argininosuccinate lyase reaction

The pH variation of the kinetic parameters, Vmax and V/K, was examined for the forward and reverse reaction of bovine liver argininosuccinate lyase. In the forward reaction the Vmax profile showed one group that must be unprotonated for activity over the pH range 5-10. The V/K profile for argininosuccinate showed one group that must be unprotonated and two groups that must be protonated for activity. The Vmax profile for the reverse reaction showed only one group that must be protonated for activity. These results support the proposal that catalysis is facilitated in the forward reaction by a general base that abstracts a proton from C-3 of argininosuccinate and a general acid that donates a proton to the guanidinium nitrogen during carbon-nitrogen bond cleavage. The enzyme is completely inactivated by diethyl pyrocarbonate or a water-soluble carbodiimide at pH 6. These experiments suggest that a histidine and a carboxyl group are at or near the active site and are essential for catalytic activity. The observed shifts of the pH profiles of the forward reaction with temperature and organic solvent (25% dioxane) were also consistent with a histidine and carboxylate group.