Substrate specificity and protonation state of ornithine transcarbamoylase as determined by pH studies

The ornithine transcarbamoylase catalyzed reaction and its inhibition by L-norvaline have been investigated between pH 5.5 and 10.5. The steady-state turnover rate (kcat) of the enzyme from Escherichia coli increases with pH and plateaus above pH 9. Its change with pH conforms to a single protonation process with an apparent pKa of 7.3. The effect of pH on the apparent Michaelis constant (KMapp) of L-ornithine suggests that this diamino acid in its cationic form is not the substrate. Treating only the zwitterions of ornithine as substrate, the pH profile of the pseudo-first-order rate constant (kcat/KMz) of the reaction is a bell-shaped curve characterized by pKa's of 6.2 and 9.1 and asymptotic slopes of +/- 1. Similar pKa's (6.3 and 9.3) are obtained for the pKi profile of zwitterionic L-norvaline, a competitive inhibitor. The pKi profile further indicates that the alpha-amino group of the inhibitor must be charged for binding. Together, these pH profiles provide sufficient information to suggest that only the minor zwitterionic species of ornithine, H2N(CH2)3CH(NH3+)COO-, binds the enzyme productively. The selection of this substrate form by the enzyme leads to a Michaelis complex in which ornithine is poised for nucleophilic attack. Following such binding, the need for deprotonation of the delta-NH3+ group is avoided, and transcarbamoylation becomes energetically more feasible. Reaction schemes accounting for the effects of pH are proposed for the enzymic reaction.