Conformational Change in Aspartate Aminotransferase on Substrate Binding Induces Strain in the Catalytic Group and Enhances Catalysis*

Aspartate aminotransferase has been known to undergo a significant conformational change, in which the small domain approaches the large domain, and the residues at the entrance of the active site pack together, on binding of substrates. Accompanying this conformational change is a two-unit increase in the pK a of the pyridoxal 5′-phosphate-Lys258 aldimine, which has been proposed to enhance catalysis. To elucidate how the conformational change is coupled to the shift in the aldimine pK a and how these changes are involved in catalysis, we analyzed structurally and kinetically an enzyme in which Val39located at both the domain interface and the entrance of the active site was replaced with a bulkier residue, Phe. The V39F mutant enzyme showed a more open conformation, and the aldimine pK a was lowered by 0.7 unit compared with the wild-type enzyme. When Asn194 had been replaced by Ala in advance, the V39F mutation did not decrease the aldimine pK a , showing that the domain rotation controls the aldimine pK a via the Arg386-Asn194-pyridoxal 5′-phosphate linkage system. The maleate-bound V39F enzyme showed the aldimine pK a 0.9 unit lower than that of the maleate-bound wild-type enzyme. However, the positions of maleate, Asn194, and Arg386 were superimposable between the mutant and the wild-type enzymes; therefore, the domain rotation was not the cause of the lowered aldimine pK a value. The maleate-bound V39F enzyme showed an altered side-chain packing pattern in the 37–39 region, and the lack of repulsion between Gly38 carbonyl O and Tyr225Oη seemed to be the cause of the reduced pK a value. Kinetic analysis suggested that the repulsion increases the free energy level of the Michaelis complex and promotes the catalytic reaction.