Hyperconjugation Promotes Catalysis in a Pyridoxal 5 '-Phosphate-Dependent Enzyme

Pyridoxal 5'-phosphate (PLP)-dependent enzymes facilitate reaction specificity by aligning the scissile sigma-bond of the PLP-substrate covalent complex perpendicular to the ring of the cofactor. Current models propose that this alignment causes a destabilization of the ground state. To test this hypothesis, quantum chemical calculations, utilizing our recent neutron diffraction models of aspartate aminotransferase, were performed. The calculations reveal that the scissile sigma-bond orbital overlaps significantly with the pi* orbital of the Schiff base. This sigma -> pi* hyperconjugation interaction stabilizes the ground state of the external aldimine and substantially contributes to transition-state stabilization by withdrawing electron density from the C alpha-H sigma bond into the pi system of PLP, enhancing the rate of catalysis.