Refined Reaction Mechanism of Aspartate Aminotransferase

Summary

The combined spectral, kinetic, and structural studies on the reaction ofE. conaspartate aminotransferase (AspAT) with aspartate and its analogue, 2-methylaspartate, provided a new, refined mechanism for this catalytic reaction. The association of the substrate aspartate and AspAT proceeds via dual routes (E + SH⁺and EH⁺+ S). The Schiff base successively increases its pK _afrom 6.8 in the unliganded state to 8.8 in the Michaelis complex, and finally to >10 in the external aldimine. These pK _ashifts are mainly brought about by relaxing the torsion around C4—C4’ of the Schiff base through the interaction of AspAT with the substrate. The significance of the low pK_avalue in the unliganded enzyme is interpreted to be the ground-state destabilization of the enzyme protein that increases thek _cat/K _mvalue.