Using N-15 NMR spectroscopy and hydrogen bond correlations, we have localized a mechanistically critical proton in aspartate aminotransferase in microcrystals and aqueous solution. It is in a H-bond between a carboxylate O of Asp222 and the pyridine nitrogen of pyridoxal-5'-phosphate. At neutral pH in water, aspartate and pyridine are unprotonated, but they share a proton in the enzyme. It is shown that such a binuclear base is typical for acid-base interactions in aprotic polar solvents. Active site H-bonds to Asp222 assist protonation of the pyridine nitrogen in the enzyme, which is considered a prerequisite for catalytic activity. We also show that acid-base behavior in enzymes should be modeled using aprotic polar solvents rather than aqueous solutions.