Mutants of 4-oxalocrotonate tautomerase catalyze the decarboxylation of oxaloacetate through an imine mechanism

A designed single amino acid substitution can alter the catalytic activity and mechanism of 4-oxalocrotonate tautomerase (4-OT). While the wild-type enzyme catalyzes only the tautomerization of oxalocrotonate, the ProlAla mutant (P1A) catalyzes two reactions - the original tautomerization reaction and the decarboxylation of oxaloacetate. Although the N-terminal amine group of P1A is involved in both reactions, our results support a nucleophilic mechanism for the decarboxylase activity, in contrast to the general acid/base mechanism that has been previously established for the tautomerase activity. These findings demonstrate that a single catalytic group in a 4-OT mutant can catalyze two reactions by two different mechanisms.