J. David Rozzell

Stereochemical imperative in enzymic decarboxylations. Stereochemical course of the decarboxylation catalyzed by acetoacetate decarboxylase

The stereochemical course of the decarboxylation of acetoacetate catalyzed by the enzyme acetoacetate decarboxylase (AAD) has been studied by using samples of optically active 2-tritioacetoacetate, prepared by enzymatic oxidation of samples of enantiomeric pairs of diastereomeric 2-tritio-3-hydroxybutyrates. A correlation is proposed connecting the stereochemical course of enzymatic decarboxylation (retention or inversion) with the structure of the substrate. Acetoacetate decarboxylase was found to catalyze decarboxylation with net racemization. The stereochemical course of an enzymic reaction is determined by the geometrical arrangement of substrates and amino acid residues in the active site. This arrangement presumably arises from evolutionary pressures selecting for enzymes that best contribute to the survival of their host organism.’-3 Thus, stereochemical choices made by enzymes are likely to reflect underlying principles of protein structure and catalysis and their role in adaptation and evolution. An important theme in recent work of ours and others is to explore how the stereochemical details of enzymic reactions might reflect underlying catalytic and structural principles that explain in evolutionary terms how stereochemical choices made by enzymes came to be.2*3 A common stereochemical course is often shared by different enzymes catalyzing similar reactions. For example, all aliphatic hydroxylases proceed with retention of configuration; all enzymes using pyridoxal phosphate catalyze the transfer of the pro-R hydrogen of pyridoxamine.2 In these uniformities, several authors have seen an underlying catalytic principle. For example, Hanson and Rose2 recently suggested that there may be an optimal stereochemical course for particular classes of reactions and that stereochemical similarities in enzymes catalyzing reactions in each class reflect convergent evolution to conform to this “mechanistic imperative”. An alternative to this teleological explanation for stereochemical uniformities within a mechanistic class is a historical explanation, also noted by Hanson and Roses2 If all modern enzymes catalyzing a certain type of reaction evolved from a common ancestral enzyme displaying a particular stereochemical preference, a common preference in all modern enzymes might reflect the conservation of the original preference during the evolutionary process. This pair of explanations is merely a molecular example of a ptoblem frequently encountered in biology: are the structural features common in different species best understood as arising by convergent evolution direct toward a purpose or by divergent evolution from a common precursor with conservation of details? This question has special importance in those classes of enzymes where stereochemical heterogeneity, not uniformity, is the rule. Hanson and Rose2 note two such classes: dehydrogenases dependent on nicotinamide cofactors, where some enzymes transfer the pro-R hydrogen of N A D H while others transfer the pro-S hydrogen, and (3-keto acid decarboxylases, where some enzymes produce inversion of configuration at the decarboxylating center, while others produce retention. In classes of enzymes not displaying a uniform stereochemical choice, it is difficult to ascribe