Exploiting Catalytic Promiscuity for Biocatalysis: Carbon-Carbon Bond Formation by a Proline-Based Tautomerase

A powerful theme that is highly relevant to the design of new biocatalysts is that of catalytic promiscuity, where an enzyme catalyzes alternative reactions in addition to its biologically relevant one. Enzyme promiscuity has great promise as a source of synthetically useful catalytic transformations. Because most enzyme active sites provide a constellation of potential catalytic groups that can act as acids, bases, and nucleophiles, the promiscuity of enzymes is likely to be widespread and tightly linked to the “chemical potential” of their active sites. Here lie formidable challenges and possibilities: the use of mechanistic reasoning to discover new promiscuous activities in existing enzymes, which could be exploited as starting points to generate novel biocatalysts.

Yufeng Miao investigated whether the enzyme 4-oxalocrotonate tautomerase (4-OT) exhibits catalytic promiscuity for carbon-carbon bond-forming reactions. 4-OT naturally catalyzes enol-keto tautomerization reactions, and is characterized by a unique catalytic N-terminal proline. Based on the presence of a nucleophilic proline in 4-OT’s active site, it was predicted and discovered that 4-OT can catalyze the Michael-type addition of aliphatic aldehydes to a wide variety of nitroolefins. The resulting γ-nitroaldehydes are important precursors for γ-aminobutyric acids, such as the marketed pharmaceuticals phenibut (tranquilizer), baclofen (anti-alcoholic), and pregabalin (anticonvulsant). 4-OT was also found to catalyze challenging inter- and intramolecular aldol reactions.

The outcomes of this work may support an exciting area in protein engineering research as the newly discovered promiscuous activities of 4-OT can serve as crucial starting points for the laboratory evolution of novel biocatalysts for carbon-carbon bond formation.