Search

The enzyme 4-oxalocrotonate tautomerase (4-OT), which in nature catalyzes a tautomerization step as part of a catabolic pathway for aromatic hydrocarbons, was found to promiscuously catalyze different types of aldol reactions. These include the self-condensation of propanal, the cross-coupling of propanal and benzaldehyde, the cross-coupling of propanal and pyruvate, and the intramolecular cyclizations of hexanedial and heptanedial. Mutation of the catalytic amino-terminal proline (P1A) greatly reduces 4-OT's aldolase activities, whereas mutation of another active site residue (F50A) strongly enhances 4-OT's aldolase activities, indicating that aldolization is an active site process. This catalytic promiscuity of 4-OT could be exploited as starting point to create tailor-made, artificial aldolases for challenging self- and cross-aldolizations.

The Michael-type addition reaction is widely used in organic synthesis for carbon-carbon bond formation. However, biocatalytic methodologies for this type of reaction are scarce, which is related to the fact that enzymes naturally catalysing carbon-carbon bond-forming Michael-type additions are rare. A promising template to develop new biocatalysts for carbon-carbon bond formation is the enzyme 4-oxalocrotonate tautomerase, which exhibits promiscuous Michael-type addition activity. Here we present mutability landscapes for the expression, tautomerase and Michael-type addition activities, and enantioselectivity of 4-oxalocrotonate tautomerase. These maps of neutral, beneficial and detrimental amino acids for each residue position and enzyme property provide detailed insight into sequence-function relationships. This offers exciting opportunities for enzyme engineering, which is illustrated by the redesign of 4-oxalocrotonate tautomerase into two enantiocomplementary 'Michaelases'. These 'Michaelases' catalyse the asymmetric addition of acetaldehyde to various nitroolefins, providing access to both enantiomers of γ-nitroaldehydes, which are important precursors for pharmaceutically active γ-aminobutyric acid derivatives.

Call me Michaelase: The enzyme 4-oxalocrotonate tautomerase (4-OT) promiscuously catalyzes the Michael-type addition of acetaldehyde to a collection of aromatic and aliphatic nitroolefins with high stereoselectivity producing precursors of γ-aminobutyric acid (GABA) analogues., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Exploiting catalytic promiscuity: The proline-based enzyme 4-oxalocrotonate tautomerase (4-OT) promiscuously catalyzes asymmetric Michael-type additions of linear aldehydes--ranging from acetaldehyde to octanal--to trans-β-nitrostyrene in aqueous solvent. The presence of 1.4 mol% of 4-OT effected formation of the anticipated γ-nitroaldehydes in fair to good yields with dr values of up to 93:7 and ee values of up to 81 %., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)