tautomerase

Tautomerase refers to a class of enzymes that catalyze tautomerization reactions, most commonly enol–keto or imine–enamine interconversions in substrates. The best-studied example is 4-oxalocrotonate tautomerase (4-OT), an enzyme from certain bacteria such as Pseudomonas species, which participates in the degradation of aromatic compounds by facilitating a tautomerization step that precedes subsequent ring-cleavage reactions.

Most tautomerases studied are small proteins, typically around 60 to 70 amino acids, and many assemble into oligomeric structures, such as hexamers. A unifying feature of the family is the presence of an essential N-terminal proline residue (Pro-1) in the catalytic site, which acts as a general acid–base catalyst to initiate proton transfer and stabilize reaction intermediates. The reactions carried out by tautomerases are often rapid relative to the small size of the enzymes, illustrating how a minimalist active site can efficiently promote chemical rearrangements.

The tautomerase family includes several related enzymes beyond 4-OT, such as malonate tautomerase, all sharing the small size and a conserved catalytic proline. Members may exhibit substrate promiscuity, catalyzing non-canonical tautomerizations and related rearrangements. This functional versatility, combined with the compact structure, has made tautomerases an area of interest for studying proton-transfer catalysis and for potential applications in biocatalysis and protein engineering.

In summary, tautomerases are a family of small, often hexameric enzymes that drive tautomerization reactions through an N-terminal proline–mediated mechanism, with 4-OT serving as the archetype and others expanding the functional scope within the superfamily.