Double-Ring Epimerization in the Biosynthesis of Clavulanic Acid.

All reaction steps during the biosynthesis of suicidal clavulanic acid (coformulated with β-lactam antibiotics and used to fight bacterial infections) are known, except for the crucial 3S,5S → 3R,5R double epimerization needed to produce a biologically active stereoisomer, for which mechanistic hypothesis is subject to debate. In this work, we provide evidence for a reaction channel for the double inversion of configuration that involves a total of six reaction steps. When mediated by an enzyme with a terminal S-H bond, this highly complex reaction is spontaneous in the absence of solvents. Polarizable continuum models introduce reaction barriers in aqueous environments because of the strong destabilization of the first transition state. Molecular geometries and electronic structures in both cases indicate that solvent-free spontaneity and aqueous medium barriers are both firmly rooted in a substantial reorganization of the electron density right at the onset of the reaction, mostly involving a cyclic evolution/involution of large regions of π delocalization used to stabilize the excess charge left after the initial proton abstraction.