Redesigning the mechanism of the lipase-catalysed aminolysis of esters

In a previous work, several 2-phenylcycloalkanamines were subjected to aminolysis catalysed by the lipase B from Candida antarctica (CAL-B). In these processes, the size of the cycle and the stereochemistry of the stereogenic centres of the amines had a strong influence on both the enantiomeric ratio and the reaction rate. Herein, molecular modelling approaches have been used to revise the lipase-catalysed aminolysis mechanism. Thus, complexes of CAL-B with the phosphonamidate analogues related to substrates in the kinetic resolution of several 2-phenylcycloalkanamines by this enzyme were built and minimised. This computational study suggests the formation of zwitterionic species (named TI-Z), resulting from the direct His-unassisted attack of the amine onto the carbonyl group of the acyl-enzyme, as the most plausible intermediate for the CAL-B-catalysed aminolysis. This proposal differs slightly from the commonly accepted serine-mediated mechanism, where removal of the proton from the amine occurs simultaneously to the nucleophile attack to the acyl-enzyme complex (TI-2). Subsequently, His-assisted deprotonation of the resulting ammonium group takes place, and a molecule of water could be necessary in some cases to facilitate the transfer of the proton to the catalytic histidine.

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