Credit: © 2008 iSTOCK

Naturally occurring enzymes can be phenomenally powerful catalysts for organic transformations, providing products having both high yield and enantioselectivity under mild conditions. However, the substrate scope of such reactions can be limited for the same reason that makes them so effective — the enzymes have evolved to catalyse very specific reactions in nature, and small changes to the substrate can have a large effect on reactivity and selectivity.

To produce an enzyme with the ideal characteristics for a particular reaction, libraries of mutant enzymes have been produced and the selectivity tested using high-throughput screening. Selection — in which the survival of the enzyme-producing cells is linked to the desired reactivity — is a popular alternative that would result in the production of only useful enzymes and eliminate the need for screening. However, although this approach has been used for activity, designing systems that produce enantioselective enzymes has been more problematic.

Now, Manfred Reetz and co-workers from the Max-Planck-Institut für Kohlenforschung in Mülheim, Germany, have designed1 a system that achieves both the reactive and enantioselective evolution of enzymes for a kinetic resolution of esters. The system relies on the application of a mixture of two substrate esters to a library of cells producing mutant versions of the enzyme Candida antartica lipase B. The first substrate has the stereochemistry that is intended for the reaction with the enzyme, during which it releases acetic acid, which the cells use as a source of energy; the second substrate has the opposite stereochemistry, and releases fluoroacetic acid, which poisons the cells. In this way, cells producing enzymes with the wrong selectivity are killed, and those with high reactivity grow more quickly because of the provided energy source.

At this stage, only moderate selectivities have been achieved, but these are all the more noteworthy given that Reetz and colleagues are producing a mutant enzyme with opposite enantioselectivity to the natural enzyme.