S164: Structure-function relationships in stereoselective alkene reductases

Alkene reductases have become increasingly useful catalysts for asymmetric reductions of C=C bonds.  Two key problems remain, however.  First, the substrate range of these enzymes is somewhat limited, generally favouring smaller alkenes.  In addition, their high stereoselectivities make it difficult to access the other product enantiomers.  We have carried out directed evolution studies of Pichia stipitis Old Yellow Enzyme 2.6 (OYE 2.6) to remedy both of the abovementioned issues.  These studies focused on three representative substrates whose reduction products are important chiral building blocks in organic synthesis.  We first surveyed residues located near the substrate binding region of the active site by site-saturation mutagenesis and found that four appeared to play important roles for two of the substrates.  On the other hand, we found no improvement in stereoselectivity for the third alkene substrate.  We solved this problem by carrying out further, parallel directed evolution studies on the other two substrates and found that a key intermediate mutant identified during the course of these efforts regained activity toward the third substrate and also showed a reversed stereoselectivity.  This process allowed us to cross a “desert” during directed evolution:  starting from an active protein and arriving at a catalytically active mutant via inactive intermediates.  Subsequent rounds of directed evolution allowed further improvements and yielded synthetically useful catalysts.  We have also carried out crystallographic studies of the wild-type and key mutant OYE 2.6 variants to understand how the mutations impacted both substrate binding and stereoselectivity and these results will be presented as well.