Active site mutations of F₄₂₀-dependent alkene reductases reverse stereoselectivity

Abstract

Ene-reductases from the Flavin/Deazaflavin Oxidoreductase (FDOR) family have potential value in biocatalysis as they typically exhibit complementary stereoselectivity to the widely utilized Old Yellow Enzyme (OYE) family, yet they are comparatively poorly understood at a mechanistic level. Here, we use a rational design approach to generate a library of 46 active site mutants of two FDORs from Mycobacterium smegmatis and examine the effects on conversion and stereoselectivity against a panel of substrates. Analysis of the effects of these mutations on stereoselectivity across all substrates revealed that the catalytic mechanism is highly sensitive to the polarity of the immediate active site. A conserved active site tyrosine in these enzymes, which does not serve as the proton donor, strongly affects stereochemical outcomes with Calpha (but not Cbeta) substituted substrates. Notably, a Tyr-Met mutation at this position reversed the diastereomeric excess (de) with (R)-carvone from 85.3% to 17.3% (cis/trans). Additionally, this mutation significantly increases activity with (1S)-verbenone. Finally, we show that the altered stereoselectivity is not due to a 'flipped' substrate binding mode in these mutants, but rather that the hydrogenation mode is altered to favor syn relative to anti addition. These results show that the FDORs are highly engineerable and that, despite their superficial similarity, the OYE and FDOR families differ in crucial mechanistic aspects.