Expansion of the Catalytic Repertoire of Alcohol Dehydrogenases in Plant Metabolism

Medium‐chain alcohol dehydrogenases (ADHs) comprise a highly conserved enzyme family that catalyse the reversible reduction of aldehydes. However, recent discoveries in plant natural product biosynthesis suggest that the catalytic repertoire of ADHs has been expanded. Here we report the crystal structure of dihydroprecondylocarpine acetate synthase (DPAS), an ADH that catalyses the non‐canonical 1,4‐reduction of an α,β‐unsaturated iminium moiety. Comparison with structures of plant‐derived ADHs suggest the 1,4‐iminium reduction does not require a proton relay or the presence of a catalytic zinc ion in contrast to canonical 1,2‐aldehyde reducing ADHs that require the catalytic zinc and a proton relay. Furthermore, ADHs that catalysed 1,2‐iminium reduction required the presence of the catalytic zinc and the loss of the proton relay. This suggests how the ADH active site can be modified to perform atypical carbonyl reductions, providing insight into how chemical reactions are diversified in plant metabolism. Medium chain alcohol dehydrogenases usually perform a reversible 1,2‐reduction of aldehydes to form the corresponding alcohols. Here we show how the active site of these dehydrogenases can be modified to perform 1,2‐reduction of iminium moieties, as well 1,4‐reduction of α,β‐unsaturated iminium groups and 1,4‐reduction of α,β‐unsaturated aldehydes.