Proton relay through an active-site network of hydrogen bonds promotes enzymatic nitrile reduction to amine via a covalent thioimidate enzyme intermediate.

Scheme1.Proposed mechanism of reduction of preQ 0 into preQ 1 ,catalyzed by the nitrile reductase QueF.The active-site dyadCys and Asp act together in covalentcatalysis. Reduction of the thioimidate enzyme adduct occurs in two NADPH-dependent stepsv ia an iminei ntermediate (not shown).Scheme2.Reactionpathways leadingtoN ADPH degradation in spontaneous and enzyme-promoted conversions.The uncoupled pathway (NADPH conversion not leading to NADP + )i nvolves hydration of NADPH to NADPHX followed by epimerization/cyclization, leading to O2'-b-6-cyclo-1,4,5,6-tetrahydronicotinamideadenine dinucleotidephosphate [(cTHN)TPN]. The enzymatic reaction identified in this study is markedb yt he box. The coupled pathway (NADPH conversion leadingt oN ADP + )entailsformation of NADP + through knowno xidationr eactionso fN ADPH: whenO 2 is present, for example.