Electronic modulation of flavin reduction and decarbonylation in the flavin-dependent monooxygenase salicylate hydroxylase (NahG)

Salicylate hydroxylase (NahG), a flavin-dependent monooxygenase, catalyzes the oxidative decarboxylation of salicylate via ipso-hydroxylation mediated by a C4a-hydroperoxyflavin intermediate. Here, we show that systematic variation of the carbonyl substituent in salicylate-derived substrates spanning amide, hydroxamic acid, ester, aldehyde, and ketone functionalities modulates catalytic efficiency and reaction outcome. Except for methyl salicylate, all substrates undergo aryl-carbonyl CC bond cleavage following aromatic hydroxylation to yield catechol, revealing decarbonylation reactivity with no known precedent among flavin-dependent monooxygenases. Apparent substrate binding correlates with the protonation state of the ortho phenol, whereas catalytic efficiency displays a strong dependence on carbonyl substituent electronics, indicating that resonance donation modulates the reductive half-reaction and defines catalytic commitment prior to oxygen activation. These findings reveal flavin reduction as an electronically modulated commitment step and decarbonylation as an intrinsic feature of NahG catalysis.