Changes in Regioselectivity of H Atom Abstraction during the Hydroxylation and Cyclization Reactions Catalyzed by Hyoscyamine 6β-Hydroxylase

Hyoscyamine 6 beta-hydroxylase (H6H) is an alpha KG-dependent nonheme iron oxidase that catalyzes the oxidation of hyoscyamine to scopolamine via two separate reactions: hydroxylation followed by oxidative cyclization. Both of these reactions are expected to involve H atom abstraction from each of two adjacent carbon centers (C6 vs C7) in the substrate. During hydroxylation, there is a roughly 85:1 preference for H atom abstraction from C6 versus C7; however, this inverts to a 1:16 preference during cyclization. Furthermore, O-18 incorporation experiments in the presence of deuterated substrate are consistent with the catalytic iron(IV)-oxo complex being able to support the coordination of an additional ligand during hydroxylation. These observations suggest that subtle differences in the substrate binding configuration can have significant consequences for the catalytic cycle of H6H.