Flavin‐N5 Covalent Intermediate in a Nonredox Dehalogenation Reaction Catalyzed by an Atypical Flavoenzyme

The flavin‐dependent enzyme 2‐haloacrylate hydratase (2‐HAH) catalyzes the conversion of 2‐chloroacrylate, a major component in the manufacture of acrylic polymers, to pyruvate. The enzyme was expressed in Escherichia coli, purified, and characterized. 2‐HAH was shown to be monomeric in solution and contained a non‐covalent, yet tightly bound, flavin adenine dinucleotide (FAD). Although the catalyzed reaction was redox‐neutral, 2‐HAH was active only in the reduced state. A covalent flavin‐substrate intermediate, consistent with the flavin‐acrylate iminium ion, was trapped with cyanoborohydride and characterized by mass spectrometry. Small‐angle X‐ray scattering was consistent with 2‐HAH belonging to the succinate dehydrogenase/fumarate reductase family of flavoproteins. These studies establish 2‐HAH as a novel noncanonical flavoenzyme.