Rapid reaction studies on the chemistry of flavin oxidation in urocanate reductase.

Urocanate reductase (UrdA) is a bacterial flavin-dependent enzyme that reduces urocanate to imidazole propionate, enabling bacteria to use urocanate as an alternative respiratory electron acceptor. Elevated serum levels of imidazole propionate are associated with the development of type 2 diabetes and, since UrdA is only present in humans in gut bacteria, this enzyme has emerged as a significant factor linking the health of the gut microbiome and insulin resistance. Here we investigated the chemistry of flavin oxidation by urocanate in the isolated FAD domain of UrdA (UrdA') using anaerobic stopped-flow experiments. This analysis unveiled the presence of a charge transfer complex between reduced FAD and urocanate that forms within the dead time of the stopped-flow instrument (∼1 ms), with flavin oxidation subsequently occurring with a rate constant of ∼60 s

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