EPR-Derived Structure of a Paramagnetic Intermediate Generated by Biotin Synthase BioB.

Biotin (vitamin B7 ) is an enzyme cofactor required by organisms from all branches of life but synthesized only in microbes and plants. In the final step of biotin biosynthesis, a radical S-adenosyl-l-methionine (SAM) enzyme, biotin synthase (BioB), converts the substrate dethiobiotin to biotin through the stepwise formation of two C-S bonds. Previous electron paramagnetic resonance (EPR) spectroscopic studies identified a semistable intermediate in the formation of the first C-S bond as 9-mercaptodethiobiotin linked to a paramagnetic [2Fe-2S] cluster through one of its bridging sulfides. Herein, we report orientation-selected pulse EPR spectroscopic results that reveal hyperfine interactions between the [2Fe-2S] cluster and a number of magnetic nuclei (e.g., 57 Fe, 15 N, 13 C, and 2 H) introduced in a site-specific manner via biosynthetic methods. Combining these results with quantum chemical modeling gives a structural model of the intermediate showing that C6, the target of the second hydrogen-atom abstraction, is now in close proximity to the nascent thioether sulfur and is ideally positioned for the second C-S bond forming event.