The novel structure and chemistry of iron–sulfur clusters in the adenosylmethionine-dependent radical enzyme biotin synthase

Biotin synthase is an adenosylmethionine-dependent radical enzyme that catalyzes the substitution of sulfur for hydrogen at the saturated C6 and C9 positions in dethiobiotin. The structure of the biotin synthase monomer is an (α/β)8 barrel that contains one [4Fe–4S]2+ cluster and one [2Fe–2S]2+ cluster that encapsulate the substrates AdoMet and dethiobiotin. The air-sensitive [4Fe–4S]2+ cluster and the reductant-sensitive [2Fe–2S]2+ cluster have unique coordination environments that include close proximity to AdoMet and DTB, respectively. The relative positioning of these components, as well as several conserved protein residues, suggests at least two potential catalytic mechanisms that incorporate sulfur from either the [2Fe–2S]2+ cluster or a cysteine persulfide into the biotin thiophane ring. This review summarizes an accumulating consensus regarding the physical and spectroscopic properties of each FeS cluster, and discusses possible roles for the [4Fe–4S]2+ cluster in radical generation and the [2Fe–2S]2+ cluster in sulfur incorporation.