title: Spectroscopic changes during a single turnover of biotin synthase: destruction of a [2Fe-2S] cluster accompanies sulfur insertion.
abstract: Biotin synthase catalyzes the insertion of a sulfur atom between the saturated C6 and C9 carbons of dethiobiotin. Catalysis requires AdoMet and flavodoxin and generates 5'-deoxyadenosine and methionine, suggesting that biotin synthase is an AdoMet-dependent radical enzyme. Biotin synthase (BioB) is aerobically purified as a dimer of 38.4 kDa monomers that contains 1-1.5 [2Fe-2S](2+) clusters per monomer and can be reconstituted with exogenous iron, sulfide, and reductants to contain up to two [4Fe-4S] clusters per monomer. The iron-sulfur clusters may play a dual role in biotin synthase: a reduced iron-sulfur cluster is probably involved in radical generation by mediating the reductive cleavage of AdoMet, while recent in vitro labeling studies suggest that an iron-sulfur cluster also serves as the immediate source of sulfur for the biotin thioether ring. Consistent with this dual role for iron-sulfur clusters in biotin synthase, we have found that the protein is stable, containing one [2Fe-2S](2+) cluster and one [4Fe-4S](2+) cluster per monomer. In the present study, we demonstrate that this mixed cluster state is essential for optimal activity. We follow changes in the Fe and S content and UV/visible and EPR spectra of the enzyme during a single turnover and conclude that during catalysis the [4Fe-4S](2+) cluster is preserved while the [2Fe-2S](2+) cluster is destroyed. We propose a mechanism for incorporation of sulfur into dethiobiotin in which a sulfur atom is oxidatively extracted from the [2Fe-2S](2+) cluster.
authors.affiliation: Johnson Research Foundation, Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104, USA.
authors.forename: N B
authors.initials: NB
authors.lastname: Ugulava
authors.forename: C J
authors.initials: CJ
authors.lastname: Sacanell
authors.forename: J T
authors.initials: JT
authors.lastname: Jarrett
mesh_terms: D001665:Binding Sites; D001710:Biotin / Q000096:biosynthesis; D002021:Buffers; D002384:Catalysis; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D004789:Enzyme Activation; D005609:Free Radicals / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013455:Sulfur / Q000737:chemistry*; D013466:Sulfurtransferases / Q000737:chemistry*
chemical_list: D002021:Buffers; D005609:Free Radicals; D007506:Iron-Sulfur Proteins; D001710:Biotin; D013455:Sulfur; D007501:Iron; D013466:Sulfurtransferases; C057985:biotin synthetase