The type and properties of the Fe-S cluster in recombinant Escherichia coli biotin synthase have been investigated in as-prepared and dithionite-reduced samples using the combination of UV-visible absorption and variable-temperature magnetic circular dichroism (VTMCD), EPR, and resonance Raman spectroscopies. The results confirm the presence of one S = 0 [2Fe-2S]2+ cluster in each subunit of the homodimer in aerobically purified samples, and the Fe-S stretching frequencies suggest incomplete cysteinyl-S coordination. However, absorption and resonance Raman studies show that anaerobic reduction with dithionite in the presence of 60% (v/v) ethylene glycol or glycerol results in near-stoichiometric conversion of two [2Fe-2S]2+ clusters to form one S = 0 [4Fe-4S]2+ cluster with complete cysteinyl-S coordination. The stoichiometry and ability to effect reductive cluster conversion without the addition of iron or sulfide suggest that the [4Fe-4S]2+ cluster is formed at the subunit interface via reductive dimerization of [2Fe-2S]2+ clusters. EPR and VTMCD studies indicate that more than 50% of the Fe is present as [4Fe-4S]+ clusters in samples treated with 60% (v/v) glycerol after prolonged dithionite reduction. The [4Fe-4S]+ cluster exists as a mixed spin system with S = 1/2 (g = 2. 044, 1.944, 1.914) and S = 3/2 (g = 5.6 resonance) ground states. Subunit-bridging [4Fe-4S]2+,+ clusters, that can undergo oxidative degradation to [2Fe-2S]2+ clusters during purification, are proposed to be a common feature of Fe-S enzymes that require S-adenosylmethionine and function by radical mechanisms involving the homolytic cleavage of C-H or C-C bonds, i.e., biotin synthase, anaerobic ribonucleotide reductase, pyruvate formate lyase, lysine 2, 3-aminomutase, and lipoic acid synthase. The most likely role for the [4Fe-4S]2+,+ cluster lies in initiating the radical mechanism by directly or indirectly facilitating reductive one-electron cleavage of S-adenosylmethionine to form methionine and the 5'-deoxyadenosyl radical. It is further suggested that oxidative cluster conversion to [2Fe-2S]2+ clusters may play a physiological role in these radical enzymes, by providing a method of regulating enzyme activity in response to oxidative stress, without irreversible cluster degradation.

Resonance Raman spectra and excitation profiles (413-676 nm) are reported for four distinct forms of Rhodobacter sphaeroides dimethyl sulfoxide (DMSO) reductase: as prepared Mo(VI), dithionite-reduced Mo(IV), dimethyl sulfide reduced Mo(IV), and glycerol-inhibited Mo(V). All of the vibrational modes in the 200-1700 cm -1 region of the Mo(VI) and Mo(IV) forms are assigned to vibrations involving atoms in the first or second coordination sphere of the bis-molybdopterin-coordinated Mo active site, the dithiolene chelate rings, or nonresonantly enhanced protein modes. On the basis of 18 O/ 16 O isotope shifts, the Mo(VI) form is shown to be mono-oxo with ν(ModO) at 862 cm -1 , and the DMS-reduced Mo(IV) form is shown to involve bound DMSO with ν(Mo-O) at 497 cm -1 and ν(SdO) at 862 cm -1 . Bands at 536 and 513 cm -1 are tentatively assigned to ν(Mo-O(Ser)) stretching modes of coordinated serinate in the Mo(VI) and Mo(IV) forms, respectively. The vibrational modes of two distinct types of dithiolene chelate rings are identified on the basis of their excitation profiles, and the frequencies indicate that one is best viewed as a dithiolate ligand, while the other has more π-delocalized character. In the low-frequency region between 335 and 405 cm -1 , the Mo-S stretching modes of a distorted square pyramidal MoS 4 unit are assigned in each of the four derivatives investigated, based on the 34 S isotope shifts and sensitivity to Mo oxidation state. The average Mo-S bond strength increases with decreasing Mo oxidation state. Taken together, the Mo-S and dithiolene vibrational assignments indicate that all four of the molybdopterin dithiolene S atoms remain coordinated in each of the four forms investigated. Structures for each of these four derivatives are proposed on the basis of the resonance Raman results, and the ability to monitor directly the origin and fate of the Mo oxo group via isotopic labeling indicates that each corresponds to a catalytically competent intermediate in the reaction cycle. Overall, the results provide direct confirmation of an oxygen atom transfer mechanism, with the active site cycling between mono-oxo-Mo(VI) and des-oxo-Mo(IV) forms via a DMSO-bound Mo(IV) intermediate, and the molybdopterin dithiolene ligands staying firmly attached throughout the catalytic cycle.