title: Role of the [Fe4S4] cluster in mediating disulfide reduction in spinach ferredoxin:thioredoxin reductase.
abstract: Thioredoxin reduction in plant chloroplasts is catalyzed by a unique class of disulfide reductases which use a one-electron donor, [Fe2S2]2+,+ ferredoxin, and has an active site involving a disulfide in close proximity to a [Fe4S4]2+ cluster. In this study, spinach ferredoxin:thioredoxin reductase (FTR) reduced with stoichiometric amounts of reduced benzyl viologen or frozen under turnover conditions in the presence of thioredoxin is shown to exhibit a slowly relaxing S = 1/2 resonance (g = 2.11, 2.00, 1.98) identical to that of a modified form of the enzyme in which one of the cysteines of the active-site disulfide is alkylated with N-ethylmaleimide (NEM-FTR). Hence, in accord with the previous proposal [Staples, C.R., Ameyibor, E., Fu, W., Gardet-Salvi, L., Stritt-Etter, A.-L., Schürmann, P., Knaff, D.B., and Johnson, M.K. (1996) Biochemistry 35, 11425-11434], NEM-FTR is shown to be a stable analogue of a one-electron-reduced enzymatic intermediate. The properties of the Fe-S cluster in NEM-FTR have been further investigated by resonance Raman and electron nuclear double resonance spectroscopies; the results, taken together with the previous UV-visible absorption, variable temperature magnetic circular dichroism, and resonance Raman data, indicate the presence of a novel type of [Fe4S4]3+ cluster that is coordinated by five cysteinates with little unpaired spin density delocalized onto the cluster-associated cysteine of the active-site disulfide. While the ligation site of the fifth cysteine remains undefined, the best candidate is a cluster bridging sulfide. On the basis of the spectroscopic and redox results, mechanistic schemes are proposed for the benzyl viologen-mediated two-electron-reduction of FTR and the catalytic mechanism of FTR. The catalytic mechanism involves novel S-based cluster chemistry to facilitate electron transfer to the active-site disulfide resulting in covalent attachment of the electron-transfer cysteine and generation of the free interchange cysteine that is required for the thiol-disulfide interchange reaction with thioredoxin.
authors.affiliation: Department of Chemistry and the Center for Metalloenzyme Studies, University of Georgia, Athens, Georgia 30602, USA.
authors.forename: C R
authors.initials: CR
authors.lastname: Staples
authors.forename: E
authors.initials: E
authors.lastname: Gaymard
authors.forename: A L
authors.initials: AL
authors.lastname: Stritt-Etter
authors.forename: J
authors.initials: J
authors.lastname: Telser
authors.forename: B M
authors.initials: BM
authors.lastname: Hoffman
authors.forename: P
authors.initials: P
authors.lastname: Schürmann
authors.forename: D B
authors.initials: DB
authors.lastname: Knaff
authors.forename: M K
authors.initials: MK
authors.lastname: Johnson
mesh_terms: D001594:Benzyl Viologen; D001665:Binding Sites; D003545:Cysteine / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D005033:Ethylmaleimide; D007506:Iron-Sulfur Proteins / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry / Q000378:metabolism*; D013059:Spectrum Analysis, Raman; D018724:Spinacia oleracea / Q000201:enzymology*
chemical_list: D004220:Disulfides; D007506:Iron-Sulfur Proteins; D001594:Benzyl Viologen; D010088:Oxidoreductases; C021588:ferredoxin-thioredoxin reductase; D003545:Cysteine; D005033:Ethylmaleimide