title: Iron-sulfur clusters of hydrogenase I and hydrogenase II of Clostridium pasteurianum.
abstract: The iron and acid-labile sulfide contents and the electron paramagnetic resonance (EPR) properties of hydrogenase I (bidirectional) and hydrogenase II (uptake) of Clostridium pasteurianum (strain W5) have been determined on the basis of quantitative amino acid analyses. The iron and acid-labile sulfide values are approximately 20 and 18 atoms per molecule of hydrogenase I and 14 and 11 atoms per molecule of hydrogenase II, respectively. These amounts are substantially greater than previously reported values, which relied on protein concentration determined by colorimetric assay. The oxidized hydrogenases exhibit unusual EPR signals that originate from a novel type of iron-sulfur center, termed the hydrogenase or H cluster, which covalently binds the inhibitor CO. This EPR signal represents approximately one unpaired electron per molecule in each enzyme with and without bound CO, which is consistent with the presence of one oxidized H cluster (S = 1/2) per enzyme molecule. The two enzymes also contain ferredoxin-type four-iron centers or F clusters. The EPR signals from the F clusters observed in the reduced forms of hydrogenase I and hydrogenase II account for approximately four and one unpaired electron per molecule, respectively. We conclude from the iron determinations and the EPR results, together with a reevaluation of previous spectroscopic data, that in both hydrogenases the H cluster probably comprises six iron atoms. Mechanistic models of the two hydrogenases are presented that account for their cluster compositions and the dramatic differences in their catalytic activities.
authors.affiliation: Department of Biochemistry, School of Chemical Sciences, University of Georgia, Athens 30602.
authors.forename: M W
authors.initials: MW
authors.lastname: Adams
authors.forename: E
authors.initials: E
authors.lastname: Eccleston
authors.forename: J B
authors.initials: JB
authors.lastname: Howard
mesh_terms: D000596:Amino Acids / Q000032:analysis; D003013:Clostridium / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006864:Hydrogenase / Q000302:isolation & purification / Q000378:metabolism*; D007501:Iron / Q000032:analysis; D007506:Iron-Sulfur Proteins / Q000302:isolation & purification / Q000378:metabolism*; D008667:Metalloproteins / Q000378:metabolism*; D008962:Models, Theoretical; D010088:Oxidoreductases / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D007501:Iron; D010088:Oxidoreductases; C029746:uptake hydrogenase; D006864:Hydrogenase