title: Redox titrations of carbon monoxide dehydrogenase from Clostridium thermoaceticum.
abstract: Redox titrations of carbon monoxide dehydrogenase (CODH) from Clostridium thermoaceticum were performed using the reductant CO and the oxidant thionin. Titrations were followed at 420 nm, a wavelength sensitive to redox changes of the iron-sulfur clusters in the enzyme. When CODH was oxidized by just enough thionin to maximize A420, two molecules of CO per mole of CODH dimer (4 equiv/mol) reduced the enzyme fully. Likewise, 4 equiv/mol of thionin oxidized the fully-reduced enzyme to the point where A420 maximized. The four n = 1 redox sites which titrated in this region were designated group I sites. They include at least two iron-sulfur clusters, [Fe/S]A and [Fe/S]B, and two other sites, A' and B'. The [Fe4S4]2+/1+ cluster in CODH is included in this group. [Fe/S]B and B' have reduction potentials (at pH 8) below -480 mV vs NHE; [Fe/S]A and A' have reduction potentials above that value. The reduction potential of either [Fe/S]B or B' is near to the CO/CO2 couple at pH 8 (-622 mV). When CODH was oxidized by more than enough thionin to maximize A420, some of the excess thionin oxidized the so-called group II redox sites. These sites have reduction potentials more positive than group I and do not exhibit changes at 420 nm when titrated. Titration of group II sites required 1-2 equiv/mol. EPR of reduced group II sites exhibited the gav = 1.82 signal. When these sites were oxidized, the only signal present had g values at 2.075, 2.036, and 1.983.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Chemistry, Texas A&M University, College Station 77843.
authors.forename: W
authors.initials: W
authors.lastname: Shin
authors.forename: P R
authors.initials: PR
authors.lastname: Stafford
authors.forename: P A
authors.initials: PA
authors.lastname: Lindahl
mesh_terms: D000445:Aldehyde Oxidoreductases / Q000378:metabolism*; D001665:Binding Sites; D002248:Carbon Monoxide / Q000378:metabolism; D003013:Clostridium / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D004789:Enzyme Activation / Q000187:drug effects; D007506:Iron-Sulfur Proteins / Q000378:metabolism; D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D010084:Oxidation-Reduction; D010640:Phenothiazines / Q000378:metabolism / Q000494:pharmacology; D013053:Spectrophotometry
chemical_list: D007506:Iron-Sulfur Proteins; D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D010640:Phenothiazines; D002248:Carbon Monoxide; D000445:Aldehyde Oxidoreductases; C019823:carbon monoxide dehydrogenase; C009469:thionine