title: Mossbauer evidence for an exchange-coupled {[Fe4S4]1+ Nip1+} A-cluster in isolated alpha subunits of acetyl-coenzyme A synthase/carbon monoxide dehydrogenase.
abstract: The active site A-cluster in the alpha subunit of the title enzyme consists of an Fe4S4 cluster coordinated to a [Nip Nid] subcomponent. The cluster must be activated for catalysis using low-potential reductants such as Ti(III) citrate. Relative to the inactive {[Fe4S4]2+ Nip2+ Nid2+} state, the activated state appears to be 2-electrons more reduced, but the location of these electrons within the A-cluster is uncertain, with {[Fe4S4]2+ Nip0 Nid2+} and {[Fe4S4]1+ Nip1+ Nid2+} configurations proposed. Recombinant apo-alpha subunits oligomerize after activation with NiCl2. The dimer fraction, upon reduction with excess Ti(III)citrate, exhibited Mössbauer spectra consisting of two quadrupole doublets representing 51% and 21% of the Fe, with parameters indicating [Fe4S4]1+ states. Spectra recorded in strong magnetic fields were typical of diamagnetic systems, indicating an exchange-coupled S = 0 {[Fe4S4]1+ Nip1+} state. Additional treatment with CO altered the doublet Mössbauer parameters, suggesting an interaction with CO, but maintaining the cluster in the {[Fe4S4]1+ Nip1+} state. Reduction with substoichiometric equivalents of Ti(III) citrate afforded an EPR signal typical of Ni1+ ions, with g parallel = 2.10 and g perpendicular = 2.02. Addition of more Ti caused the signal intensity to decline, suggesting that it arises from the semireduced {[Fe4S4]2+ Nip1+} state.
authors.affiliation: Department of Chemistry and Department of Biochemistry and Biophysics, Texas A&M University, College Station, Texas 77843, USA.
authors.forename: Xiangshi
authors.initials: X
authors.lastname: Tan
authors.forename: Marlène
authors.initials: M
authors.lastname: Martinho
authors.forename: Audria
authors.initials: A
authors.lastname: Stubna
authors.forename: Paul A
authors.initials: PA
authors.lastname: Lindahl
authors.forename: Eckard
authors.initials: E
authors.lastname: Münck
mesh_terms: D000445:Aldehyde Oxidoreductases / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D019343:Citric Acid / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D009097:Multienzyme Complexes / Q000737:chemistry* / Q000378:metabolism; D009532:Nickel / Q000737:chemistry*; D021122:Protein Subunits; D015204:Spectroscopy, Mossbauer / Q000379:methods
chemical_list: D007506:Iron-Sulfur Proteins; D009097:Multienzyme Complexes; D021122:Protein Subunits; C419387:titanium citrate; D019343:Citric Acid; D009532:Nickel; D000445:Aldehyde Oxidoreductases; C019823:carbon monoxide dehydrogenase