Mossbauer evidence for an exchange-coupled {[Fe4S4]1+ Nip1+} A-cluster in isolated alpha subunits of acetyl-coenzyme A synthase/carbon monoxide dehydrogenase. Academic Article uri icon


  • 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 Mssbauer 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 Mssbauer 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.

published proceedings

  • J Am Chem Soc

author list (cited authors)

  • Tan, X., Martinho, M., Stubna, A., Lindahl, P. A., & Mnck, E.

citation count

  • 22

complete list of authors

  • Tan, Xiangshi||Martinho, Marlène||Stubna, Audria||Lindahl, Paul A||Münck, Eckard

publication date

  • May 2008