FUEL 184 |
| Acetyl-CoA decarbonylase/synthase (ACDS) is a multienzyme complex found in methanogens and certain other Archaea that carries out the overall synthesis and cleavage of the acetyl C-C and C-S bonds of acetyl-CoA. The reaction is involved both in the autotrophic fixation of carbon and in the process of methanogenesis from acetate, and is catalyzed by a unique active site metal center known as the A cluster, which is located on the beta subunit of the ACDS complex and is composed of an Fe4S4 center bridged to a binuclear Ni-Ni site. In this work, a high rate of acetyl-CoA synthesis was achieved with the recombinant ACDS beta subunit by use of methylcobinamide as an effective mimic of the physiological base-off corrinoid substrate. The redox dependence of acetyl-CoA synthesis exhibited one-electron Nernst behavior, and the influence of pH on the observed midpoint potential suggested that reductive activation of the A cluster also involves a protonation step. Initial burst kinetic studies indicated the formation of stoichiometric amounts of an A cluster acetyl species, and titration experiments established that the A cluster acetyl species undergoes reductive elimination of the acetyl group with the simultaneous release of two, low potential electrons. The data also show that two electrons are taken up in reductive activation of the enzyme. The one-electron Nernst behavior is interpreted as the sum of two independent, low potential, one-electron steps. The results effectively exclude reaction mechanisms involving either one- or three-electron reduced forms of the A cluster. A mechanism involving a [Fe4S4]1+-Ni1+-Ni2+ species is favored over [Fe4S4]2+-Ni0-Ni2+. |
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Hybrid Nanotechnologies for an Enhanced CO2 Fixation
8:50 AM-11:45 AM, Wednesday, April 9, 2008 Morial Convention Center -- Rm. 240/241, Oral
Division of Fuel Chemistry |