Spectroscopic, redox, and structural characterization of the Ni-labile and nonlabile forms of the acetyl-CoA synthase active. Site of carbon monoxide dehydrogenase
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abstract
The subunit of carbon monoxide dehydrogenase from: Clostridium thermoaceticum Was isolated, treated as described below, and examined by XAS, EPR, and UV-vis spectroscopies. This subunit contains the active site for acetyl-coenzyme A synthesis, the A-cluster, a Ni ion bridged to an Fe4S4 cube. Populations of subunits contain two major forms of A-clusters, a catalytically active form called Ni-labile and an inactive form called nonlabile. The objective of this stud was to elucidate the redox and spectroscopic properties of could be reduced either by CO and a catalytic amount of native enzyme or by electrochemically reduced triquat in the presence of CO. The Ni2+ component of the Ni-labile form reduced to Ni1+ and bound CO. CO-binding raised E(o') for the Ni2+/Ni1+ couple, thereby rendering CO and triquat effective reductants. Dithionite did not reduce the Ni-labile form, though its addition to CD/CODH-reduced Ni-labile clusters caused an intracluster electron transfer from the Ni1+ to the [Fe4S4]2+ cluster. Dithionite reduced the [Fe4S4]2+ component of the nonlabile form, as well as the cluster of the Ni-labile form once Ni was removed. Ni may not be bridged to the cube in the nonlabile form. XAS reveals that the Ni in the nonlabile form has a distorted square-planar geometry with two N/O scatters at 1.87 and two S scatters at 2:20 . The [Fe4S4]2+ portion of Ni-labile A-clusters may maintain the Ni in a geometry conducive to reduction, CO and methyl group binding, and the migratory-insertion step used in catalysts. It may also transfer electrons to and from the redox-active D site during reductive activation.
