Spectroscopic, redox, and structural characterization of the Ni-labile and nonlabile forms of the acetyl-CoA synthase active site of carbon monoxide dehydrogenase

The α 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 α 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 study was to elucidate the redox and spectroscopic properties of these A-cluster forms and thereby understand their structural and functional differences. The Ni-labile form 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°‘ for the Ni2+/Ni1+ couple, thereby rendering CO and triquat effective reductants. Dithionite did not reduce the Ni-labile form, though its addition to CO/C...