Identification and characterization of the CO Oxidation/CO2 reduction site of Ni-containing carbon monoxide dehydrogenases

Abstract

The active site for the CO Oxidation/CO2 reduction activity of carbon monoxide dehydrogenase, called the C-cluster, consists of a Fe4S4 cluster bridged to a Ni ion. One of the irons, called FCII, is either 5 or 6 coordinate and is thought to be bridged to the Ni. The cluster is coordinated by at least two cysteines and one histidine ligand. An exchangeable proton also binds to the Fe-S cluster. 'The C-cluster can be isolated in three oxidation states. The Co state is the most oxidized. Co is diamagnetic with a system spin of S = 0. A one electron reduction of Co yields the C182 state. C182 is paramagnetic with S = 1/2. Lowering of solution potentials eliminates the C182 State and causes development ofthe C186 state. C186 also has a system spin of S = 1/2. C186 is thought to be two electrons more reduced than C182. The midpoint potential for the C182/C186 conversion is -530 mV. The C-cluster is connected to the other redox sites in the enzyme via electron pathways. The B-cluster is the conduit for electron transfer between the redox-active clusters in the enzyme and external redox agents. Electrons are transferred from the C-cluster to external electron acceptors via the B-cluster. Oxidation of CO to C02 is a two electron process. The C-cluster must accept two electrons at the same time from CO as it is oxidized. The B-cluster only does one electron redox reactions. This requires that the C-cluster exist in at least three sequential redox states. Cn+ is the state that binds and is reduced by CO. C(n-2)+ is the state that binds and is oxidized by CO2. Cint is a Stable one electron intermediate. The reaction is reversible. This means that the midpoint potential of the Cn+/C(n-2)+ couple ,should be close to that of the CO/CO2 couple, - -520 mV at pH 7. The Midpoint potential for the C182/C186 couple is in this region. Thus, C182 is probably the Cn+ state and the C186 is probably the C(n-2)+. The Cint state has not yet been observed.