A Multinuclear ENDOR Study of the C-Cluster in CO Dehydrogenase from Clostridium thermoaceticum:  Evidence for HxO and Histidine Coordination to the [Fe₄S₄] Center

The C-cluster of carbon monoxide dehydrogenase (CODH) catalyzes the reversible oxidation of CO to form CO2. This study reports electron nuclear double resonance (ENDOR) spectroscopy of the one-electron reduced (Cred1), the CN--inhibited, and the CO (or dithionite)-reduced (Cred2) forms of the C-cluster from Clostridium thermoaceticum CODH (CODHCt). The observed hyperfine interactions of 1,2H, 14N, 13C, and 57Fe support and extend the current Ni−X−[Fe4S4] C-cluster model in which a [Fe4S4] center is linked to a Ni ion through a unique iron, FCII. The unpaired electron spin apparently is localized on the [Fe4S4] component of the cluster, and thus the hyperfine interactions observed by ENDOR most probably reflect species associated with that component. A solvent-exchangeable proton with a maximum hyperfine coupling of A(1H) = 16 MHz is detected in the Cred1 form, but not in the CN--inhibited or Cred2 forms. The exchangeable proton is assigned to a probable solvent-derived (HxO, x = 1, 2) ligand to FCII of the Cred1 [Fe4S4]1+ center and is predicted to be a substrate in CO/CO2 catalysis. For both Cred1 and Cred2, we find ENDOR features in the region expected for a nitrogen-donor ligand which likely arise from a histidine ligand to the [Fe4S4] center. 57Fe ENDOR detects at least two classes of Fe in Cred1 that most likely arise from the (Fe2.5+)2 mixed-valence pair. Their large maximum couplings of A(57Fe) > 40 MHz support the unusual nature of the cluster; these do not change dramatically between the Cred1 and Cred2 forms of the enzyme. Cred2 formed by reduction with 13CO reveals no new 13C features, strongly suggesting that neither CO nor its oxidized products are bound to the [Fe4S4] center in Cred2. Taken together, these ENDOR assignments suggest that in the Cred1 state, the unique Fe ion of the CODH C-cluster has an available coordination site that stably binds HxO or CN- and that reduction of the C-cluster results in rearrangement at that site, causing loss of the bound aqueous ligand.