CO-Induced structural rearrangement of the c cluster in Carboxydothermus hydrogenoformans CO dehydrogenase -: Evidence from NiK-edge X-ray absorption spectroscopy

We have examined the C cluster in type II CO dehydrogenase (CODH) from Carboxydothermus hydrogenformans using Ni K-edge X-ray absorption near edge spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The enzyme was studied under three conditions: "as-isolated" and after treatment with CO or Ti-III. The shape of the Ni K-edge changes slightly between the different conditions, but no significant edge shift is seen, suggesting that the C cluster contains Ni-II in both forms. The Ni EXAFS of as-isolated CODH can be simulated with 4 Ni-S interactions at 2.20 Angstrom with a large spread in distances. A light atom (C, N, 0) is not required to fit the spectrum. After CO treatment, significant changes are observed in the EXAFS. A new feature appears at similar to2.7 Angstrom; this component is consistent with a Ni-Fe interaction. The average Ni-S distance also expands to similar to2.25 Angstrom. The changes between the two forms suggest that the active site (C cluster) undergoes structural rearrangement after CO treatment, and the observed changes help reconcile the two different crystal structures. The implications of the structural change for the enzyme activation and mechanism are discussed.