On the Structure of the Nickel/Iron/Sulfur Center of the Carbon Monoxide Dehydrogenase from Rhodospirillum rubrum: An X-Ray Absorption Spectroscopy Study

The nickel/iron/sulfur center of the carbon monoxide dehydrogenase (carbon monoxide:(acceptor)oxidoreductase; EC 1.2.99.2) enzyme from Rhodospirillum rubrum (Rr-CODH) was studied by x-ray absorption spectroscopy at the Ni K edge. Extended x-ray absorption fine structure data show that the first Ni coordination shell consists of 2 S atoms at 2.23⚬A and 2-3 N/O atoms at 1.87 Å. The edge structure indicates a distorted tetrahedral or five-coordinate Ni environment in both oxidized and reduced Rr-CODH. By comparing second-shell extended x-ray absorption fine structure data of Rr-CODH to that of (Et4N)3[NiFe3S4(SEt)4], a cubane-type cluster, it was clearly established that Ni in the Rr-CODH center is not involved in the core of a NiFe3S4cubane cluster. One model consistent with the results is a mononuclear Ni2+site, bridged by S-Cys or sulfide to one or both of the Fe4S4clusters of the enzyme, with the remaining coordination sites occupied by additional S-Cys or N/O-liganding amino acid residues.