CN and CO ligands

CN and CO ligands keeps the Fe in a low-spin state (S = 0) and prevents the detection of hyperfine splitting of Fe in EPR and Fe-ENDOR (Huyett et al. 1997). Also, very good agreement with experimental data was obtained for model clusters of the Ni-A, Ni-C and Ni-L states. 

A similar reaction can be written for the [Fe] hydrogenases with a Fe-[4Fe-4S] complex replacing the nickel. Note that the nickel atom in the NiFe cluster, and the Fe-[4Fe-4S] sites are nearest to the electron carrier [4Fe-4S] clusters, indicating that electron transfer occurs through these atoms. The other atom in each of the centres is an iron atom with -CN and -CO ligands, and it seems likely that this is a binding site for hydride (Fig. 8.1). 

The presence of CN and CO ligands of the active site Fe raises the question as to how these potentially toxic molecules are incorporated in the enzyme. Significant progress has been made in the elucidation of the biosynthetic pathway of cyanide from a carbamoyl phosphate precursor [61]. On the other hand, the origin of the CO ligand remains unclear [62]. Many proteins are involved in the biosynthesis and the maturation of the active site of [NiFe]-... 

Pierik AJ, Hulstein M, Hagen WR, Albracht SPJ. 1998a. A low-spin iron with CN and CO as intrinsic ligands forms the core of the active site in [Fe]-hydrogenases. Fur J Biochem 258 572-8. 

A number of low-molecular-weight inorganic ligands are involved in metal binding in metalloproteins. These include HCO) and P04, as ligands to Fe in proteins involved in iron transport, in association with amino acid residues of the protein. We will discuss the role of CN and CO as ligands to Fe in bacterial hydrogenases, where they are part of more complex metal centres in a later section. 

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