Sandbox Reserved 760: Difference between revisions

Each of the two FeMo-cofactor units contains 1 Molybdedum, 7 iron, 9 sulfur and one homocitrate molecule, and also serves as the site of substrate binding and subsequent reduction. The M-cluster is formed from Fe4S3 and MoFe3S3 partial cubanes that are joined together by three nonprotein ligands. The FeMo-cofactor is tucked -10 A beneath the protein surface, in an environment primarily provided by the α subunit. Only two protein ligands, Cys a275 and Hisa442, organize the cofactor to the protein. The octahedral ordered sphere of the Molybdenum is completed by the addition of homocitrate which together displaces a terminal Fe atom from the P-cluster. The remaining Fe and S are used to make up the P-cluster pairs. The role of the P-cluster is to transfer the electrons from the Fe-protein to the FeMo-cofactor or M-cluster. It is connected to the backbone of the protein via 7 amino acid residues: six cysteines and one serine. Three cysteine residues are provided by α subunit while the rest come from the β subunit. There are two copies of the P-cluster in the MoFe protein which contain two Fe4S4 clusters that are reductively coupled and are now considered as a Fe8S8 cluster. What makes the P-cluster pair an unusual inorganic structure is that the two Fe4S4 clusters are connected by the thiol side chains of Cysteine α88 and β95, and a disulfide bond between two cluster inorganic sulfurs. This disulfide bond is located on the side of the P-cluster pair closest to the surface of the protein. The presence of the disulfide bond in the P-cluster pair shows that the center of the P-cluster serves as a two electron redox group. Since the Fe-protein is considered to be a one-electron donor with the P-cluster pair as the immediate acceptor, the P-cluster pair may be considered to be both.one- and two-electron redox reactant.