MOLECULAR AND STRUCTURAL BASIS OF ELECTRON-TRANSFER IN TETRA-HEME AND OCTA-HEME CYTOCHROMES

The first three-dimensional structure of a dimeric, octa-heme cytochrome c(3) (M(r) 26000) from Desulfovibrio desulfuricans Norway, established at 2.2 Angstrom resolution, is briefly presented and compared to the known 3-D-structures of different c(3)-type tetraheme cytochromes, in order to contribute to a better understanding of the function of multiheme clusters and of the role of conserved amino acids implicated in possible electron transfer pathways. The dimeric protein crystallizes in the space group P3(1)21 with a = 73.01 Angstrom, c = 61.81 Angstrom and the asymmetric unit contains one monomer subunit, the dimer being generated by the crystallographic two-fold axis. The 3-D-structure was solved using the molecular replacement method with a model based an the structure of the tetraheme cytochrome c(3) (M(r) 13000) from D desulfuricans Norway, presently refined at 1.7 Angstrom resolution. The monomeric subunit has the same overall fold as all cytochromes c(3) (M(r) 13000). Moreover, the heme core of all examined cytochromes c(3) is highly conserved, but differences appear concerning the heme environments and the histidines, axial ligands of the heme-iron atoms.