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

The first three-dimensional structure of a dimeric, octa-heme cytochro me c(3) (M(r) 26000) from Desulfovibrio desulfuricans Norway, establis hed at 2.2 Angstrom resolution, is briefly presented and compared to t he known 3-D-structures of different c(3)-type tetraheme cytochromes, in order to contribute to a better understanding of the function of mu ltiheme clusters and of the role of conserved amino acids implicated i n possible electron transfer pathways. The dimeric protein crystallize s in the space group P3(1)21 with a = 73.01 Angstrom, c = 61.81 Angstr om and the asymmetric unit contains one monomer subunit, the dimer bei ng generated by the crystallographic two-fold axis. The 3-D-structure was solved using the molecular replacement method with a model based a n the structure of the tetraheme cytochrome c(3) (M(r) 13000) from D d esulfuricans 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 enviro nments and the histidines, axial ligands of the heme-iron atoms.