THE CYTOCHROME B(6)F COMPLEX - NOVEL ASPECTS

The Cyt b(6)f complex from plant chloroplasts, the green alga Chlamydo monas reinhardtii, and the thermophilic cyanobacterium, Mastigocladus laminosus, can be isolated in a highly active state, in which it is di meric and contains one bound chlorophyll a molecule per monomeric unit . The latter feature is a distinguishing trait compared to the bc(1) c omplex of bacterial photosynthesis and the respiratory chain. In contr ast to the trans-membrane domains of the b(6)f complex, and of most ot her integral membrane proteins, which are characterized by an a-helica l structure, the p-side peripheral domains, consisting of Cyt f and th e Rieske protein, have a predominantly beta-strand secondary structure motif. One consequence of this motif is an extension of these polypep tides from the membrane surface. For example, the length of Cyt f is 7 5 Angstrom. The heme Fe is 45 Angstrom from the a-carbon of Arg250 at the membrane bilayer interface and, even though Cyt f may be tilted re lative to the membrane plane, the heme electron transfer reactions are carried out far from the membrane surface. The presence of an interna l 5 water chain, which has the properties of a proton wire, with one w ater H-bonded to the histidine-25 heme ligand, also suggests that the pathway of long distance H+ translocation traverses the extended p-sid e protein domain of the b(6)f complex. A mechanism of H+ transfer in t he chain that is coupled to the redox state of the heme, in which a pr oton is transferred into the chain to compensate the extra electron in the ferro-heme, is proposed.