The cytochrome b6f complex. Novel aspects

The Cyt b6f complex from plant chloroplasts, the green alga Chlamydomonas reinhardtii, and the thermophilic cyanobacterium, Mastigocladus laminosus, can be isolated in a highly active state, in which it is dimeric and contains one bound chlorophyll a molecule per monomeric unit. The latter feature is a distinguishing trait compared to the b6f complex of bacterial photosynthesis and the respiratory chain. In contrast to the trans‐membrane domains of the b6f complex, and of most other integral membrane proteins, which are characterized by an a‐helical structure, the p‐side peripheral domains, consisting of Cyt f and the Rieske protein, have a predominantly β‐strand secondary structure motif. One consequence of this motif is an extension of these polypeptides from the membrane surface. For example, the length of Cyt f is 75 Å. The heme Fe is 45 Å from the α‐carbon of Arg250 at the membrane bilayer interface and, even though Cyt f may be tilted relative to the membrane plane, the heme electron transfer reactions are carried out far from the membrane surface. The presence of an internal 5 water chain, which has the properties of a proton wire, with one water H‐bonded to the histidine‐25 heme ligand, also suggests that the pathway of long distance H+ translocation traverses the extended p‐side protein domain of the b6f complex. A mechanism of H+ transfer in the chain that is coupled to the redox state of the heme, in which a proton is transferred into the chain to compensate the extra electron in the ferro‐heme, is proposed.