ISSN:
1399-3054
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1399-3054.1997.tb00011.x
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