Photosynthesis | Structure-Function of the Cytochrome b6f Lipoprotein Complex

The cytochrome b6f complex, embedded in the chloroplast thylakoid membrane, is a central hub in the proton-coupled electron transfer chain of oxygenic photosynthesis. It transfers electrons coupled to trans-membrane proton translocation in (1) a "linear" or "non-cyclic" pathway involving electron transfer between the two light-driven "reaction center" complexes, photosystem I (PSI) and II (PSII), from water to NADP+, and (2) in the ferredoxin/FNR-dependent "cyclic" pathway using electrons transferred from PSI (Fig. 1). When light intensity is not limiting, oxidation and deprotonation of plastoquinol by the cytochrome (b6f) complex determine the rate-limiting step in the electron transport chain. The electron transfer and proton translocation functions of the complex are discussed in the context of its high resolution (2.50 Å) crystal structure (PDB entry 4OGQ), a structure determination unique in studies of membrane proteins through its description of details of intra-protein lipid. Excluding lipid and bound detergent, the dimeric cytochrome complex consists of a 250 kDa lipoprotein complex containing, per monomer, thirteen trans-membrane α-helices and seven prosthetic groups per monomer. The prosthetic groups consist of four hemes and an enigmatic bound pair of pigments, one chlorophyll a and one β-carotene separated by 14 Å in each monomer. In the 2.50 Å structure each monomer contains at least 23 lipid binding sites. The major presence of the lipid suggests a change in the label and perspective for membrane protein structures, which should properly be described as "lipoproteins". This label applies not only to the cytochrome b6f complex, but also to the set of trans-membrane oligomeric membrane proteins which function in oxygenic photosynthesis and, more generally, in membrane-based transport.