Cytochrome b6 f complex

EJ Boekema, AF Boonstra, JP Dekker and M R gner (1994) Electron microscopic structural analysis of photosystem I, photosystem II, and the cytochrome b6/f complex from green plants and cyanobacteria. J Bioenerg Biomembr 26 17-29... 

ISOLATION AND CHARACTERIZATION OF PS I. PS II AND CYTOCHROME B6/F COMPLEXES FROM SYNECHOCYSTIS PCC 6803 AND A CP43-LESS MUTANT... 

A correlation between the freeze-fracture data, and biochemical characterisation of thylakoid membrane complexes, has been made to produce a hypothetical model of the higher plant photosynthetic membrane, shown in Figure 2. An attempt has been made to indicate whether a polypeptide is extrinsic or intrinsic, and if extrinsic, on which side of the thylakoid it is likely to be located. For intrinsic polypeptides, the number of times they span the membrane is reflected in their width, and the stoichiometry is also indicated, where known. The organisation of the cytochrome b6/f complex is based on the model of O Keefe (7), and of the FNRl on that of Pschorn et al. (8). 

Plastocyanin (PC) mediates electron transfer from the cytochrome f (cyt f) component of the cytochrome b6/f complex to the Photosystem I reaction center during photosynthetic electron transport. At the preceeding International Congress on Photosynthesis, predictions were made regarding regions on cyt f which might be likely candidates for the PC interaction site (1). To further examine this hypothesis, we have further characterized a covalently linked PC-cyt f adduct. This adduct was prepared by incubation of the two proteins in the presence of EDC, a water soluble carbodiimide, and has been demonstrated to have a 1 1 stoichiometry (2). 

LHCb phosphorylation which is a prerequisit for further phosphorylation of LHCa by Kb. Kb activity depends on the presence of b6/f complexes in a way which may involve activation by cytbh. This cytochrome has a midpoint potential of Omv (Joliot and Joliot, 1987) consistent with that required for Kb activation (Horton et al., 1981). It is worth noting that such a heme-regulated kinase is involved in the initiation of translation (Edelman et al., 1987). In the latter case, SH groups are involved in the regulation of the enzyme activity, a feature that has also been recognized for the LHC-kinase (Millner et al., 1982). 

Plant plastocyanins are synthesized in the cytosol as 160-170-ammo acid precursor polypeptides consisting of a 60-70-residue transit peptide followed by a 97 99-amino acid mature protein. The transit peptide imports the precursor plastocyanin molecule across the chloroplast envelope and thylakoid membranes to its final destination in the thylakoid lumen, where it shuttles electrons by accepting them from the membrane bound cytochrome / (cyt /) of the cyt b6/f complex and donating them to the photooxidized reaction center P700-I- of photosystem I. Cyanobacterial plastocyanins use an 30-amino acid leader seqnence for thylakoid membrane translocation. Currently, there are more than 100 plant and cyanobacterial plastocyanin sequences that are available either by direct protein sequencing or deduced from the nucleotide sequences of their genes. 

Figure 2.4 illustrates electron and proton transport processes. Electrons are initially energized by sunlight hitting photosystem II (PSII see figure legend) and transported to photosystem I (PSI). In PSI, sunhght energy is again imparted and the electrons are transferred by ferredoxin, another electron carrier, to NADPH. Electron transport from PSII to PSI is via plastoquinone (PQ), cytochrome b6/cytochrome f complex, and plastocyanin (blue arrows in Fig. 2.4). During electron transport, protons are taken up by plastoquinone (similar to...

It has been proposed before that the cytochrome b (cytb f) complex is involved in the activation of the redox controlled LHCn kinase (1-5). Cytochrome b /f-less mutants did not phosphorylate the LHCII polypq)tides (2-5). The putative enzyme was isolated and its properties characterised (6). However, the activity of the isolated enzyme was not increased by addition of reductants, such as duroquinol, known to activate the kinase in situ (6). To verify that the cytochrome complex is required for the activation of LHCII kinase, we attempted to reconstitute isolated cytbe/f complex into thylakoid membranes of mutants lacking this complex and thus restore kinase activity. For this purpose, the complex was isolated as reported before (7). However, preliminary assays demonstrated that a highly enriched cyt.b6/f preparation exhibits kinase activity. The properties of this kinase, including a residual response to redox control, have been investigated. 

Cytochrome b6f is a is part of the electron transport chain that transfers electrons from photosystem II to photosystem I. Cytochrome b6f is a complex of proteins that includes cytochromes f, b6, and an iron sulfur protein. Cytochrome b6f accepts electrons from plastoquinone QH2 and passes them to plastocyanin (Figure 17.12). In addition to transferring electrons, the cytochrome b6f complex pumps protons into the thylakoid lumen, helping to build the proton gradient, which is used by the CFO-CFl complex to make ATP. 

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