3D LHCII-PSII supercomplex
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Schematic models for the major protein complexes involved in photosynthesis

Summary (more detail below figure): A schematic model (cartroon) detailing the main photosynthetic complexes engaged in oxygenic photosynthesis situated within the higher plant / green algal (eukaryotic) thylakoid membrane, where Photosystem Two (PSII) is shown as the first major complex in the electron transport chain, after light (hv) has been absorbed by its bound light-harvesting components, the Lhcb proteins. Click on each complex within image below to gain more information. Clicking on the PSII complex will zoom it, and then its individual subunits will become active for further investigation.

A larger version of this image (1800 x 868; 464 kb).
For details regarding PSII subunit composition, please view this table (1024 x 768).

A detailed model (Nov. 2011 update) of the Z scheme including structural information on the organisation of the protein complexes involved in electron (e-) and proton (H+) transport within the thylakoid membrane of green plants. The electron transport scheme is based on the literature, numerous reviews / text book figures, as well as numerous valuable discussions with local, national and international colleagues - updated by Jon since 1996.

In higher plants (eukaryotes), photosynthetic and carbon fixation reactions are housed in the chloroplasts. The so-called 'light reactions' of photosynthesis take place in the chloroplast's thylakoid membranes and are driven by light, as captured by the Light-Harvesting Complex (LHCI (Lhca) / LHCII (Lhcb)) antenna that are bound to PSI and PSII, respectively. 'Linear electron transport' involves electrons (e-) being derived from the splitting of water, by PSII, and sequentially passed along the photosynthetic e- transport chain by plastoquinone (PQ), cytochrome b6f (Cytb6f), plastocyanin (PC), Photosystem I (PSI) and PSI-bound ferredoxin (Fd), before being used for producting NADPH by ferredoxin–NADP+ oxido-reductase (not shown) in the Stromal matrix (Stroma). From this splitting of water (water oxidation), and from the Q-cycle operating about Cytb6f, protons (H+) accumulate in the thylakoid's lumenal space (lumen), thereby generating a gradient which provides for ATP production by proton motive force (chemiosmosis) via chloroplastic ATP synthase (CFoF1 ATP synthase). Under conditions causing the phenomenon of 'cyclic e- transport' that operates about Photosystem One (PSI), protons H+ are reduced to molecular hydrogen (H2) via hydrogenases (not shown). The products of the above, ATP and NADPH, are consumed during carbon dioxide (CO2) fixation, where a separate enzyme, RuBisCO, is responsible for incorporating CO2 into Ribulose bisphosphate (RuBP), ultimately forming sugar phosphates (our food, the global biomass).

see: e.g. Umena et al., (2011; T. elongatus PSII; cyanobacterial), Standfuss et al., (2005; spinach LHCII; higher plant), Stroebel et al., (2003; Chlamydomonas Cyt b6f; green algal), Jordan et al., (2001; T. elongatus PSI; cyanobacterial) and Amunts et al., (2007-2010; pea LHCI-PSI; higher plant).
 
A larger version of this image (1800 x 1013; 347 kb).

Adapted from Allen J.F., de Paula W.B.M, Puthiyaveetil S. & Nield J. (2011)
Trends in Plant Science, Vol. 16(12):645-655
http://www.sciencedirect.com/science/journal/13601385

Major proteins and protein complexes of the chloroplast photosynthetic apparatus of a higher plant exemplified by Arabidopsis thaliana. Photosystem II (PS II); cytochrome b6f (Cyt b6f); photosystem (PS I); ATP synthase; and Rubisco. See below for functions and major components. Polypeptide subunits encoded in the chloroplast are coloured green; polypeptide subunits encoded in the nucleus are coloured yellow. After Race, H.L., et al. (1999) Why have organelles retained genomes? Trends Genet. 15, 364-370

The major protein complexes with Protein Data Bank (PDB) accession numbers for atomic co-ordinates.

Photosystem II (PS II) is a membrane-intrinsic, light-dependent water:plastoquinone oxidoreductase. The primary electron donor chlorophyll a, P680, passes electrons to Phe (for pheophytin), and the bound plastoquinone molecules QA and QB. Lhcb refers to the outer, peripheral, light-harvesting, chlorophyll a– and b– and carotenoid–binding polypeptides of photosystem II. Cyanobacterial reaction centre : 3ARC.pdb; 3BZ1.pdb; 3BZ2.pdb. Lhcb, Chloroplast LHC II: 1rwt.pdb; 2bhw.pdb.

Cytochrome b6f complex (Cyt b6f ) is a membrane-intrinsic, light-independent, proton14 translocating plastoquinol:plastocyanin oxidoreductase. Cytochrome b6f is a close structural and functional homologue of respiratory complex III, catalysing the proton-motive Q-cycle, in which each pair of electrons (e–16 ) moves four protons (H+17 ) from the N-phase (in chloroplasts, the stroma) to the P-phase (in chloroplasts, the thylakoid lumen). Chlamydomonas: 1Q90.pdb. Cyanobacteria, Mastigocladus: 1UM3.pdb. Cyanobacteria, Nostoc: 2ZT9.pdb.

Photosystem I (PS I) is a membrane-intrinsic, light-dependent plastocyanin:ferredoxin oxidoreductase. Ferredoxin transfers single electrons to the flavoprotein ferredoxin:NADP+ reductase (FNR), while FNR transfers two electrons to NADP+ and H+ to give NADPH, terminating non-cyclic electron transport. Ferredoxin also transfers electrons to plastoquinone to give cyclic electron transport, and to oxygen, giving pseudocyclic electron transport. Cyanobacterial, Thermosynechococcus elongatus, reaction centre: 1JB0.pdb Pea, Pisum sativum, chloroplast reaction centre: 3LW5.pdb; 2WSC.pdb).

ATP synthase is a proton-translocating ATP hydrolase. Photosynthetic electron transport establishes a transmembrane gradient of proton concentration, a proton motive force, that serves to drive the ATPase reaction in the direction of ATP synthesis. The 10 ATPase activity itself is in a membrane-anchored but water-soluble domain, CF1. Proton translocation is by the hydrophobic, membrane-intrinsic CFo. Spinach chloroplast CFo has 14-fold rotational symmetry, corresponding to 14 copies of subunit III in the complete ring. The corresponding number of Fo subunits in other systems can be 8, 10 , 11, 14, or 15. Bovine mitochondria. Fo: 1C17.pdb. F1: 1E79.pdb; 2A7U.pdb; 1L2P.pdb.

Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) catalyses carboxylation of ribulose-1,5-bisphosphate to give two molecules of 3-phosphoglycerate, the initial step of the reductive pentose phosphate pathway and autotrophic metabolism in plants, algae, and many bacteria. The oxidase reaction gives, instead, one 3- phosphoglycerate and one 2-phosphoglycollate – no CO2 is fixed, and light-dependent O2 uptake gives “photorespiration” . Rubisco is a membrane-extrinsic and weakly water soluble oligomeric protein consisting of 8 large subunits containing the catalytic site, and 8 small subunits, giving L8S8, though L2 forms are known. Spinach, Spinacea oleracea: 1RCX.pdb. Chlamydomonas: 1GK8.pdb. Red alga Galdieria partita:1IWA.pdb. Cyanobacterium Synechococcus PCC 6301: 1RBL.pdb.

© Jon Nield, Mechanistic and Structural Biology, SBCS, Queen Mary, University of London, 2007-2011