Photosystem catalyzed electron transport

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

Spectroscopic and crystallographic studies have identified four Fe S clusters in the membrane-bound photosynthetic electron transport chain of plant and cyanobacterial chloro-plasts. One is the Rieske-type [2Fe-2S] + + center in the cyt b(,f complex, which catalyzes electron transfer from plasto-quinol to plastocyanin with concomitant proton translocation, and is functionally analogous to the cyt bc complex, with cyt / in place of cyt The remainder are low-potential [4Fe 4S] + + centers in Photosystem I which constitute the terminal part of the electron transfer chain that is initiated by the primary donor chlorophyll. One is a very low-potential [4Fe S] + + center, Fx (Em =-705 mV), that bridges two similar subunits (PsaA and PsaB) and is coordinated by two cysteines from each subunit in a C-Xg-C arrangement. This cluster transfers electrons to the 2Fe-Fd acceptor via an electron transfer chain composed of Fa, a [4Fe S] + + cluster with Em = -530 mV, and Fb, a [4Fe S] + + clusters with Em = -580 mV. Fa and Fb are in a low-molecular weight subunit (PsaC, 9 kDa) that shows strong sequence and structural homology with bacterial 8Fe-Fds. The center-to-center distance between Fx and Fa and between Fa and Fb are 14.9 A and 12.3 A, respectively, well... 

Photosy.stem II, an enormous transmembrane assembly of more than 20 subunits, catalyzes the light-driven transfer of electrons from water to plas toquinone. This electron acceptor closely resembles ubiquinone, a component of the mitochondrial electron-transport chain. Plastoquinone cycles between an oxidized form (C and a reduced form (QH, plastoquinol). Fhe overall reaction catalyzed by photosystem II is... 

FNR catalyzes the transfer of electrons from ferredoxin to NADP+ in the last step of electron transport of photosystem II in photosynthesis. 

Photosystem II (PS II) catalyzes the light-induced reduction of plastoquinone by water in thylakoids in plants and cyanobacteria. The PS II complex is composed of a number of polypeptides, some of which bind pigments and/or redox components of the photosynthetic electron transport chain. Fig. 1 presents a schematic model of PS II. 

Figure 1.1. Simplified scheme showing electron transport in a portion of a chloroplast thylakoid membrane. Electrons flow from water via an oxygen-evolving complex (OEC) to photosystem II (PS2), pheophytin (PHEO), plastquinone (PQ), plastocyanin (PC) to photosystem I (PSI). Aq, Chlorophyll FeS, iron-sulfur centres FD, ferredoxin. Phosphorylation is catalyzed by proton transport through a transmembrane proton channel (CFq) to the ATP-synthetase complex (CF,).

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