Pheophytin-quinone

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. 

Bacteria have a single reaction center in purple bacteria, it is of the pheophytin-quinone type, and in green sulfur bacteria, the Fe-S type. 

The importance of bioexcimers (bioexciplexes) in the photochemistry of biological compounds has been also emphasized. Computation of potential energy curves modeling the complex pheophytin-quinone shows the relevance that stabilization caused by the formation of rr-stacked excited dimers, that is, excimers (exciplexes) and the corresponding presence of conical intersections, have to provide... 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

Figure 1. Pheophytin-quinone and iron-sulfiir Reaction Centers. The dotted line represents the absorption of light by theprimary electron donor (Chl2 or BChl2).Thelineshows the energy transfers in the Reaction Center, from the PSII tyrosine residue (Yz), through the monomer bacttriochlorophyll (BChl), A) the monomer bacterio-pheophytin (BPhe), or B) pheophytin (Phe) and quinone transfer components, QA and QB, in the pheophytin-quinone type of Reaction Center, and Q through the monomer chlorophyll (Chi), quinone (Q) and F components in the iron-sulfur Reaction Centers.

The reaction center of photosystem II (PSII) consists of three proteins The 32-kDa protein (32K, also referred to as D,), Dj, and cytochrome bjs, (1,2). It has several features in common with the reaction center from purple bacteria (3,4), including amino acid sequence homology in functional regions (3,5), arrangement of the transmembrane helices (6,7,8), and conservation of the binding sites for chlorophylls, pheophytins, quinones and a non-heme iron (3,4,6,7,9). Furthermore, 32K and the L-subunit of the bacterial reaction center are the site of triazine herbicide action (10-12), and point mutations at conserved residues in these proteins can confer herbicide resistance (3,13-15). 

Radicals are formed in electron transfer reactions involving organic molecules such as chlorins [P, (bacterio)chlorophylls, (bacterio)pheophytins], quinones (Q, Q, A, of Type I RCs), tyrosine (TyrZ and TyrD in Photosystem II), carotenoids, etc. Classical EPR spectroscopy, now complemented by more elaborate techniques such as pulse EPR, ENDOR, or high-field EPR, is the most efficient way to study them (see examples of applications in References 3,4,14,42—45). 

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