Reaction center cofactors

Structure of the Rps. viridis photosynthetic reaction-center cofactors. The black dots delineate the outward (O)- and inward (I)-facing portions of the membrane. Adapted from Reference 36. 

Allen, J.R, et al. Structure of the reaction center from Rhodobacter sphaeroides R-26 the cofactors. Proc. Natl. Acad. Sd. USA 84 5730-5734, 1987. 

Photosystem II (Fig. 1) bears many similarities to the much simpler reaction center of purple bacteria. Remarkable is, however, the increase in complexity at the protein level. In a recent review on the evolutionary development of the type 11 reaction centres340 this was attributed to the invention of water-splitting by PS II and the necessity to protect and repair the photosynthetic machinery against the harmful effects of molecular oxygen. The central part of PS II and the bRC show a highly conserved cofactor arrangement,19 see Fig. 1. These cofactors are arranged in two branches bound to two protein subunits, L/M and D1/D2 in bRC and PS II, respectively. On the donor side a closely related pair of chlorophylls or bacteriochlorophylls exists the acceptors comprise monomeric chlorophylls, pheophytins (Ph) and 2 quinones QA and QB. Qa and Qb are plas-... 

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. 

Energy of Cofactors at the Quinone Q(a) Site of the Photosynthetic Reaction Center of... 

The primary photochemical processes of photosynthesis take place within membrane bound complexes of pigments and protein, reaction centers (Shuvalov and Krasnovsky, 1981 Deisenhofer et al., 1986, Rees et al., 1989 Norris and Shiffer, 1990 Kirmaier and Holten, 1991 Feher et al, 1992 Stowell et al, 1997). One mole of a reaction center from different bacteria contains 4 moles of bacteriochlorophyl (Bchl), 2 moles of bacteriopheophytin (Bph), two moles of ubiquinone (Q) and a non-heme Fe atom. In RC from Rhodobacter speroides, a total of 11 hydrophobic a-helixes create a framework that organizes the cofactor and a hydrophobic band approximately 35 A wide. RC from Rhodopseudomonus viridus has three polypeptides having pronounced hydrophobic properties. The molecular mass of the polypeptides are 37 571 (L), 35902 (M) and 28902 (H). The H subunit does not carry pigments but it is sufficient for the photochemical activity. The protein components of reaction centers from different-bacteria are similar. 

Kirmaier, C., Cua, A. He, C. Holten, D. Bocian, David F. (2002) Probing M-branch electron transfer and cofactor environment in the bacterial photosynthetic reaction center by addition of a hydrogen bond to the M-side bacteriopheophytin, Journal of Physical Chemistry B 106, 495-503. 

A view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

Chains of redox cofactors for long range electron transfer are clearly the way electrons are transferred over the tens of angstroms dimensions of membranes and their proteins. Once again, purple photosynthetic bacterial reaction centers provide an archetype for understanding electron transfer chain design and behavior. The heme chain in Rps. viridis... 

FIGURE 7. Two redox cofactor chains meet at the bacteriochlorophyl dimer in the photosynthetic reaction center of Rp. viridis. Electron transfer takes place by tunneling between cofactors diat are spaced by no more dian 14, assuring overall elech on transfer rates in the msec or faster range, even though a total distance of 70 is crossed by the c heme chain. 

Wamcke, K., and Dutton, P. L., 1993, Influence of QA site redox cofactor structure on equilibrium binding, in situ electrochemistry, and electron transfer performance in the photosynthetic reaction center protein Biochemistry 32 4769n4779. 

Rischel, C., Spiedel, D., Ridge, J. P., Jones, M. R., Breton, J., Lambry, J. C., Martin, J. L., and Vos, M. H., 1998, Low-frequency vibrational modes in proteins large frequency-shifts induced by point-mutations in ftie protein-cofactor matrix of bacterial reaction centers. Proc. Natl. Acad. Sci. USA, 95 12306nl2311. 

Vos, M. H., Breton, J., and Martin, J. L., 1997, Electronic energy transfer within the hexamer cofactor system of bacterial reaction centers. J. Phys. Chem., 101 9820fi9832. 

Yeates, T. O., Komiya, H., Chirino, A., Rees, D. C., Allen, J. P., and Feher, G., 1988, Structure of the reaction center from Rhodobacter sphaeroides R-26 and 2.4.1 Protein-cofactor (bac-teriochlorophyll, bacteriopheophytin and carotenoid) interactions. Proc. Natl. Acad. Sci. USA, 8 5 799397997. 

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