The Bacterial Photosynthetic Reaction Centre

Many key protein ET processes have become accessible to theoretical analysis recently because of high-resolution x-ray stmctural data. These proteins include the bacterial photosynthetic reaction centre [18], nitrogenase (responsible for nitrogen fixation), and cytochrome c oxidase (the tenninal ET protein in mammals) [19, 20]. Although much is understood about ET in these molecular machines, considerable debate persists about details of the molecular transfonnations. 

Figure 10.17 The core of the bacterial photosynthetic reaction centre. (From Berg et al., 2001. Reproduced with permission from W.H. Freeman and Co.)...

Figure 12.9 Schematic view of the bacterial-photosynthetic reaction centre and the energy transfers which occur. The groups are held in a fixed geometry by the surrounding proteins...

TOWARDS AN ANALOGUE OF THE BACTERIAL PHOTOSYNTHETIC REACTION CENTRE SYNTHESIS OF AN OBLIQUE BIS-PORPHYRIN SYSTEM CONTAINING Al, 10-PHENANTHROLINE... 

Figure 11.4 View of the bacterial photosynthetic reaction centre showing inorganic cofactors. (Data from Protein Data Bank (DO 10.2210/pdb2l5N/pdb))...

The semiquinone biradical produced in the photocycle of bacterial photosynthetic reaction centres was trapped at 77 K and examined at 9.6, 35, and 94 GHz.16 Simulations of spectra at the multiple resonant frequencies using the simulated annealing method revealed the spatial and electronic structure of the biradical. The value of r was found to be 17.2 + 0.2 A, which is in good agreement with the value of 17.4 0.2 A obtained in an X-ray crystal structure. This study shows the power of high-frequency EPR combined with data obtained at lower frequencies. 

A schematic view of bacterial photosynthetic reaction centre with a chain of electron-carrying metal prosthetic groups is shown in Figure 11.4. The metal ions involved are magnesium (bacteriochlorophylls) and iron (haem and non-haem iron species). 

Fyfe, P. K., McAuley-Hecht, K. E., Jones, M. R., and Cogdell, R. J., 1998a, Purple bacterial photosynthetic reaction centres. In Biomembrane Structures, (P. I. Haris and D. Chapman, eds.) 64987, lOS Press, Amsterdam, The Netherlands. 

Photoinduced charge separation processes in the supramolecular triad systems D -A-A, D -A -A and D -A-A have been investigated using three potential energy surfaces and two reaction coordinates by the stochastic Liouville equation to describe their time evolution. A comparison has l n made between the predictions of this model and results involving charge separation obtained experimentally from bacterial photosynthetic reaction centres. Nitrite anion has been photoreduced to ammonia in aqueous media using [Ni(teta)] " and [Ru(bpy)3] adsorbed on a Nafion membrane. 

Some support for these ideas comes from recent results about the relative placement of chromophores in a bacterial photosynthetic reaction centre. ... 

The fact that the singlet-triplet mixing in radical pairs becomes faster at high fields, due to the increase of the Zeeman interaction, can also permit modelling of the sequential electron-transfer process of both the primary and secondary pairs. The importance of protein dynamics on the electron-transfer rate was noted in a 95 GHz study of bacterial photosynthetic reaction centres with slow electron-transfer rates. ... 

P Beroza, DR Fredkin, MY Okamura and G Feher (1992) Proton transferpathways in the reaction center of Rhodobacter sphaeroldes A computational study. In J Breton and A Vermeglio (eds) The Photosynthetic Bacterial Reaction Center II Structure, Function and Dynamics, pp 363-374. Plenum U Ermler, G Fritzsch, SK Buchanan and H Michel (1994) Structure of the photosynthetic reaction centre from Rhodobacter sphaeroldes at 2.65 resolution cofactors and protein-cofactor Interactions. Structure 2 925-936... 

The photosynthetic reaction centres (RCs) are transmembrane protein-pigment complexes that perform light-induced charge separation during the primary steps of photosynthesis. RCs from purple bacteria consist of three protein subunits, L, M and H, and bind four bacteriochlorophylls, two bacteriopheophytins, two quinones, one non-haem iron and one carotenoid. The elucidation at atomic resolution of the three-dimensional structures of the bacterial RCs from Rhodopseudomonas (Rps.) viridis (1) and Rhodobacter (Rb,) sphaeroides (2-4) has provided impetus for theoretical and experimental work on the mechanism of primary charge separation in the RCs. The structures revealed that the cofactors are bound at the interface between the L and M subunits and are organised around a pseudo C2 symmetry axis. However, the structural symmetry does not result in functional symmetry as the electron transfer proceeds only along the L branch (5). 

The photochemically active pigments of photosystem II (PSII) are housed in an apoprotein environment provided by the D1 and D2 polypeptides which also contain binding sites for the acceptor quinones (Q and Q ). The organisation of the polypeptides and chromophores has been inferred (1-3) from various similarities and homologies between PSII reaction centres and the photosynthetic reaction centres of purple bacteria, such as Rhodopseudomonas viridis, which have been structurally resolved in considerable detail (e.g. 4). By analogy with the L and M polypeptides of bacterial reaction centres, D1 and D2 each contain 5 transmembrane helical spans. There is strong sequence homology... 

Figure Bl.15.16. Two-pulse ESE signal intensity of the chemically reduced ubiqumone-10 cofactor in photosynthetic bacterial reaction centres at 115 K. MW frequency is 95.1 GHz. One dimension is the magnetic field value Bq, the other dimension is the pulse separation x. The echo decay fiinction is anisotropic with respect to the spectral position.

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