Bacterial reaction centre

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.

EPR studies of paramagnetic species occurring in the bacterial reaction centre (bRC) have been comprehensively reviewed in this series by Weber45. This chapter covered the literature up to 1999, and also gave an introduction into structure and function of this interesting membrane protein. Due to an enormous international research effort (for leading references see ref. 3, 46-52 many of the questions about the structure and function of the bRC were already answered by the end of the last millennium. Advanced EPR techniques have... 

DNA sequencing of the plant genome is now common and leads immediately to sequences of certain proteins. They can frequently be identified with the pigment-protein complexes isolated as described in 2. Remarkable homologies are found e.g. between proteins of the bacterial reaction centres (the M and L subunits) and the plant PS2 reaction centre (proteins D1 and D2), see Barber (1987) for a review. 

In order to build synthetic analogues of the bacterial reaction centre, several approaches using multiporphyrin systems have recently been reported (Dubowchik and Hamilton, 1985, 1986 Wasielewski et al., 1982). However, the number of molecular arrays whose rigidity is sufficient to ensure control of the distance and orientation of the respective tetrapyrrolic subunits is highly limited (Dubowchik and Hamilton, 1987 Cowan etal., 1987, Sessler and Johnson, 1987). 

M.E. Michel-Beyerle and G.J. Small, eds, Chem. Phys. 197 (1995) 223-472, special issue on photosynthesis and the bacterial reaction centre. 

THE MECHANISM OF ENERGY STORAGE BY THE BACTERIAL REACTION CENTRE... 

Light excitation of the bacterial reaction centre triggers a series of electron transfer reactions that, due to variations in distance between the cofac-... 

The application of time-resolved spectroscopy has revealed many of the details and complexities of the picosecond time-scale electron transfer reactions that are catalysed by the bacterial reaction centre. Most studies... 

One of the most intriguing features of the bacterial reaction centre is the asymmetry of trans-membrane electron transfer. Although the BChl, BPhe and UQjq cofactors are arranged in two approximately symmetrical trans-membrane branches, only the so-called A-branch is used for transmembrane electron transfer. The factors determining this functional asymmetry continue to be the subject of great interest, as the reaction centre presents a chain of cofactors that catalyses electron transfer with great efficiency, and a similar chain of cofactors that is much less effective. This functional asymmetry is due to small but crucial differences in the structure of the proteinxofactor system along the two branches. 

Site-directed mutagenesis has proven to be a very powerful tool for the study of eleetron transfer in the bacterial reaction centre. It has been used to investigate the role played by specific amino acid residues during eleetron transfer, and as a means of modifying the energetics of electron transfer. A number of reviews of the applieation of mutagenesis to study of the bacterial reaction centre have been published (Fyfe et al., 1998 Parson, 1996 Woodbury and Allen, 1995 Takahashi and Wraight, 1994 Kirmaier and Holten, 1993 Coleman and Youvan, 1990). In this section, we concentrate on two heavily-studied elasses of reaction centre mutant that have... 

FIGURE 13. Energy and electron transfer in the Tyr M210 —> Trp mutant reaction centre. (A) The fluorescence excitation spectrum of this reaction centre (dotted) does not match the absorbance spectrum (solid) in the H and B Qy bands, indicating that some energy delivered to the BPhes and monomeric BChls is not passed to P. Despite this, excitation of the BPhes and monomeric BChls does lead to efficient formation of P Q/ (filled boxes). (B) Schematic of routes of energy and electron transfer in the bacterial reaction centre. Excitation of Ba either results in energy transfer to P or in direct formation of P El via the intermediates P+Ba" and/or B/Ha. ... 

Timpmann, K., Ellervee, E., Laisaar, A., Jones, M. R., and Freiberg, A., 1997, High pressure-induced acceleration of primary photochemistry in membrane-bound wild type and mutant bacterial reaction centres in Ultrafast Processes in Spectroscopy (R. Karli, P. Freiberg, and P. Saari, eds.) pp. 236n247. 

When the components of the PS II reaction centre are drawn on a redox scale and compared in this way to those of the purple bacterial reaction centre, a remarkable similarity can be seen between the electron acceptors in each system (Fig. 4). The chemical natures of these components are extremely similar, being made up of a complex of two quinones, an iron atom and a pheophytin (a bacteriopheo-phytin in bacteria). The donor side of PS II in the redox scheme is, however, not comparable to that in bacteria. P-680 may appear to be structurally similar to P-870 in bacteria in that it is made up of chlorophyll (bacteriochlorophyll in bacteria) and that is acts as the primary electron donor however, the P-680/P-680+ redox couple is approximately 600-800 mV more oxidizing than the equivalent bacterial redox couple P-870/P-870, = +450 mV). In addition, PS II has an array of high-potential components which make up the 02-evolving enzyme and which are clearly unique to that system. 

The redox properties of Qg are also unlike those of plastoquinone in the pool. The semiquinone form, Qb . is tightly bound to a protein of the reaction centre and is thus stabilized. Qb is much more stable than Qa . since forward electron transfer does not take place from Qb - The lifetime of Qb, like that of Qa in the presence of DCMU, is determined by the stability and availability of positive charges on the donor side. For example, Qb recombination occurs with S2 or S3 (Ref. 115, and see section 3.5) with a ty2 of approximately 30 s [116] but when Qb is present in centres where the stable S states, S,) and Sj, are present, Qb is stable for hours. This probably explains why a certain proportion of Qg is present even in PS II which has been dark-adapted for long periods. A number of measurements have indicated the involvement of proton uptake when Qb is reduced to semiquinone form [117], although the optical spectrum is more compatible with Qb being the unprotonated anion. This can be explained by the protonation of a group on the protein close to Qg, as first proposed in purple bacterial reaction centres to explain similar phenomena [118]. 

In the bacterial reaction centre the iron is situated between the two quinones [133], hence the almost identical interactions between and Fe " and between Qb and Fe ". The distance between the quinone and the iron estimated from considerations of the magnetic interaction [134,135] was verified as being 7 A by X-ray crystallography [133]. This value can probably be directly applied to the distance between and Fe in PS II, since the EPR signal is so similar to that in bacteria. 

The function of the iron remains unknown in both the bacterial reaction centre and PS II. In bacteria the iron can be replaced by other divalent transition metals with no apparent effect on the electron transfer reactions [136]. Removal of the metal slows down (by 2-fold) the electron transfer rate from to Qg but does not block electron transfer [136]. Despite these observations the conservation of this metal within the quinone complex throughout the evolutionary processes that separate the purple bacteria from higher plants indicates an important role for this component. As yet we are still ignorant of that role. 

Although our current understanding of the Qh/Ql phenomenon is not yet clear (in fact the low-potential wave is absent in some titrations e.g. Ref. 152), it seems that the more plausible explanations require no modification of the structural model of PS II based on the bacterial reaction centre. 

Fig. 5. A possible structure of the PS II reaction centre. The model leans heavily on the analogy with the bacterial reaction centre. Discussion of the location of the chromophores within the polypeptides is given in the text. The orientation of some of the components is shown. The role of the extrinsic polypeptides and the possible structure of the manganese cluster are discussed in Chapter 6.

The model in Fig. 5 is based on the X-ray structure of the purple bacterial reaction centre. Since no analogies to Z and D are present in purple bacteria it is reasonable to suggest that these components originate in polypeptides other than the 32 kDa (Dj and D2) polypeptides. An obvious candidate is the 47 kDa polypeptide which forms part of the PS II core. 

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