Purple bacterial antennas

Good spectroscopic data for well-defined complexes from purple bacteria are available (Refs. 28, 29 and references therein), but general agreement is lacking on what, if anything, constitutes a minimum complex. One candidate is the so-called photoreceptor unit , consisting of core antenna plus reaction center [30]. This cyclic unit structure has features in common with secondary-antenna cyclic structures [31]. Some exciton analyses apply to any of these membrane antennas, others are specific. A key issue is the explanation of spectral data on the B850 part of the 

B800-850 complex (see Chapter 11). Two approaches so far have been tried. 1. First rationalize the BChl geometry in terms of spectral observations, then attempt to justify the geometric model in terms of independent structural information. 2. First introduce all available structural information into a geometric model, then attempt to deduce remaining, uncertain, geometric features from spectral observations. The first approach is considered in Section 3.1, the second in Section 3.2. 

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This is the most promising approach so far to a geometric model of a purple bacterial antenna complex [18]. Its basic ideas are simple and ingenious. If correct, it would explain much. However, the model has some significant problems. 

Of the other ten alternatives on the Pearlstein-Hemenger list, three deserve further discussion. One of these, interactions involving higher electronic excited states, has been invoked recently as important for understanding purple bacterial antenna and RC spectra [18,20,22]. It has also been resurrected recently as a possible explanation for the spectral anomalies of the Fenna-Matthews structure [4]. However, both from the 1978 calculations and again from very recent ones (R.M. Pearlstein, unpublished results), it is quite clear that simultaneous inclusion in the... 

In the ten years prior to the publication of the first 3-D structure of a purple bacterial antenna complex, a considerable accumulation of biochemical and biophysical data was used to construct models of these complexes. Included in this work was the determination of the primary structures of a large number of antenna polypeptides from both LHl and LH2 (Zuber Brunisholz, 1991). Analyses of these primary structures showed that all purple bacterial antenna complexes are built on the same... 

Figure 7 Rogue s gallery of structures of peripheral anteima complexes. As labelled these include Chlorosomes from green sulfur bacteria, fused antenna domains of the Photosystem I core, the CP43 and CP47 proteins of Photosystem II, the Fenna-Matthew-Olson (FMO) protein associated with chlorosomes, LHI proteins surrounding a purple bacterial photo synthetic core, the peridinin-chlorophyll a protein of dinoflagellate algae, the LHCI and LHCII proteins found in plants and many algae, and the LHII protein complex that is associated with LHI in purple bacteria...

A typical purple bacterial photosynthetic unit consists of two types of antenna complexes, called LHl and LH2, which transfer absorbed solar radiation to the reaction center, where the light-energy is trapped and converted into useful chemical energy (for reviews see Zuber and Cogdell, 1995 Cogdell et al., 1996 Papiz et al., 1996 Hu et al., 1998). The LHl complexes surround the RC and form the so-called core complex. The LH2 complexes are arranged more peripherally around the core complexes (Fig. 1). 

The observations made here indicate that there are differences between PSII and the purple bacterial reaction centre as regards the triplet yield when is singly reduced. However an extra electron acceptor (1) is not nessecary to explain these differences. It is more likely that differences in antenna size and in the interaction between Q. and the radical pair are responsible. 

Initial attempts to separate the photochemical activity associated with PS II produced complexes which carried 100 or more antenna Chi molecules together with the reaction center (12-13). Recently, much more highly enriched complexes have become available (14), and the current stoichiometry of 4-5 Chi, 2 Pheo and 1 copy of each of the D1 and D2 polypeptides per reaction center is essentially the same as that of the purple bacterial reaction centers. Active PS II reaction centers also include two small polypeptides (ca. 5 and 9 kDa) associated with cytochrome b-559, although the role of this heme-protein is far from clear at present. (It does not appear to be directly involved in electron donation to the oxidized primary donor, by contrast with the situation for the cytochrome in the reaction center complexes of Rps. viridis.)... 

While the functional distinction between the two classes of antenna complex remains valid, it appears, from more recent biochemical analyses of a wider range of purple bacterial light-harvesting complexes, that structurally each of those two classes has at least two subdivisions (Table II). 

In purified preparations of antenna complexes obtained from several phototrophic purple bacteria, mixtures of metabolically related carotenoids are usually found (Cogdell and Thomber, 1979). The sole reported exception to this rule seems to be a B880 complex of Rhodo spirillum mbmm in which only spirilloxanthin was detected, although Duysens (in Cogdell and Thomber, 1979, pp. 77) suggested that its uniform carotenoid composition could be due to the use of old bacterial cultures as the source of the... 

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