Reaction centers of purple bacteria

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. 

Deisenhofer, J. Michel, H. (1991) Structures of bacterial photosynthetic reaction centers. Annu. Rev. Cell Biol. 7, 1-23. Description of the structure of the reaction center of purple bacteria and implications for the function of bacterial and plant reaction centers. 

There are many systems of different complexity ranging from diatomics to biomolecules (the sodium dimer, oxazine dye molecules, the reaction center of purple bacteria, the photoactive yellow protein, etc.) for which coherent oscillatory responses have been observed in the time and frequency gated (TFG) spontaneous emission (SE) spectra (see, e.g., [1] and references therein). In most cases, these oscillations are characterized by a single well-defined vibrational frequency, It is therefore logical to anticipate that a single optically active mode is responsible for these features, so that the description in terms of few-electronic-states-single-vibrational-mode system Hamiltonian may be appropriate. 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

Reaction centers of purple bacteria typically contain three polypeptides, four molecules of bacteriochlorophyll, two bacteriopheophytins, two quinones, and one nonheme iron atom. In some bacterial species, both quinones are ubiquinone. In others, one of the quinones is menaquinone (vitamin K2), a naphthoquinone that resembles ubiquinone in having a long side chain (fig. 15.10). Reaction centers of some species, such as Rhodopseudomonas viridis, also have a cytochrome subunit with four c-type hemes. 

Although the structures of the plant reaction centers are not yet known in detail, photosystem II reaction centers resemble reaction centers of purple bacteria in several ways. The amino acid sequences of their two major polypeptides are homologous to those of the two polypeptides that hold the pigments in the bacterial reaction center. Also, the reaction centers of photosystem II contain a nonheme iron atom and two molecules of plastoquinone, a quinone that is closely related to ubiquinone (see fig. 15.10), and they contain one or more molecules of pheophytin a and several... 

If the reaction centers of photosystem I and photosystem II are segregated into separate regions of the thylakoid membrane, how can electrons move from photosystem I to photosystem II Evidently the plastoquinone that is reduced in photosystem II can diffuse rapidly in the membrane, just as ubiquinone does in the mitochondrial inner membrane. Plastoquinone thus carries electrons from photosystem II to the cytochrome b6f complex. Plastocyanin acts similarly as a mobile electron carrier from the cytochrome b f complex to the reaction center of photosystem I, just as cytochrome c carries electrons from the mitochondrial cytochrome bct complex to cytochrome oxidase and as a c-type cytochrome provides electrons to the reaction centers of purple bacteria (see fig. 15.13). 

Hoff, A.J. and Deisenhofer, J. (1997) Photophysics of photosynthesis. Structure and spectroscopy of reaction centers of purple bacteria, Physics Reports 287, 1-247. 

Kolbasov,d. and Scherz, A. (2000) Asymetric electron transfer in reaction centers of purple bacteria strongly depends on different electron matrix elements in the active and inactive brunches, J. Phys. Chem. B 104, 1802-1809. 

C. R. D. Lancaster and H. Michel, in Photosynthetic Reaction Centers of Purple Bacteria, in Handbook of MetaUoproteins , eds. A. Messerschmidt, R. Huber, T. Poulos, and K. Wieghardt, John Wiley Sons, New York, 2001, p. 119. 

It should be mentioned that electron transfer to the quinone pool, both by PS II and by the reaction centers of purple bacteria, now proceeds via a two-electron gating mechanism after one electron has arrived at the temporarily bound quinone Qb, the semiquinone remains in its unprotonated, negatively charged form and tightly bound to the reaction center only after a second photoreaction does its full reduction, protonation and release as a quinol take place [19]. This procedure may also have played a role in the selection of the dimeric reaction center structure, but its importance most likely has to do with the reactivity of semiqui-nones with molecular oxygen and in that case it probably appeared much later. 

The resemblance between PS II and the reaction centers of purple bacteria and filamentous green bacteria has been established by extensive biochemical and spectroscopic studies. In addition to the analogous electron carriers (bacterio)pheophytin and quinones present in purple bacteria and PS II, their reaction-center-core protein subunits, the L and M subunits in purple bacteria and the DI and D2 subunits in PS II, show significant amino-acid sequence homologies. 

The reaction center of purple bacteria contains three protein subunits (L, M, and H) located in the plasma membrane (Figure 8-35). Bound to these proteins are the prosthetic groups that absorb light and transport electrons during photosynthesis. The prosthetic groups include a special pair of bacteriochlorophyll a molecules equivalent to the reaction-center chlorophyll a molecules in plants, as well as several other pigments and two quinones, termed Qa and Qb, that are structurally similar to mitochondrial ubiquinone. 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Bkodopseudomoms viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33, 28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution " (Fig. 

The photosystems of green plants and photosynthetic bacteria appear to function with basically the same sort of mechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, but the molecular constituents of the reaction center are quite diflFerent. Photosystem I contains iron-sulphur proteins as electron acceptors so can be called iron-sulphur (FeS) type reaction center, while photosystem II contains pheophytin as the primary electron acceptor and quinone as the secondary acceptors so it can be called pheophytin-quinone (4>-Q) type . The reaction center of purple bacteria, green nonsulphur bacteria, and PSII are (4>-Q) type. Green sulphur bacteria, heliobacteria, and PSI have (FeS) type reaction centers. ... 

When a chromophore is excited, the absorption spectrum of a nearby chromophore may also be influenced owing to an interaction with the excited molecule, which is observed, for instance, in the photosynthetic bacterial reaction center of purple bacteria. In the same complex the... 

Several laboratories now make use of the ultrafast continuum pump-probe technique in the study of ultrafast processes in biological molecules or molecular complexes.Notable molecules and complexes under study are the photosynthetic reaction centers of purple bacteria, the reaction centers of photosystems I and II of green plants. 

Another important feature of the electron transport chain is that the macromolecular protein complexes responsible for electron transfer along the electron transport chain have a dynamic structure. That was clearly shown, for example, in the investigation of electron transfer in the reaction centers of purple bacteria Rhodospirileum rubrum in oxidizing conditions.6... 

Compared to the structure of the reaction center of purple bacteria, the knowledge about Photosystem II is still in a very early stage. As a first step in the unravelling of the structure of PS II, we report in this communication the preparation of two-dimensional crystals of the PS II reaction center material comprised of CP47-Dl-D2-cytochrome b559, and of the subsequent analysis of these crystals by electron microscopy and image reconstruction techniques. 

Oxygenic photosynthesis takes place in two photochemical reaction centers Photosystem I (PSI) and Photosystem II (PSII). While PSn is believed to be closely related to the reaction center of purple bacteria (1), PSI spears to be unique to oxygenic photosynthetic organisms. It is clear that this photosynthetic complex has no structural or functional similarities to the bacterial reaction center, nevertheless it plays a major role in the oxygenic photosynthetic process. Its function in the reducing site of the electron transfer chain enables the reduction of ferredoxin, and eventually the reduction of one of the energy components formed in the process, NADPH. 

Delayed fluorescence also can report on the energies and dynamics of metastable states that are created photochemically by electron transfer or other processes. In photosynthetic reaction centers of purple bacteria or plant photosystem 11, the amplitude of delayed fluorescence from an early ion-pair state decreases on picosecond and nanosecond time scales, while the population of the state remains essentially constant [290-293]. Both structural heterogeneity and relaxations of the protein around the ion-pair probably contribute to the complex time dependence of the delayed fluorescence. 

Analysis of Transient Absorption Data from Reaction Centers of Purple Bacteria... 

The kinetics of primary processes occuring in the photosynthetic reaction center (RC) has been studied so far almost exclusively by transient absorption spectroscopy in the femto- to nanosecond time range. Most of these studies were carried out furthermore on the reaction centers of purple bacteria like, e.g., R sphaeroides and Rs. rubrum (for a review see (1)). Despite a large number of transient absorption studies being carried out over the last decade, no agreement has been reached as to the nature of the primary processes and the identity of the intermediates. At present in particular two topics are highly controversial i) Is the accessory bacteriochlorophyll (BChl) a anion an intermediate in the electron transfer process (2), and ii) are... 

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