Bacterial photosystems

All photosynthetic cells contain some form of photosystem. Photosynthetic bacteria, unlike cyanobacteria and eukaryotic phototrophs, have only one photosystem. Interestingly, bacterial photosystems resemble eukaryotic PSII more than PSI, even though photosynthetic bacteria lack Og-evolving capacity. 

Another method used to vary the AG° of the recombination reaction without chemical modification of the centers, consists of placing the system in an electric field whose orientation and intensity are well defined [141]. However, the energy level shifts induced by the field also change the electronic factors, so that the interpretation of the experimental results is not straightforward. Bixon and Jortner have proposed using electric field effects to elucidate the nature of the primary electron step in bacterial photosystems [142], a problem that will be discussed in Sect. 3.5. One basic difficulty encountered in this method is the evaluation of the internal field effectively seen by the redox centers in the membrane. 

Photosystem I (PS I) in the cyanobacterium Synechococcus elongatus is the first system of this type for which the structure has been solved in atomic detail. Although the bacterial photosystem differs slightly from the systems in higher plants, the structure provides valuable hints about the course of the light reactions in photosynthesis (see p. 128). The functioning of the photosystem is discussed in greater detail on p. 130. 

The bacterial photosystem functions without dioxygen production which simplifies the task at hand. Namely, electrons are obtained from more easily oxidized terminal electron donors such as H2S instead of water. Nonetheless, the basic design needed to transform solar energy into stored chemical energy is present. The protein subunits and cofactors that comprise the photosystem in purple bacteria, such as Rhodobacter (Rb.) sphaeroides and Rhodopseudomonas (Rps.) viridis,33 are shown schematically in Fig. 1 which is based on a crystal structure of this assembly.34... 

The structure and function of this bacterial photosystem reveals important principles for the design of artificial photosystems. First, the sensitizer needs to be posi tioned close to secondary acceptors and donors which themselves are spatially iso lated from each other such that photoexcitation leads to rapid spatial separation of the electron hole pair. Second, compartmentalization of the photosynthetic assembly is likely to be necessary so as to prevent wasteful back reactions. For water splitting, a system in which H2 and O2 are generated in separate compartments would have both safety and efficiency advantages. 

Photosystem I forms the second light-absorbing component in the Z scheme for green plants and algae and like PS II, the structure of the protein complexes has been determined by X ray crystallography.34,49 51 PS I is similar in function to that of the purple bacterial photosystem in that the oxidation potential generated is modest (P700VP700 at +0.5V), however, the primary function of this photosystem is to... 

The primary acceptor in PS II is a plastoquinone, PQ, as ascertained from optical absorbance difference spectroscopy [46], Until recently, the EPR spectrum of the semiquinone escaped observation, and only the advent of preparation methods for PS II subchloroplast particles made its recording possible. As surmised earlier, the spectrum of the intact acceptor [47] very much resembled the very broad qui-none-iron acceptor complex in purple bacteria, whereas in iron-depleted PS II particles the narrow spectrum typical of an immobilized semiquinone was found [48], As in the bacterial photosystem, flash-induced reduction of Q, of the second quinone, Qb, or of both resulted in somewhat different EPR spectra, indicative of structural changes that influence the magnetic interaction between the semiquinone and the non, and/or between the two semiquinones [49],... 

Both the cyclic and linear pathways of electron flow in the bacterial photosystem generate a proton-motive force. As... 

Electrons flow through PSII via the same carriers that are present in the bacterial photosystem. In contrast to the bacterial system, photochemically oxidized Peso in PSII Is regenerated to Peso by electrons derived from the splitting of H2O with evolution of O2 (see Figure 8-37, lefi). 

The proton-motive force generated by photoelectron transport In plant and bacterial photosystems Is augmented by operation of the Q cycle In c3Aochrome bf complexes associated with each of the photosystems. 

Summarize the common features of diverse photosynthetic reaction centers, including bacterial, photosystem It, and photosystem I. 

Bacterial and mitochondrial porins and ion channels Bacterial spore coat Bacterial photosystems (PS-I)... 

Computer power is now no longer a serious limitation for many applications in the area of small molecules. We have carried out studies of molecular modeling and energy calculations using a simplified model of the D1 binding niche, based on the X-ray analysis of the bacterial photosystem by Deisenhofer et al. (6) and using the inhibition ta obtained with the phenols. The results, however, have to be regarded as preliminary. 

The observed asymmetry of the spin density distribution in favor of Dl has been explained using a model that assumes an energetic difference between the dimer halves Dl and Dj of a magnitude comparable to that of the interdimer interaction energy. Significant shifts of spin density between Dl and Df have been observed, when RC s of different native bacteria and mutants were compared, indicating differences in the energetics of the primary donors in the different bacteria. The orbital asymmetry of the primary donor is obviously a common feature in many bacterial photosystems and may play a functional role for the unidirectional electron transfer in bacterial photosynthesis. 

---