Photosynthetic complexes

Light-driven electron transfer in plant chloroplasts during photosynthesis is accomplished by multienzyme systems in the thylakoid membrane. Our current picture of photosynthetic mechanisms is a composite, drawn from studies of plant chloroplasts and a variety of bacteria and algae. Determination of the molecular structures of bacterial photosynthetic complexes (by x-ray crystallography) has given us a much improved understanding of the molecular events in photosynthesis in general. 

We are investigating further the nature of the carotenoids present in the bacterial photosynthetic complexes. In this report we shall present evidence showing that whereas the R. rubrum antenna contains a mixture of carotenoids the composition of which changes with the nutritional state of the culture, only spirilloxanthin and, in minor amounts, monodemethylated spirilloxanthin are found in association with the R. rubrum reaction center. The current progress of our work on the functional bases of the differences in carotenoid composition between the antenna and the reaction center shall also be presented. 

In order to investigate whether the carotenoid composition of a culture has any influence on the nature of the pigments which are inserted in the photosynthetic complexes, these were solubilized and purified from several cultures, and their visible absorption spectra were compared with those of the corresponding native membranes. The results are summarized in Table 1, which shows the location of the carotenoid central peak in the spectra of some of the preparations. 

The results of this study demonstrate that the antenna and the reaction center of R rubrum differ in then-specificities of carotenoid binding. Thus, the microorganism follows in this respect the pattern of other related phototrophic bacteria (Cogdell and Thomber, 1979 Cogdell et al., 1976). Such difference suggests strongly that the functional role of the carotenoid in each type of photosynthetic complex has differential aspects of importance sufficient to impose distinctive structural requirements. The available information on... 

The esterifying alcohol, in most cases phytol (Fig. 1), comprises about 1/3 of the mass of Chls, yet its influence on the chemistry (and function) is still poorly understood. With the exception of the nonesterified Chls c, it renders Chls amphotoeric, and is important both in aggregation in polar environments, and in the positioning of Chls in photosynthetic complexes. Variations of the alcohol are frequent in phototrophic bacteria, in particular in the chlorosomes, where they contribute to formation of the fluorescent BChl c, d, and e aggregates that are unique for these pigments (16). 

The overall process of photosynthetic electron transfer is promoted by an array of catalytic proteins, only a few of which are real photochemical enzymes. It is now realized that these proteins form a number of well-defined complexes, partially independent from each other, but nevertheless interacting through redox carriers, freely diffusable either in the membrane lipids or at the membrane-water interface. The concept of membrane photosynthetic complex is experimentally justified by the possibility of isolating specific multiprotein associations following micellization of the membrane with mild detergents. In general these associations are characterized by well-defined catalytic activities, which are lost, however, if the complex is dissociated into the individual polypeptides by more drastic detergent treatments. 

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

Since the first isolation of a reaction center preparation from the membrane of a facultative photosynthetic bacterium [6] our knowledge on the structure and function of these complexes has made great advances. Today the RC from purple bacteria, and particularly from the carotenoid-less strain R26 of Rhodopseudomonas sphaeroides, are by far the best known examples of photosynthetic complexes studied. Other RC from different bacteria species have also been studied and differences in components sometimes observed these differences will be mentioned below, whenever necessary, while discussing the properties of the preparations from Rp. sphaeroides R26. 

Fig. 18. Freeze-fracture electron micrography of thylakoid membrane. (A) A portion of the chioroplast thylakoids (B top) a schematic view of the stacked region of thylakoids frozen in freon at liquid-nitrogen temperature ("freeze etch") and (B bottom) after fracture along the thick dashed line by the impact of a microtome knife [freeze fracture] (C) an electron micrograph of a replica of the EF and PF faces such as those shown in (B) bottom (D) distribution of the four photosynthetic complexes in the various fracture faces. (A) kindly furnished by Dr. Andrew Staehelin Source for (B) and (C) Miller (1978) The photosynthetic membrane. SciAm241 107.

Freeze-fracture electron microscopy of thylakoid membranes has clearly revealed an asymmetric lateral distribution of the various photosynthetic complexes in the granal and stromal membranes, i.e., the distribution of the protein complexes in the membrane is nonrandom. This lateral asymmetry was further substantiated by the results of electron microscopy of the inside-out vesicles discussed in Section Vll. These findings by electron microscopy are summarized by the model shown in Fig. 21 (A). It is a transverse cross section of the thylakoids shown earlier in Fig. 13 (D) and (D ), with the various photosynthetic protein complexes appropriately placed in the granal and stromal regions. 

Judging from available experimental results as reported so far, one can expect STM/STS to be a potentially powerful tool for further applications. The high resolution of STM should in general be able to yield single-molecule images of photosynthetic complexes as has already been shown here. Further... 

Resonance Raman (RR) spectroscopy yields detailed information about the structure and ground-state environmental interactions assumed by the chlorin pigments within photosynthetic complexes (4-5). We have studied photosystem II RCs using this method and we report here the selective observation of the RR contributions of the acceptor pheophytin (Pheo) and of the chlorophyll that reach the triplet state within these particles. 

A Method for Studying Pigment Organization in Photosynthetic Complexes... 

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. 

All crystal forms were also tested for typical photochemical properties characteristic for PS-I-RC. Here again it is important to show that in the crystals, all the cofactors retain both their composition and function so that the complex will have a similar photochemical activity in the crystalline form as in the soluble form. When it comes to photosynthetic complexes, two major parameters regarding the unique photochemical property need to be proven a. that indeed the pigmental content was preserved in the crystals, and b. that the crystals have the ability to carry out the photochemistry, i.e., the charge separation and electron transfer reactions. 

The goal of much current research is to understand at the molecular level the mechanisms by which O2 tension regulates the induction of the photosynthetic apparatus. One of the mechanisms operative in R. capsulatus consists of an increase in the frequency of transcription initiation of genes encoding peptides of the photosynthetic complexes, and of genes encoding carotenoid and bchl biosynthetic enzymes (5,6,7,8,9,10). 

Shaul Mukamel, who is currently the C. E. Kenneth Mees Professor of Chemistry at the University of Rochester, received his Ph.D. in 1976 from Tel Aviv University, follot by postdoctoral appointments at MIT and the University of California at Berkeley and faculty positions at the Weizmann Institute and at Rice University. He has b n the recipient of the Sloan, Dreyfus, Guggenheim, and Alexander von Humboldt Senior Scientist awards. His research interests in theoretical chemical physics and biophysics include developing a density matrix Liouville-space approach to femtosecond spectroscopy and to many body theory of electronic and vibrational excitations of molecules and semiconductors multidimensional coherent spectroscopies of sbucture and folding dynamics of proteins nonlinear X-ray and single molecule spectroscopy electron transfer and energy ftrnneling in photosynthetic complexes and Dendrimers. He is the author of over 400 publications in scientific journals and of the textbook. Principles of Nonlinear OfMical Spectroscopy (Oxford University Press), 1995. 

Panitchayangkoon G, Hayes D, Fransted KA, Caram JR, Harel E, Wen J, Blankenship RE, Engel GS (2010) Ixmg-lived quantum coherence in photosynthetic complexes at physiological temperature. Proc Natl Acad Sci USA 107 12766-12770... 

Gloeobacter violaceus is a unicellular cyanobacteria that besides the outer wall layer only has one membrane the cell membrane. It performs oxygenic photosynthesis and the photosynthetic complexes are localised in the cell membrane (Rippka et al, 1974). Long phycobilisomes are oriented perpendicular to the cell membrane and the phycobiliproteins carry phycoerythrobilin, phycourobilin, phycocyanin and allophycocyanin (Guglielmi et al, 1981, Bryant era/., 1981)... 

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