The reaction centers of photosynthetic bacteria

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. 

The location of the complex in respect with the membrane bilayer has been studied either with hydrophobic and hydrophilic probes, or with monospecific antibodies against single subunits. In general these studies have shown that both the M and H subunits are accessible on both sides of the membrane to hydrophylic reagents, while the L subunit can be labelled only on the periplasmic side [9]. On the other hand, labelhng with the hydrophobic marker 5-iodonaphthyl-l-azide, showed that the most heavily labelled, and therefore the most extensively exposed to an hydrophobic environment, are the L and M subunits [10,11]. 

The exact location of the functional molecules in these subunits is still rather uncertain. The association of one ubiquinone molecule with the M subunit has been demonstrated by covalent linkage with the photoaffinity-reactive quinone analogue 2-azido-anthraquinone [12]. Similarly the association of the second quinone molecule 

Following the scheme of Fig. 4.2, the primary photochemical reaction within the reaction center complex will promote the transfer of one electron from the primary 

most of the data collected on bacterial RC seem to agree on the presence of a bacteriochlorophyll pair which can be oxidized upon illumination. Thermodynamically this donor has been characterized by studying the dependence of photooxidation from the ambient redox potential in Rp. sphaeroides the E -j was found to be 0.44 V and pH independent [14]. 

---

Vos M H, Jones M R, Hunter C N, Breton J, Lambry J C and Martin J L 1996 Femtosecond spectroscopy and vibrational coherence of membrane-bound RCs of Rhodobacfe/ sp/raero/des genetically modified at positions M210 and LI 81 The Reaction Center of Photosynthetic Bacteria—Structure and Dynamics ed M E Michel-Beyerle (Berlin Springer) pp 271-80... 

Koyama, Y., Kito, M., Takii, T., Saili, K., Tsukida, K., and Yamashita, J. 1983. Configuration of the carotenoid in the reaction centers of photosynthetic bacteria. 2. Comparison of the resonance Raman lines of the reaction centers with those of the 14 different cis-trans isomers of (i-carotene. Photobiochem. Photobiophys. 5 139-150. 

G. Hartwich, H. Lossau, A. Ogrodnik, and M. E. Michel-Beyerle, in The Reaction Center of Photosynthetic Bacteria, M. E. Michel-Beyerle, ed., Springer-Verlag, Berlin, 1996, pp. 199. 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

M. E. Michel-Beyerle, The Reaction Center of Photosynthetic Bacteria Structure and Dynamics, Springer, Berlin, 1996. 

The nonheme iron enzymes discussed so far in this section either utilize oxygen as a substrate or form it as a product. Other nonheme iron sites that do not bind O2 as part of their catalytic function have similar ligand environments. An example of such a system is the QFe site associated with the reaction centers of photosynthetic bacteria and with photosystem II of chloroplasts (Feher et al., 1989). 

Turzo, K., Laczko, G., Filus, Z., and Maroti, P. (2000) Quinone-dependent delayed fluorescence from the reaction center of photosynthetic bacteria, Biophys. J. 79, 14-25. 

These observations led to the prediction that accessory carotenoid pigments would be found in van der Waals contact with bacteriochlorophylls in the reaction centers of photosynthetic bacteria [58]. Indeed, the crystal structure of wild-type Rb. sphaeroides clearly shows spheroidene to be in contact with the adjacent monomer bacteriochlorophyll (Figure 1) [8]. 

The three-dimensional structure of the reaction center of photosynthetic bacteria has been known for well over a decade now [see Chapter 3], but structural information on the photosystem-1 reaction center is still preliminary, although some tentative but important information regarding the distances and orientation between the various electron-transport cofactors and the placement of the protein helices is now available. 

Lancaster, C.R.D. Michel, R In The Reaction Center of Photosynthetic Bacteria - Structure and Dynamics, Michel-BeyCTle, M.E., Ed. Springer Berlin, 1996 pp23-35. 

Zinth W, Arlt T, Schmidt S., Penzkofer H, Wachtveitl J, Huber H, Nagele T, Hamm P, Bibikova M, Oesterhelt D, Meyer M and Scheer H (1996) The first femtoseconds of primary photosynthesis—the process of the initial electron transfer reaction. In Michel-Beyerle ME (ed) The Reaction Center of Photosynthetic Bacteria, pp 160-173. Springer-Velag, Berlin... 

The reaction center of photosynthetic bacteria can be easily isolated and it is particulariy stable. gainst denaturation. Therefore, its photosynthedc proteins are suitable for realization of devices, promoting a li t-induced electron transfer across lipid membranes. ... 

Michel-Beyerle ME (eds) (1996) The reaction center of photosynthetic bacteria. Springer, Berlin Heidelberg New York... 

Figure 40 The charge separation within the iecial pair and successive electron transfers in the reaction center of photosynthetic bacteria. (BChl)2, bacteriochl< -q)hyll dimer, BChl, bacteriochlorophyll monomer QA, QB, ubiquinones [424].

The photophysical and electron transfer properties of bacteriochlorophylls (Bchl) and bacteriopheophytins (Bpheo) found in the reaction centers of photosynthetic bacteria have been directly associated with the mechanism of charge separation which underlies photosynthesis [1]. The appearance of the Bpheo anion (Bpheo ) within 3-5 ps after excitation of the special pair of Bchl (P) is well documented from transient absorption spectroscopy [2-4]. The 200 ps lifetime of Bpheo which is primarily determined by the electron transfer process to a quinone also has been established by picosecond changes in absorption [5,6], Thus, the general kinetic time scale for the primary processes in bacterial photosynthesis has been determined by the transient differences in electronic state properties. 

---