Reaction centers proteins, structure

Figure 3. The bacterial reaction-center protein model from Rhodopseudomonas sphaeroides the structure and positioning of components are highly speculative.

The primary photochemical reaction is an electron transfer reaction which occurs within a highly structured reaction-center protein which spans the thylakoid membrane. 

Very few integral membrane proteins have been crystallized. The reaction-center proteins purified from membranes of photosynthetic bacteria are a notable exception. These proteins were discussed in chapter 15. Before their crystal structures were elucidated, analysis of hydropathy plots suggested that each of the two main protein subunits is folded into five transmembrane a helices, and one such helix was predicted to occur in another subunit. The crystal structures provided a beautiful confirmation of these predictions (see fig. 15.11a). Successful crystallization of the reaction-center proteins was achieved by including small,... 

Wamcke, K., and Dutton, P. L., 1993, Influence of QA site redox cofactor structure on equilibrium binding, in situ electrochemistry, and electron transfer performance in the photosynthetic reaction center protein Biochemistry 32 4769n4779. 

As is apparent in Fig. 3, considerable similarity exists in the arrangement of the electron transfer cofactors in PS I and PS n. The main differences between the two systems are as follows 1) PS I has three Pe4S4 iron-sulfur clusters. Ex, Ea, and Eb, located on the stromal side of the complex 2) In PS I the primary acceptor is a chlorophyll, not pheophytin and 3) the distance between the primary acceptor (Aqa3 ) and phylloquinone (Aia,b) in PS I is significantly shorter than the corresponding distance between PheoA,B and Qa.b in PS II and Type II reaction centers. These structural differences correlate with functional differences between the two types of reaction centers. In PS II, the mobile electron carrier on the stromal side of the complex is Qb, which is a lipid-soluble, two-electron acceptor. In contrast, the mobile electron carrier in PS I is ferredoxin, which is a water-soluble, one-electron acceptor. The three iron-sulfur clusters in PS I provide a chaimel by which electrons are funneled out of the reaction center to ferredoxin. On the donor side of the complex, plastocyanin, the reductant that replenishes electrons removed from P700, is also a water-soluble protein and is a one-electron donor. Thus, each photon absorbed by the PS I complex leads to the transfer of one electron from plastocyanin to ferredoxin. In Fig. 2, it is apparent that the midpoint potentials of the acceptors in PS I are about 500 to 700 mV more negative than those in PS II, and the... 

Even where structures of quinone-protein complexes are available from X-ray diffraction experiments, the structures, side-chain conformations, and intermolecular contacts with proteins for the corresponding quinoidal radicals must usually be inferred indirectly from spectroscopic data. The primary spectroscopic methods used to infer structures of quinoidal radicals in photosynthetic reaction center proteins are designed to probe molecular vibrations and spin properties. Directly measurable quantities that are also... 

Proton transfer is closely linked to the structure of the reaction-center protein. Since protons are present in the external aqueous medium, the (reduced) quinone molecules are buried inside the interior ofthe reaction-center protein, therefore protonation would seem to require some kind of channel for the passage of water molecules. However, at least until recently (see below), there was no evidence for the presence of channels large enough to accommodate water molecules. An alternative mechanism might involve a chain of ionizable amino acids which extends from the surface of the protein to the interior where the reduced quinone is located, forming a pathway along which protons may be transported. Such a mechanism has been likened to a bucket brigade or relay station and shown to exist in such proteins as bacteriorhodopsin, ATP synthase and cytochrome oxidase. 

P Beroza, DR Fredkin, MY Okamura and G Feher (1992) Proton transferpathways in the reaction center of Rhodobacter sphaeroldes A computational study. In J Breton and A Vermeglio (eds) The Photosynthetic Bacterial Reaction Center II Structure, Function and Dynamics, pp 363-374. Plenum U Ermler, G Fritzsch, SK Buchanan and H Michel (1994) Structure of the photosynthetic reaction centre from Rhodobacter sphaeroldes at 2.65 resolution cofactors and protein-cofactor Interactions. Structure 2 925-936... 

As seen earlier in Chapter 2 on bacterial reaction centers, crystallization of the reaction-center protein of the photosynthetic h iCttn xm Rhodopseudomonas viridis by Michel in 1982 and subsequent determination ofthe three-dimensional structure ofthe reaction center by Deisenhofer, Epp, Miki, Huber and Michel in 1984 led to tremendous advances in the understanding ofthe structure-function relationship in bacterial photosynthesis. Furthermore, because of certain similarities between the photochemical behavior of the components of some photosynthetic bacteria and that of photosystem II, research in photosystem-II was greatly stimulated to its benefit by these advances. In this way, it became obvious that the ability to prepare crystals from the reaction-center complexes of photosystems I and II would be of great importance. However, it was also recognized that, compared with the bacterial reaction center, the PS-I reaction center is more complex, consisting of many more protein subunits and electron carriers, not to mention the greater number of core-antenna chlorophyll molecules. 

As summarized in Table IV, the evidence thus far obtained with various glycosidases, phosphorylases, and pyrophosphorylases indicates that retaining glycosylases would probably be more precisely characterized as proteins with a 1-MCO type of structure, which restricts both substrates and cosubstrates to one mode of orientation to the reaction center. The structurally directed approach of all incoming cosubstrates, either always from the a- or always from the /3-side of the center, can account for the constancy... 

The reaction center protein (RC) stores the energy of a photon as reduced quinone and oxidized cytochrome . The protein is designed so that ov r 98% of the photons absorbed yieid product, but these products store less than 500 meV of the 1,400 meV initially absorbed. As electron transfer rates are controlled by both the distance between donors and acceptors and the free energy difference between the reactants and products, the protein must control both factors to achieve the near unity quantum yield (1-5). The positions of the cofactors relative to each other are now well known thanks to the solution of the x-ray crystal structure of RCs from Rp. viridis (6, 7) and from Rb. sphaeroides (8, 9). This information has allowed consideration of the effects of the distance between and orientation of donor and acceptor on the electron transfer rates (10, 11). Knowledge of the structure also allows exploration of the interaction of cofactors with the protein that determine the reaction free energy (12). 

CD spectra of borohydride-treated and native Rb, sphaeroides R26 reaction centers are presented in Fig. 4. For comparison, the CD spectra of Rb, sphaeroides wild type strain 2.4.1 reaction centers are presented in Fig. 5. Because CD is a sensitive probe of the structure and environment of bound chromophores, it provides an opportunity to examine whether or not spheroidene is bound in the same manner for all the complexes. Furthermore, it is known that carotenoids are not optically active unless physically bound to the reaction center protein [8]. This is shown in Fig. 5 which demonstrates that spheroidene, incorporated into Triton X-100 micelles at a concentration equal to that of spheroidene bound to the reaction centers, does not display a CD spectrum. Upon binding to the reaction center protein, spheroidene becomes optically active, presumably either through exciton interactions with the amino acid residues in proximity to the chromophore or by an asymmetry in the carotenoid configuration or conformation, and displays a pronoimced CD spectrum. 

The relation of the structure and organization of the Photosystem II reaction centers to those from Photosystem I or from the green or purple bacteria presents an interesting example of comparative biochemistry. Similarities between PS II and purple bacterial reaction centers include aspects of the reaction center proteins, the stoichiometry of chlorophyll and pheophytin in the reaction center and the complex of iron with quinones as the primary electron acceptor. In each of these respects the reaction centers of PS I or green bacteria, however, have no obvious similarity. 

Effect of Cofactor Structure on Control of Electron Transfer Rates at the Site of the Reaction Center Protein... 

The bacterial photosynthetic reaction center protein (RC) catalyzes the conversion of light to electrochemical energy through a sequence of photon-initiated electron transfer reactions between redox cofactors held at fixed distances in the protein [1-4]. The primary processes are elicited by absorption of a photon by the primary donor, a dimer of bacteriochlorophyll ([BChIjg). The first excited singlet state of the dimer, [BChljg, transfers an electron over 10 A in 3 ps to a bacteriopheophytin (BPh). The BPh" in turn reduces a quinone bound at the site to form an anionic semiquinone in 0.2 ns (for a review, see [5]). Numerous experimental efforts have aimed to identify the factors which control the remarkable near unit quantum yield and temperature independence of the RC processes [6-9], often with an eye toward emulation in artificial photosynthetic devices [10]. Here, we examine the role of structural components of the quinone cofactor in determining electron transfer rates at the RC Q/y site. 

This is precisely the situation that occurs for radical pairs produced in photosynthetic organisms. Since the spin-polarized EPR signal from 2-TAPD -ZP-l-NQ, Figure 9, closely resembles those from photosynthetic organisms, [24,25] and since we know the structure of this molecule, the magnitudes of the spin-spin interactions and the relative orientations of the donors and acceptors in the proteins should be similar to those in 2-TAPD -ZP-l-NQ. Further comparisons of the EPR signals from our supramolecular arrays with those fi om the donor-acceptor arrays within photosynthetic reaction center proteins for which no x-ray structures are known will yield information concerning the distances and orientations of the donors and acceptors within these proteins. 

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