Reaction center proteins design

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... 

Photosynthetic antenna and reaction center proteins convert light energy, normally from sun, into chemical energy that can be later released to fuel organisms activities. Research on artificial photosynthesis is important both fundamentally and for practical applications [42-44]. A critical consideration in designing integrated artificial photosynthetic systems is the ability to create large ordered arrays of... 

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). 

Photosystem II (PS II) of higher plant possess unique properties with respect to function, organization and protein turnover. It is a multisubunit protein complex which Is composed of at least 20 different polypeptides (1). The two reaction center polypeptides, designated D1 and D2, appear to carry all the redox components necessary for the primary photochemistry of PS II (2) and possibly also the Mn (3). The great majority of the PS II units is located in the appres-sed thylakold regions in association with its chlorophyll a/b antenna (4). PS II has a central catalytic role, but It also plays a central role In the long and short term acclimation of the photosynthetic apparatus. It Is also the target for the photoinhibition process which leads to Impaired electron transport capacity and the subsequent breakdown of the two reaction center subunits. In particular the Dl-protein. 

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. 

Photosystem I contains three iron-sulfur clusters firmly associated with the reaction center. These are designated Fe-Sx, Fe-SA, and Fe-SB in figure 15.17. The cysteines of Fe-Sx are provided by the two main polypeptides of the reaction center, which also bind P700 and its initial electron acceptors Fe-SA and Fe-SB are on a separate polypeptide. The quinone that is reduced in photosystem I probably transfers an electron to Fe-Sx, which in turn reduces Fe-SA and Fe-SB. From here, electrons move to ferredoxin, a soluble iron-sulfur protein found in the chloroplast stroma, then to a flavoprotein (ferredoxin-NADP oxidoreductase), and finally to NADP+. 

Chains of redox cofactors for long range electron transfer are clearly the way electrons are transferred over the tens of angstroms dimensions of membranes and their proteins. Once again, purple photosynthetic bacterial reaction centers provide an archetype for understanding electron transfer chain design and behavior. The heme chain in Rps. viridis... 

Chains also contribute to the robustness of natural electron transfer protein design because the close spacing between successive redox centers means that the driving force of the reaction can usually vary widely with relatively little effect on the overall electron transfer rate through the protein. Indeed, many naturally occurring chains have uphill electron transfer steps of hundreds of meV. 

The most basic design requirements of the bacterial photosynthetic reaction centers then, are a light energy absorbing center and two chains of redox centers that connect this light-activatable center to cytochrome c and quinone on opposite sides of the membrane. The physical process by which electrons are transferred between members of the chains in reaction centers, and indeed in the vast majority of electron transfer proteins, was also revealed by Chance, together with Devault, in these same years [4]. 

As shown in both Fig. 21 (A) and (B), there are five major protein complexes in the thylakoid-mem-brane network (1) the photosystem-11 core with bound inner antennae (collectively designated as PS II ), (2) the photosystem-I core with bound anteimae LHC I (collectively designated as PS I ), (3) the cytochrome (jg/complex, (4) theCFo CF, ATP-synthase and, finally (5) a separate, peripheral lightharvesting, chlorophyll-protein complex called LHC 11 that supplements the inner antennae bound to the photosystem-II reaction center. ... 

The chemical composition of the highly purified reaction-center complex from several photosynthetic bacteria is now well established. In addition to protein subunits designated as L (light), M (medium) and H (heavy), some reaction centers also contain a c-type cytochrome (C) subunit. The chemical composition of reaction-center complexes of several of these bacteria is shown in Table 1 ... 

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