3.E.2 The Photosynthetic Reaction Center (PRC) Family
Photosynthesis converts solar energy to chemical energy by means of photosystem (PS)s I and II. They use the energy of absorbed photons to translocate electrons across the membrane. Similarly, bacterial reaction centers (RCs) together with the cytochrome b6f complex mediate the conversion of electromagnetic energy (light) into electrochemical energy (pmf) by transmembrane electron and proton transport during photosynthesis. In this process electron transfer to quinone is coupled to proton transfer. Both PSI and PSII of plants and cyanobacteria belong to the PRC superfamily, but they are more complex than the purple bacterial members. For example, cyanobacterial PSI exists as a trimer (3 x 360 kDa), and each monomer consists of at least eleven dissimilar protein subunits. These proteins coordinate more than 100 cofactors.
Only six bacterial phyla contain chlorophototrophs: Cyanobacteria, Chlorobi, Proteobacteria, Chloroflexi, Firmicutes and Acidobacteria (Bryant et al., 2006; 2007). In chlorophototrophs, light energy is transduced into chemical potential energy by reaction centers, photo-oxidoreductases that form two families of BChl/Chl-containing, pigment-protein complexes (Golbeck, 1993). Type 1 reaction centers include cyanobacterial Photosystem I and the homodimeric reaction centers of Chlorobi and heliobacteria (Firmicutes). Type 2 reaction centers include cyanobacterial Photosystem II and the reaction centers of Proteobacteria and Chloroflexi. Although their subunits are not discernibly similar in sequence, the two reaction-center types probably share a common evolutionary origin because their electron-transfer domains have similar structures and cofactor arrangements (Schubert et al., 1998).
In its photocycle, PSI captures light energy by a large internal antenna system and guides it to the core of the reaction center with high efficiency. After primary charge separation initiated by excitation of the chlorophyll dimer P700, the electron passes along the electron transfer chain (ETC) consisting of the spectroscopically identified cofactors A0 (Chla), A1 (phylloquinone) and the Fe4S4 clusters FX, FA and FB. At the stromal (cytoplasmic) side, the electron is donated by FB to ferredoxin (or flavodoxin) and then transferred to NADP+ reductase. The reaction cycle is completed by re-reduction of P700 by cytochrome c6 (or plastocyanin) at the inner (lumenal) side of the membrane. The electron carried by cytochrome c6 is provided by PSII by way of a pool of plastoquinones and the cytochrome b6/f complex.
Oxygenic photosynthesis in plants, algae and cyanobacteria is initiated at photosystem II, a homodimeric multisubunit protein-cofactor complex embedded in the thylakoid membrane. Photosystem II captures sunlight and powers the unique photo-induced oxidation of water to atmospheric oxygen. Crystallographic investigations of cyanobacterial photosystem II had provided several medium-resolution structures (3.8 to 3.2 Å) that explain the general arrangement of the protein matrix and cofactors, but do not give a full picture of the complex. A more complete cyanobacterial photosystem II structure has been obtained by Loll et al. (2005). It shows locations of and interactions between 20 protein subunits and 77 cofactors per monomer. Assignment of 11 β-carotenes yielded insights into electron and energy transfer and photo-protection mechanisms in the reaction centre and antenna subunits. The high number of 14 integrally bound lipids reflects the structural and functional importance of these molecules for flexibility within and assembly of photosystem II. A lipophilic pathway was proposed for the diffusion of secondary plastoquinone that transfers redox equivalents from photosystem II to the photosynthetic chain. The structure provides information about the Mn4Ca cluster, where oxidation of water takes place. These studies uncover near-atomic details necessary to understand the processes that convert light to chemical energy (Loll et al., 2005).
Bacterial RCs consist of three subunits (L, M and H) containing 5, 5 and 1 transmembrane α-helical spanners (TMSs), respectively. The L and M chains are homologous to each other and to the D1 and D2 proteins of plant photosystem II. The 3-dimensional structures of RC from Rhodopseudomonas viridis and Rhodobacter sphaeroides (Deisenhofer and Michel, 1989, 1991; Feher et al., 1992) as well as photosystem I of the cyanobacterium, Synechococcus elongatus (Jordan et al., 2001) have been solved. The cofactors in the former are (1) a bacteriochlorophyll dimer (D), two bacteriochlorophyll monomers (BA and BB), two bacteriopheophytins (φA and φB), two ubiquinones (QA and QB) and a non-heme ferrous iron. Light ejects an electron from D, and the electron is transferred across the membrane, preferentially along the A branch, in a series of steps via φA and QA to QB. After reduction of D+ by cytochrome c2, light ejects a second electron from D to produce the fully reduced QB2-.
The proton transfer events involve protonation of QB using protons from the aqueous solution on the cytoplasmic side of the membrane as QB is reduced by electrons derived from D. The two protons are proposed to be transferred through the hydrophobic domain of RC to QB by proton transfer via two pathways, the first involving Asp213 and Ser223 in the L subunit, the second involving Asp213 and Glu212 in the L subunit. QH2 then dissociates from RC, diffuses to the periplasmic side of the membrane, and is oxidized by the cytochrome bc1 (b6f) complex (TC #3.D.3) with the release of the two protons into the periplasm. In this overall process, protons are therefore transported across the membrane generating a pmf (Cogdell et al., 1999; Okamura and Feher, 1992; Miksovska et al., 1999).
Because proton transport is not mediated solely by the RC and depends on the functioning of the cytochrome bc complex as well as diffusible QH2, the RC is a 'partial' H+ transport system that initiates and provides the energy for H+ flux but does not by itself catalyze transmembrane H+ transport. For this reason, RC plus cytochrome b6f plus quinone comprises the multicomponent transport system.
The overall reaction is thus:
2H+ (in) + 2hν 2H+ (out).