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. Light harvesting (LH) complexes feed photons into the PSs. The structure of the pea PSI-LHCI complex of 600 KDa has been solved to 2.8 Å resolution showing pigments and co-factors (Qin et al. 2015).
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.
In photosynthetic membranes of cyanobacteria, algae, and higher plants, photosystem I (PSI) mediates light-driven transmembrane electron transfer from plastocyanin or cytochrome c6 to the ferredoxin-NADP complex. The oxidoreductase function of PSI is sensitized by a reversible photooxidation of primary electron donor P700, which launches a multistep electron transfer via a series of redox cofactors of the reaction center (RC) (Melkozernov et al. 2006). The excitation energy for the functioning of the primary electron donor in the RC is delivered via the chlorophyll core antenna in the complex with peripheral light-harvesting antennas. Supermolecular complexes of the PSI acquire remarkably different structural forms of the peripheral light-harvesting antenna complexes, including distinct pigment types and organizational principles. The PSI core antenna, being the main functional unit of the supercomplexes, provides an increased functional connectivity in the chlorophyll antenna network due to dense pigment packing resulting in a fast spread of the excitation among the neighbors. Functional connectivity within the network as well as the spectral overlap of antenna pigments allows equilibration of the excitation energy in the depth of the whole membrane within picoseconds and loss-free delivery of the excitation to primary donor P700 within 20-40 ps (Melkozernov et al. 2006). Low-light-adapted cyanobacteria under iron-deficiency conditions extend this capacity via assembly of efficiently energy coupled rings of CP43-like complexes around the PSI trimers. In green algae and higher plants, less efficient energy coupling in the eukaryotic PSI-LHCI supercomplexes is probably a result of the structural adaptation of the Chl a/b binding LHCI peripheral antenna that not only extends the absorption cross section of the PSI core but participates in regulation of excitation flows between the two photosystems as well as in photoprotection.
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. Chromatophores containing the RC and LH1 can interface with a conducting support to allow capture and conversion of solar energy as an alternative fuel (Harrold et al. 2013).
Reaction center-light harvesting 1 (RC-LH1) complexes are the fundamental units of bacterial photosynthesis, which use solar energy to power the reduction of quinone to quinol prior to the formation of the proton gradient that drives ATP synthesis. The dimeric RC-LH1-PufX complex of Rhodobacter sphaeroides is composed of 64 polypeptides and 128 cofactors, including 56 LH1 bacteriochlorophyll a (BChl a) molecules that surround and donate energy to the two RCs. The 3D structure of the complex has been determined to 8 Å by X-ray crystallography (Qian et al. 2013). Each half of the dimeric complex consists of a RC surrounded by an array of 14 LH1 αβ subunits, with two BChls sandwiched between each αβ pair of transmembrane helices. The N- and C-terminal extrinsic domains of PufX promote dimerization by interacting with the corresponding domains of an LH1 β polypeptide from the other half of the RC-LH1-PufX complex. Close contacts between PufX, an LH1 αβ subunit, and the cytoplasmic domain of the RC-H subunit prevent the LH1 complex from encircling the RC to create a channel connecting the RC QB site to an opening in the LH1 ring, allowing Q/QH2 exchange with the external quinone pool. Qian et al. 2013 also identified a channel that connects the two halves of the dimer, potentially forming a long-range pathway for quinone migration along rows of RC-LH1-PufX complexes in the membrane. The structure of the RC-LH1-PufX complex explains the crucial role played by PufX in dimer formation and shows how quinone traffic traverses the LH1 complex as it shuttles between the RC and the cytochrome bc1 complex.
Channels and pathways for the transfer of substrates (water, plastoquinone (PQ)) and products (electrons, protons, oxygen, reduced PQ (PQH2)) to and from the redox active catalytic sites of photosystem II (PSII) have been identified (Murray and Barber 2007). A putative oxygen channel is about 21 Å in length, leading from the water splitting site to the lumen. This channel follows a path along the lumenal surface of CP43, passing across the interface of the large extrinsic loop which joins the fifth and sixth transmembrane helices of this chlorophyll binding protein. In so doing it seems to minimise interactions with the excited states of chlorophylls bound within the PSII complex, especially those that constitute the primary electron donor, P680. Two additional channels leading from the water splitting site, and also exiting at the lumen, were also identified. Their hydrophilic nature suggests that they probably facilitate the delivery of water to, and protons from, the catalytic site (Murray and Barber 2007).
A series of integral membrane proteins of known structure and varying degrees of sequence identity have been compared. These proteins include photosynthetic reaction centers from proteobacteria and cyanobacterial photosystems I and II, as well as cytochrome oxidase, bacteriorhodopsin and cytochrome b. The reaction center complexes show conservation of the core structure of 5 transmembrane helices, strongly implying common ancestry, whereas the other proteins have structures that are unrelated (Sadekar et al. 2006).
The overall reaction is thus:
2H+ (in) + 2hν → 2H+ (out)