Photosystem II
Created by Austin Looney
The light-harvesting reactions of photosynthesis form the basis of all life on Earth with very few exceptions (1). These principle reactions occur in Photosystem II (PSII, PDB ID = 1S5L) and result in the energetically demanding oxidation of water to molecular oxygen, hydrogen ions, and free electrons. The corresponding half reaction for the oxidation is as follows:
2H2O(l) --> O2(g) + 4H+(aq) + 4e- Eox°= -1.23 V (2).
The exact mechanism is not entirely understood, but relies on a number of cofactors that shuttle electrons around the reaction center (RC) proteins, from the
oxygen evolving center (OEC), a Mn3CaO4 cluster where the oxidation occurs. The PSII RC of Thermosynechococcus elongates contains two subunits,
D1 and D2, that accommodate these cofactors and the
OEC with
specific binding configurations and electronic environments (2, 3, 4). As determined by the ExPASy Proteomics Server, the D1 (38.24 kDa) and D2 (39.36 kDa) subunits have pIs of 5.72 and 5.74 respectively (11); the excess of negatively charged acidic residues in these subunits satisfies the liganding requirements of the RC metal ions. In particular, the OEC ligands both stabilize intermediates in the O=O bond formation and accommodate the various S state transitions that occur throughout the oxidation (2). Additionally, the structure contains channels to allow certain molecules like water, bicarbonate, and oxygen access to and from the RC, as well as a protonation route for the H+ ions formed as a byproduct of the reaction (4).
The ligands that participate in the oxygen-forming reaction are housed in the two reaction center (RC) subunits of PSII, D1 (or Q(B) protein) and D2 (7). The whole protein derives stability from the maintenance of hydrophilic residues on the outside of trans-membrane spans and on cytosolic faces, and nonpolar/hydrophobic residues within the membrane-intrinsic regions (4). All five of the main RC alpha-helices tilt against the membrane plane (7), and lipids occupy several cavities around the RC. The high lipid content allows for flexibility and motion, and facilitates the replacement of the D1 subunit, which can suffer oxidative damage from the T state P680*+ chlorophyll radical pair. These lipids share structural similarities with those found in the thylakoid membrane, the location of PSII (8). In vivo, PSII exists as a homodimer, with each
monomer containing nineteen subunits. Four large subunits (D1, D2, CP43, and CP47) comprise the monomer cores (2), and dimerisation occurs only between the PsbM subunits of each dimer through energetically driven
hydrophobic interactions , which occur more or less down a straight
axis of contact. The association is energetically favorable, as such a large concentration of hydrophobic residues would demand a high degree of order from surrounding polar solvent molecules, prior to incorporation in the thylakoid membrane. Additionally, the monomer-monomer interface contains several prosthetic groups and lipids (9).
Helices predominate in the membrane-intrinsic region, while the luminal and cytosolic faces exhibit mostly random coil; the luminal face has one subunit composed of beta-sheets (2, 7, 8, 9). A Ramachandran plot clearly demonstrates the structural predominance of alpha-helices (see Image 1), most of which serve the structural purpose of spanning the hydrophobic thylakoid membrane. Accordingly, the exposed residues in this region tend toward
hydrophobicity.
The photo-oxidation reaction begins with the
alpha-chlorophyll A special pair, termed P680. Each RC subunit
coordinates one of the chlorophylls at its central magnesium (II) ion via two highly conserved histidine residues,
D1 H-198 and D2 H-197. The RC proteins of all photosynthetic plants, algae, and cyanobacteria exhibit these specific residues, and mutation of either confers loss of RC activity. Mostly hydrophobic residues comprise the rest of the P680 binding pocket, and accommodate the ring structures of the chlorophyll pair (4). Upon illumination P680 undergoes charge separation and ejects an electron to form the cationic radical P680*+ (7); the charge separation occurs across four chlorophylls: P680 and two accessory chlorophylls, ChlD1 and ChlD2, on either side of the pair. This radical has an oxidising potential of about 1.3-1.4 V, enough to drive the energetically demanding oxidation of water (2).
Ejected electrons flow from the chlorophyll pair around the RC towards the stromal surface via ChlD1, redox-active pheophytin (PheoD1), plastoquinone A (QA), and past the bicarbonate-coordinated non-heme Fe ion to plastoquinone B (QB) (2).
D1 anchors all the prosthetic groups up to the bicarbonate/iron, which both D1 and D2 coordinate (4). D1 R27 and D1 Q130 H-bond the keto groups of the PheoD1 (4), while a highly conserved hydrophobic pocket accommodates QA. H-bonding by the main-chain amide of D2 F261 and D2 H214 further contribute to QA stability (2). The
QB hydrophobic binding pocket is less conserved, and is slightly larger in T. elongates models than in bacterial counterparts (4). D1 S264, D1 H215, and possibly a main-chain amide of D1 F265 H-bond this pastoquinone (2). Two sequential reductions and proton transfers reduce QB to plastoquinol, which diffuses into the lumen; these reduced products contribute to the reaction series of photosystem I (9). Water molecules presumably play a role in QB protonation, and a putative channel enclosed by charged residues allows for diffusion of bicarbonate anions to the non-heme iron (II) coordination sphere and water to the quinone pocket (4).
The OEC replaces the electrons transferred from the P680*+ radical via redox-active tyrosine residues; the fast route (marked by a shorter van der Waals distance to the chlorophyll pair) involves D1 Y-161 (termed Z), and the slower route involves D2 Y-160 (termed D). A series of acidic residues located between Z and P680*+ likely play a role in this electron transfer (4), resulting in neutral tyrosine radicals which directly drive the oxidation of water by the Mn4CaO4 cluster (2). While coordination models for the OEC vary depending on the modeling of the Mn atoms in the cluster (typically either a cubane structure or a planar hook arrangement (2, 8, 9)), they always involve negatively charged residues that can accommodate the various S state transitions of the OEC throughout the oxidation and dioxygen formation (2, 3, 7, 9). Proton transfer from the OEC occurs via a coordinated water molecule and chlorine anion (9), and residues D1 D-61, D2 K-317, D2 E-312, and D1 E-65 provide a
proton transfer pathway to the lumen (2). These protons contribute to the proton gradient needed to drive the phosphorylation of ADP (7). D1, CP43, PsbO, and several integral lipids contain channels that allow for the diffusion of water to and protons and molecular oxygen away from the OEC, facilitating the reaction (9).
Overall, the reaction center is highly conserved across all species (which accounts for the 0.00 E value from the NCBI Protein BLAST (10)):
the transmembrane regions and quinone binding sites of PSII are almost identical to the L and M subunits of the reaction center of the photosynthetic purple bacteria Rhodopseudomonas viridis (bRC, 2WJM) (2, 4). A comparison with Q(B) protein of T. vulcanus reveals that primary as well as tertiary tertiary structural features are essentially conserved throughout the genus, and even a Dali Server comparison with Rhodobacter sphaeroides gives a Z score of 19.4 with RMSD 2.6 (5). Minute variation in primary structure and the presence or absence of bicarbonate as an iron (II) ligand comprise the only real differences between the PSII RC and that of R. Sphaeroides (6). Experiments have confirmed a low tolerance for mutations in the RC proteins of multiple photosynthetic species (2, 3, 4, 7, 9), which corroborates the notion that such complex photochemistry requires a very specific structure.