PSII and PSI: Structure, Function and Organization

The PSII and PSI photosynthetic complexes are very similar in eukaryotic algae (and plants) and cyanobacteria, as are many elements of the light capture, electron transport, and carbon dioxide (CO₂) fixation systems. The PSI and PSII complexes contain an internal antenna-domain carrying light harvesting chlorophylls and carotenoids, both non-covalently bound to a protein moiety, and a central core domain where biochemical reactions occur. In the internal antenna complexes, chlorophylls do most of the light harvesting, whereas carotenoids and xanthophylls mainly protect against excess light energy, and possibly transfer the absorbed radiation. In all photosynthetic eukaryotes, PSI and PSII form a supercomplex because they are associated with an external antenna termed LHC. The main function of LHCs is the absorption of solar radiation and the efficient transmittance of excitation energy towards reaction center chlorophylls. LHCs are composed of a protein moiety to which chlorophylls and carotenoids are non-covalently bound.

In eukaryotic algae, ten distinct light harvesting apoproteins (Lhc) can be distinguished. Four of them are exclusively associated with PSI (Lhca1–4), another four with PSII (Lhcb3–6), and two (Lhcb1 and Lhcb2) are preferentially but not exclusively associated with PSII, that is they can shuttle between the two photosystems. The apoproteins are three membrane-spanning a-helices and are nuclear-encoded. LHCs are arranged externally with respect to the photosystems. In Cyanophyta, Glaucophyta, Rhodophyta, and Cryptophyta, no LHCs are present and the light-harvesting function is performed by phycobiliproteins organized in phycobilisomes peripheral to the thylakoid membranes in the first three divisions, and localized within the lumen of thylakoids in the latter division. The phycobilisome structure consists of a three-cylinder core of four stacked molecules of allophycocyanin, close to the thylakoid membrane, on which converge rod-shaped assemblies of coaxially stacked hexameric molecules of only phycocyanin or both phycocyanin and phycoerythrin. Phycobilisomes are linked to PSII but they can diffuse along the surface of the thylakoids, at a rate sufficient to allow movements from PSII to PSI within 100 ms. Among prokaryotes, Prochlorophyta (Prochlorococcus sp., Prochlorothrix sp. and Prochloron sp.), differ from cyanobacteria in possessing an external chlorophyll a and b antenna, like eukaryotic algae, instead of the large extrinsic phycobilisomes.

PSII complex can be divided into two main protein superfamilies differing in the number of membrane-spanning a-helices, that is, the six-helix protein superfamily, which includes the internal antennae CP43 and CP47 (CP stands for Chlorophyll–Protein complex), and the five-helix proteins of the reaction center core D1 and D2 (so-called because they were first identified as two diffuse bands by gel electrophoresis and staining) where ETC components are located. External antenna proteins of Prochlorophyta belong to the six-helix CP43 and CP47 superfamily and not to the three-helix LHCs superfamily.

PSII is a homodimer, where the two monomers in the dimers are almost identical. The monomer consists of over 20 subunits. All the redox active cofactors involved in the activity of PSII are bound to the reaction center proteins D1 and D2. Closely associated with these two proteins are the chlorophyll a binding proteins CP43 and CP47 and the extrinsic luminally bound proteins of the oxygen evolving complex.

Each monomer also includes one heme b, one heme c, two plastoquinones, two pheophytins (a chlorophyll a without Mg₂₊), and one non-heme Fe and contains 36 chlorophylls a and 7 all-trans carotenoids assumed to be β-carotene molecules. Eukaryotic and cyanobacterial PSII are structurally very similar at the level of both their oligomeric states and organization of the transmembrane helices of their major subunits. The eukaryotic PSII dimer is flanked by two clusters of Lhcb proteins. Each cluster contains two trimers of Lhcb1, Lhcb2, and Lhcb3 and the other three monomers, Lhcb4, Lhcb5, and Lhcb6.

The reactions of PSII are powered by light-driven primary and secondary electron transfer processes across the reaction center (D1 and D2 subunits). Upon illumination, an electron is dislodged from the excited primary electron donor P₆₈₀, a chlorophyll a molecule located towards the luminal surface. The electron is quickly transferred towards the stromal surface to the final electron acceptor, a plastoquinone, via a pheophytin. After accepting two electrons and undergoing protonation, plastoquinone is reduced to plastoquinol, and it is then released from PSII into the membrane matrix. The cation P₆₈₀⁺ is reduced by a redox active tyrosine, which in turn is reduced by a Mn ion within a cluster of four. When the (Mn)4 cluster accumulates four oxidizing equivalents (electrons), two water molecules are oxidized to yield one molecule of O₂ and four proton. All the redox active cofactors involved in the electron transfer processes are located on the D1 side of the reaction center.

PSI complex possesses only eleven-helix PsaA and PsaB protein superfamilies. Each 11 transmembrane helices subunit has six N terminal transmembrane helices that bind light-harvesting chlorophylls and carotenoids and act as internal antennae and five C terminal transmembrane helices that bind Fe₄S₄ clusters as terminal electron acceptors. The N terminal part of the PsaA and PsaB proteins are structurally and functionally homologues to CP43 and CP47 proteins of PSII; the C terminal part of the PsaA and PsaB proteins are structurally and functionally homologues to D1 and D2 proteins of PSII. Eukaryotic PSI is a monomer that is loosely associated with the Lhca moiety, with a deep cleft between them. The four antenna proteins assemble into two heterodimers composed of Lhca1 and Lhca4 and homodimers composed of Lhca2 and Lhca3. Those dimers create a half-moon-shaped belt that docks to PsaA and PsaB and to other 12 proteic subunits of PSI, termed PsaC to PsaN that contribute to the coordination of antenna chromophores. On the whole PSI binds approximately 200 chromophore molecules. The cyanobacterial PSI exists as a trimer. One monomer consists of at least 12 different protein subunits, (PsaA, PsaB, PsaC, PsaD, PsaE, PsaF, PsaI, PsaJ, PsaK, PsaL, PsaM, and PsaX) coordinating more than 100 chromophores.

After primary charge separation initiated by excitation of the chlorophyll a pair P₇₀₀, the electron passes along the ETC consisting of another chlorophyll a molecule, a phylloquinone, and the Fe₄S₄ clusters. At the stromal side, the electron is donated by Fe₄S₄ to ferredoxin and then transferred to NADP⁺ reductase. The reaction cycle is completed by re-reduction of P₇₀₀⁺ by plastocyanin (or the interchangeable cytochrome c₆) at the inner (lumenal) side of the membrane. The electron carried by plastocyanin is provided by PSII by the way of a pool of plastoquinones and the cytochrome b₆f complex.

Photosynthetic eukaryotes such as Chlorophyta, Rhodophyta, and Glaucophyta have evolved by primary endosymbiosis involving a eukaryotic host and a prokaryotic endosymbiont. All other algae groups have evolved by secondary (or higher order) endosymbiosis between a simple eukaryotic alga and a non-photosynthetic eukaryotic host. Although the basic photosynthetic machinery is conserved in all these organisms, it should be emphasized that PSI does not necessarily have the same composition and fine-tuning in all of them. The subunits that have only been found in eukaryotes, that is, PsaG, PsaH, and PsaN, have actually only been found in plants and in Chlorophyta. Other groups of algae appear to have a more cyanobacteria-like PSI. PsaM is also peculiar because it has been found in several groups of algae including green algae, in mosses, and in gymnosperms. Thus, the PsaM subunit appears to be absent only in angiosperms. With respect to the peripheral antenna proteins, algae are in fact very divergent. All photosynthetic eukaryotes have Lhcs that belong to the same class of proteins. However, the Lhca associated with PSI appear to have diverged relatively early and the stoichiometry and interaction with PSI may well differ significantly between species. Even the green algae do not possess the same set of four Lhca subunits that is found in plants.

Are all those light harvesting complexes necessary? They substantially increase the light harvesting capacity of both photosystems by increasing the photon collecting surface with an associated resonance energy transfer to reaction centers, facilitated by specific pigment–pigment interactions. This process is related to the transition dipole–dipole interactions between the involved donor and acceptor antenna molecules that can be weakly or strongly coupled depending on the distance between and relative orientation of these dipoles. The energy migrates along a spreading wave because the energy of the photon can be found at a given moment in one or the other of the many resonating antenna molecules. This wave describes merely the spread of the probability of finding the photon in different chlorophyll antenna molecules. Energy resonance occurs in the chromophores of the antenna molecules at the lowest electronic excited state available for an electron, because only this state has a life time (10^-8 sec) long enough to allow energy migration (10^-12 sec). The radiationless process of energy transfer occurs towards pigments with lower excitation energy (longer wavelength absorption bands). Within the bulk of pigment–protein complexes forming the external and internal antenna system, the energy transfer is directed to chlorophyll a with an absorption peak at longest wavelengths. Special chlorophylls (P₆₈₀ at PSII and P₇₀₀ at PSI) located in the reaction center cores represent the final step of the photon trip, because once excited (P₆₈₀ + hν→ P*₆₈₀ ; P₇₀₀ + hν → P*₇₀₀) they become redox active species (P*₆₈₀ → P₆₈₀⁺ + e^-; P*₇₀₀ → P₇₀₀⁺ + e^-), that is, each donor releases one electron per excitation and activates different ETCs.

For an image gallery of the three-dimensional models of the two photosystems and LHCs in prokaryotic and eukaryotic algae refer to the websites of Jon Nield and James Barber at the Imperial College of London (U.K.).