The Central Photochemical Event: Light-Driven Electron Flow:- In Plants, Two Reaction Centers Act in Tandem
The photosynthetic apparatus of modern cyanobacteria, algae, and vascular plants is more complex than the one center bacterial systems, and it appears to have evolved through the combination of two simpler bacterial photocenters. The thylakoid membranes of chloroplasts have two different kinds of photosystems, each with its own type of photochemical reaction center and set of antenna molecules. The two systems have distinct and complementary functions (Fig. 19–49). Photosystem II (PSII) is a pheophytin-quinone type of system (like the single photosystem of purple bacteria) containing roughly equal amounts of chlorophylls a and b. Excitation of its reaction center P680 drives electrons through the cytochrome b6 f complex with concomitant move ment of protons across the thylakoid membrane. Photosystem I (PSI) is structurally and functionally related to the type I reaction center of green sulfur bacteria. It has a reaction center designated P700 and a high ratio of chlorophyll a to chlorophyll b. Excited P700 passes electrons to the Fe-S protein ferredoxin, then to NADP+, producing NADPH. The thylakoid membranes of a single spinach chloroplast have many hundreds of each kind of photosystem.
These two reaction centers in plants act in tandem to catalyze the light-driven movement of electrons from H2O to NADP+ (Fig. 19–49). Electrons are carried between the two photosystems by the soluble protein plastocyanin, a one-electron carrier functionally simi lar to cytochrome c of mitochondria. To replace the electrons that move from PSII through PSI to NADP+, cyanobacteria and plants oxidize H2O (as green sulfur bacteria oxidize H2S), producing O2 (Fig. 19–49, bottom left). This process is called oxygenic photosynthesis to distinguish it from the anoxygenic photosynthesis of purple and green sulfur bacteria. All O2-evolving photosynthetic cells—those of plants, algae, and cyanobacteria—contain both PSI and PSII; organisms with only one photosystem do not evolve O2. The dia gram in Figure 19–49, often called the Z scheme because of its overall form, outlines the pathway of electron flow between the two photosystems and the energy relationships in the light reactions. The Z scheme thus describes the complete route by which electrons flow from H2O to NADP+, according to the equation 2H2O+2NADP++8 photons → O2+2NADPH+2H+ For every two photons absorbed (one by each photo system), one electron is transferred from H2O to NADP+. To form one molecule of O2, which requires transfer of four electrons from two H2O to two NADP+, a total of eight photons must be absorbed, four by each photosystem.
The mechanistic details of the photochemical reac tions in PSII and PSI are essentially similar to those of the two bacterial photosystems, with several important additions. In PSII, two very similar proteins, D1 and D2, form an almost symmetrical dimer, to which all the electron-carrying cofactors are bound (Fig. 19–50). Ex citation of P680 in PSII produces P680*, an excellent electron donor that, within picoseconds, transfers an electron to pheophytin, giving it a negative charge (Pheo-). With the loss of its electron, P680* is trans formed into a radical cation, P680. Pheo- very rap idly passes its extra electron to a protein-bound plastoquinone, PQA (or QA), which in turn passes its electron to another, more loosely bound plastoquinone, PQB (or QB). When PQB has acquired two electrons in two such transfers from PQA and two protons from the solvent water, it is in its fully reduced quinol form, PQBH2. The overall reaction initiated by light in PSII is
4P680+4H++2PQB+4 photons→4P680++2PQBH2 (19–12) Eventually, the electrons in PQBH2 pass through the cytochrome b6 f complex (Fig. 19–49). The electron initially removed from P680 is replaced with an electron obtained from the oxidation of water, as described below. The binding site for plastoquinone is the point of action of many commercial herbicides that kill plants by blocking electron transfer through the cytochrome b6 f complex and preventing photosynthetic ATP production. The photochemical events that follow excitation of PSI at the reaction center P700 are formally similar to those in PSII. The excited reaction center P700* loses an electron to an acceptor, A0 (believed to be a special form of chlorophyll, functionally homologous to the pheophytin of PSII), creating A0- and P700+ (Fig. 19–49, right side); again, excitation results in charge separation at the photochemical reaction center. P700 is a strong oxidizing agent, which quickly acquires an electron from plastocyanin, a soluble Cu-containing electron-transfer protein. A0 is an exceptionally strong reducing agent that passes its electron through a chain of carriers that leads to NADP+. First, phylloquinone (A1) accepts an electron and passes it to an iron-sulfur protein (through three Fe-S centers in PSI). From here, the electron moves to ferredoxin (Fd), another iron-sulfur protein loosely associated with the thylakoid membrane. Spinach ferredoxin (Mr 10,700) contains a 2Fe-2S center (Fig. 19–5) that undergoes one-electron oxidation and reduction reactions. The fourth electron carrier in the chain is the flavoprotein ferredoxin: NADP+ oxidoreduc tase, which transfers electrons from reduced ferredoxin (Fdred) to NADP+: 2Fdred+2H++ NADP+→2Fdox+NADPH+H+ This enzyme is homologous to the ferredoxin: NAD reductase of green sulfur bacteria.
FIGURE 19–49 Integration of photosystems I and II in chloroplasts. This “Z scheme” shows the pathway of electron transfer from H2O (lower left) to NADP (far right) in noncyclic photosynthesis. The position on the vertical scale of each electron carrier reflects its standard reduction potential. To raise the energy of electrons derived from H2O to the energy level required to reduce NADP+ to NADPH, each electron must be “lifted” twice (heavy arrows) by photons absorbed in PSII and PSI. One photon is required per electron in each photosystem. After excitation, the high-energy electrons flow “downhill” through the carrier chains shown. Protons move across the thylakoid membrane during the water-splitting reaction and during electron transfer through the cytochrome b6f complex, producing the proton gradient that is central to ATP formation. The dashed arrow is the path of cyclic electron transfer (discussed later in the text), which involves only PSI; electrons return via the cyclic pathway to PSI, instead of reducing NADP+ to NADPH.
FIGURE 19–50 Photosystem II of the cyanobacterium Synechococcus elongates. The monomeric form of the complex shown here has two major transmembrane proteins, D1 and D2, each with its set of co factors. Although the two subunits are nearly symmetric, electron flow occurs through only one of the two branches of cofactors, that on the right (on D1). The arrows show the path of electron flow from the Mn ion cluster (Mn4, purple) of the water-splitting enzyme to the quinone PQB (orange). The photochemical events occur in the sequence indicated by the step numbers. Notice the close similarity between the positions of cofactors here and the positions in the bacterial photore action center shown in Figure 19–48. The role of the Tyr residues is discussed later in the text.
