Photoelectron transport in the PSII reaction center (see Figure 1) resembles that in the purple bacterial reaction center described earlier (see Figure 2). Excitation by a photon with a wavelength of <680 nm triggers the loss of an electron from a P680 chlorophyll. That electron is transported rapidly to a quinone (QA) and then to the primary electron acceptor, QB, on the stromal surface of the thylakoid membrane (see Figure 1). PSII differs from the bacterial reaction center, however, in that it has an additional three protein subunit, the oxygen-evolving complex, that faces the thylakoid lumen (in this context, “oxygen-evolving” refers to generating O2 from water).
Fig1. Linear electron flow in plants, which requires both chloroplast photosystems, PSI and PSII. Blue arrows indicate flow of electrons; red arrows indicate proton movement. LHCs are not shown. (Left) In the PSII reaction center, two sequential light-induced excitations of the same special-pair P680 chlorophyll result in a two-step reduction of the primary electron acceptor QB to QH2. On the luminal side of PSII, electrons removed from H2O by the oxygen-evolving complex are transferred to P680 +, restoring the reaction-center chlorophylls to the ground state after each excitation. The oxygen-evolving complex contains a cluster of four manganese ions (Mn, violet), a Ca2+ ion (green), and a Cl− ion (teal). These bound ions function in the split ting of H2O and maintain the environment essential for high rates of O2 evolution. Four sequential light-induced excitations of the P680 (double the number illustrated here) are required to oxidize two water molecules and release four protons and one molecule of molecular oxygen (O2). A tyrosine on the protein helps conducts electrons from the Mn ions to the oxidized reaction-center chlorophyll (P680 +), reducing it to the ground state (P680) after excitation by each photon. (Center) The cytochrome bf complex then accepts electrons from QH2 and transports two protons into the lumen. Operation of a Q cycle in the cytochrome bf complex translocates additional protons across the membrane to the thylakoid lumen, increasing the proton-motive force. (Right) In the PSI reaction center, each electron released from light-excited P700 chlorophylls moves via a series of carriers in the reaction center to the stromal surface, where soluble ferredoxin (an Fe-S protein) transfers the electron to ferredoxin-NADP+ reductase (FNR). This enzyme uses the prosthetic group flavin adenine dinucleotide (FAD) and a proton to reduce NADP+, forming NADPH. P700 + is restored to its ground state by addition of an electron carried from PSII via the cytochrome bf complex and plastocyanin, a soluble electron carrier.
Fig2. Cyclic electron flow in the single photosystem of purple bacteria. Cyclic electron flow generates a proton-motive force but no O2. Blue arrows indicate flow of electrons; red arrows indicate proton movement. (Left) Energy absorbed directly from light or fun neled from an associated LHC (not illustrated here) energizes one of the special-pair chlorophylls in the reaction center. Photoelectron transport from the energized chlorophyll, via an accessory chlorophyll, pheophytin (Ph), and quinone A (QA), to quinone B (QB), which forms the semi quinone Q.- and leaves a positive charge on the chlorophyll. Following absorption of a second photon and transfer of a second electron to the semiquinone, the quinone rapidly picks up two protons from the cytosol to form QH2. (Center) After diffusing through the membrane and binding to the Qo site on the periplasmic (exoplasmic) face of the cytochrome bc1 complex, QH2 donates two electrons and simultaneously gives up two protons to the external medium in the periplasmic space, generating a proton electrochemical gradient (proton-motive force) that drives ATP synthesis (right). Electrons are transported back to a reaction-center chlorophyll via a soluble cytochrome, which diffuses in the periplasmic space. Note the cyclical path (blue) of electrons. Operation of a Q cycle in the cytochrome bc1 complex pumps additional protons across the membrane to the external medium, as in mitochondria. See J. Deisenhofer and H. Michael, 1991, Annu. Rev. Cell Biol. 7:1.
The photochemically oxidized special-pair reaction- center chlorophyll a of PSII, P680 +, is the strongest biological oxidant known. The reduction potential of P680 + is more positive than that of H2O, and thus it can oxidize H2O to generate O2 and H+ ions. The oxidation of H2O provides the electrons for reduction of P680 + in PSII, thus replacing the electron released by light absorption. The oxygen-evolving complex is a protein that contains a manganese (Mn), calcium, and oxygen cluster (Mn4CaO5) that directly mediates the conversion of H2O to O2, as well as a bound Cl− ion that influences the reaction rate (see Figure 12-44). The Mn ions cycle through five different oxidation states during O2 generation. A nearby tyrosine side chain helps transfer electrons from H2O to the P680 +. The protons released from H2O remain in the thylakoid lumen. The oxidation of two molecules of H2O to form O2 requires the removal of four electrons, but absorption of each photon by PSII results in the transfer of just one electron. Thus a single PSII must lose an electron and then oxidize the oxygen-evolving complex four times in a row for an O2 molecule to be formed. Channels in the protein of the oxygen-evolving complex have been proposed to serve as conduits through the complex for the de livery of H2O to and the removal of O2 from the active site.
The herbicide atrazine is one of the most commonly used weed killers in US agriculture. Atrazine binds to PSII, blocks the binding of oxidized QB, and thus blocks downstream electron transport.
