F

Ca2+ Cl

17 kDa

Pheo ^Chl^

17 kDa

Special-pair " chlorophylls

„ O2-evolving complex

▲ FIGURE 8-38 Electron flow and O2 evolution in chloroplast PSII. The PSII reaction center, comprising two integral proteins, D1 and D2, special-pair chlorophylls (P6so), and other electron carriers, is associated with an oxygen-evolving complex on the luminal surface. Bound to the three extrinsic proteins (33, 23, and 17 kDa) of the oxygen-evolving complex are four manganese ions (red), a Ca2+ ion (blue), and a CP ion (yellow). These bound ions function in the splitting of H2O and maintain the environment essential for high rates of O2 evolution. Tyrosine-161 (Y161) of the D1 polypeptide conducts electrons from the Mn ions to the oxidized reaction-center chlorophyll (P680+), reducing it to the ground state P680. See the text for details. [Adapted from C. Hoganson and G. Babcock, 1997, Science 277:1953.]

one nonheme iron atom. These small molecules are bound to two PSII proteins, called D1 and D2, whose sequences are remarkably similar to the sequences of the L and M peptides of the bacterial reaction center, attesting to their common evolutionary origins (see Figure 8-35). When PSII absorbs a photon with a wavelength of <680 nm, it triggers the loss of an electron from a P680 molecule, generating P680+. As in photosynthetic purple bacteria, the electron is transported via a pheophytin and a quinone (QA) to the primary electron acceptor, QB, on the outer (stromal) surface of the thylakoid membrane (Figures 8-37 and 8-38).

The photochemically oxidized reaction-center chlorophyll of PSII, P680 + , is the strongest biological oxidant known. The reduction potential of P680+ is more positive than that of water, and thus it can oxidize water to generate O 2 and H+ ions. Photosynthetic bacteria cannot oxidize water because the excited chlorophyll a+ in the bacterial reaction center is not a sufficiently strong oxidant. (As noted earlier, purple bacteria use H2S and H2 as electron donors to reduce chlorophyll a+ in linear electron flow.)

The splitting of H2O, which provides the electrons for reduction of P680+ in PSII, is catalyzed by a three-protein complex, the oxygen-evolving complex, located on the luminal surface of the thylakoid membrane. The oxygen-evolving complex contains four manganese (Mn) ions as well as

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