Redox Potentials of Chlorophylls and β-Carotene in the Antenna Complexes of Photosystem II

2005-02-16T00:00:00Z (GMT) by Hiroshi Ishikita Ernst-Walter Knapp
Electron transfer (ET) processes in reaction centers (RC) of photosystem II (PSII) are prerequisites of oxygen generation. They are promoted by energy transfer from antenna to RC. Here, we calculated the redox potentials of chlorophyll<i>a</i>/β-carotene (Chl<i>a</i>/Car) in PSII CP43/CP47 antenna complexes, solving the linearized Poisson−Boltzmann (LPB) equation based on the PSII crystal structure. The majority of antenna Chl<i>a</i> redox potentials for reduction/oxidation were lower than those of RC Chl<i>a</i>. Hence, ET events with excess electrons remain localized in the RC. Simultaneously antenna Chl<i>a</i> can serve as an efficient cation sink to rereduce RC Chl<i>a</i> if normal PSII function is inhibited. Especially three antenna Chl<i>a</i> (Chl-47, Chl-18, and Chl-12) and two Car bridging the space between Chl<sub>Z(D1)</sub> and cytochrome (cyt) <i>b</i>559 have the same level of oxidation redox potential. Together with Chl<sub>Z(D2)</sub> they form an electron hole transfer pathway and temporary storage device guiding from the oxidized P680<sup>+•</sup> Chl<i>a</i> to the cyt <i>b</i>559. This path may play a photoprotective role as efficient electron hole quencher.