Function of two β-carotenes near the D1 and D2 proteins in photosystem II dimers

Abstract

The antenna proteins in photosystem II (PSII) not only promote energy transfer to the photosynthetic reaction center (RC) but provide also an efficient cation sink to re-reduce chlorophyll a if the electron transfer (ET) from the Mn-cluster is inhibited. Using the newest PSII dimer crystal structure (3.0 Å resolution), in which 11 β-carotene molecules (Car) and 14 lipids are visible in the PSII monomer, we calculated the redox potentials ( E m) of one-electron oxidation for all Car ( E m(Car)) by solving the Poisson–Boltzmann equation. In each PSII monomer, the D1 protein harbors a previously unlocated Car (Car D1) in van der Waals contact with the chlorin ring of Chl Z(D1). Each Car D1 in the PSII dimer complex is located in the interface between the D1 and CP47 subunits, together with another four Car of the other PSII monomer and several lipid molecules. The proximity of Car bridging between Car D1 and plastoquinone/Q A may imply a direct charge recombination of Car +Q A −. The calculated E m(Car D1) and E m(Chl Z(D1)) are, respectively, 83 and 126 mV higher than E m(Car D2) and E m(Chl Z(D2)), which could explain why Car D2 + and Chl Z(D2) + are observed rather than the corresponding Car D1 + and Chl Z(D1) +.