Influence of the Axial Ligand on the Cationic Properties of the Chlorophyll Pair in Photosystem II from Thermosynechococcus vulcanus.

Abstract

Influence of the axial ligand of P(D1) chlorophyll (D1-His-198) on the E(m) of monomer chlorophylls P(D1) and P(D2), and the P(D1)(•+)/P(D2)(•+) charge ratio was investigated by theoretical calculations using the PSII crystal structure of Thermosynechococcus vulcanus analyzed at 1.9-Å resolution. It was found that the E(m)(P(D1))/E(m)(P(D2)) values and P(D1)(•+)/P(D2)(•+) ratio remained unchanged upon D1-H198Q mutation. However, E(m)(P(D1)) was increased in the D1-H198A mutant, resulting in a more even distribution of the positive charge over P(D1)/P(D2). Introduction of a water molecule as an axial ligand resulted in equal E(m) values and P(D1)(•+)/P(D2)(•+) ratios between the mutant and wild-type, thus confirming the presence of the water ligand in the mutant.