Involvement of histidine 190 on the D1 protein in electron/proton transfer reactions on the donor side of photosystem II

Flash-induced chlorophyll fluorescence kinetics from photosystem II in thylakoids from the dark-grown wild type and two site-directed mutants of the D1 protein His190 residue (D1-H190) in Chlamydomonas reinhardtii have been characterized. Induction of the chlorophyll fluorescence on the first flash, reflecting electron transport from Y-Z to P680(+), exhibited a strong pH dependence with a pK of 7.6 in the dark-grown wild type which lacks the Mn cluster. The chlorophyll fluorescence decay, measured in the presence of DCMU, which reflects recombination between Q(A)(-) and Y-Z(ox), was also pH-dependent with a similar pK of 7.5. These results indicate participation by the same base, which is suggested to be D1-H190, in oxidation and reduction of Y-Z in forward electron transfer and recombination pathways, respectively. This hypothesis was tested in the D1-H190 mutants. Induction of chlorophyll fluorescence in these H190 mutants has been observed to be inefficient due to slow electron transfer from Y-Z to P680(+) [Roffey, R. A., et al. (1994) Biochim. Biophys. Acta 1185, 257-270]. We show that this reaction is pH-dependent, with a pK of 8.1, and at pH greater than or equal to 9, the fluorescence induction is efficient in the H190 mutants, suggesting direct titration of Y-Z. The efficient oxidation of Y-Z (approximate to 70% at pH 9.0) at high pH was confirmed by kinetic EPR measurements. In contrast to the wild type, the H190 mutants show little or no observable fluorescence decay. Our data suggest that H190 is an essential component in the electron transfer reactions in photosystem II and acts as a proton acceptor upon Y-Z oxidation. In the H190 mutants, this reaction is inefficient and Y-Z oxidation only occurs at elevated pHs when Y-Z itself probably is deprotonated. We also propose that H190 is able to return a proton to Y-Z(ox) during electron recombination from Q(A)(-) in a reaction which does not take place in the D1-H190 mutants.