Formate-induced inhibition of the water-oxidizing complex of photosystem II studied by EPR

The effects of various formate concentrations on both the donor and the acc eptor sides in oxygen-evolving PS II membranes (BBY particles) were examine d. EPR, oxygen evolution and variable chlorophyll fluorescence have been ob served. It was found that formate inhibits the formation of the S-2 state m ultiline signal concomitant with stimulation of the Q(A)(-)Fe(2+) signal at g = 1.82. The decrease and the increase in intensities of the multiline an d Q(A)(-)Fe(2+) signals, respectively, had a linear relation for formate co ncentrations between 5 and 500 mM. The g = 4.1 signal formation measured in the absence of methanol was not inhibited by formate up to 250 mM in the b uffer. In the presence of 3% methanol the g = 4.1 signal evolved as formate concentration increased. The evolved signal could be ascribed to the inhib ited centers. Oxygen evolution measured in the presence of an electron acce ptor, phenyl-p-benzoquinone, was also inhibited by formate proportionally t o the decrease in the multiline signal intensity. The inhibition seemed to be due to a retarded electron transfer from the water-oxidizing complex to Y-z(+) which was observed in the decay kinetics of the Y-z(+) signal induce d by illumination above 250 K. These results show that formate induces inhi bition of water oxidation reactions as well as electron transfer on the PS II acceptor side. The inhibition effects of formate in PS II were found to be reversible, indicating no destructive effect on the reaction center indu ced by formate.