Investigation of the low-affinity oxidation site for exogenous electron donors in the Mn-depleted photosystem II complexes

In the manganese-depleted photosystem II (PSII[-Mn]) preparations, oxidation of exogenous electron donors is carried out through the high-affinity (HA) and the low-affinity (LA) sites. This paper investigates the LA oxidation site in the PSII(-Mn) preparations where the HA, Mn-binding site was blocked with ferric cations [[11] B.K. Semin, M.L. Ghirardi, M. Seibert, Blocking of electron donation by Mn(II) to Y_Z^• following incubation of Mn-depleted photosystem II membranes with Fe(II) in the light, Biochemistry 41 (2002) 5854-5864.]. In blocked (PSII[-Mn,+Fe]) preparations electron donation by Mn(II) cations to Y_Z^• was not detected at Mn(II) concentration 10 μM (corresponds to K_m for Mn(II) oxidation at the HA site), but detected at Mn concentration 100 μM (corresponds to K_m for the LA site) by fluorescence measurements. Comparison of pH-dependencies of electron donation by Mn(II) through the HA and the LA sites revealed the similar r{cyrillic}K_a{cyrillic} equal to 6.8. Comparison of K_m for diphenylcarbazide (DPC) oxidation at the LA site and K_d for A_T thermoluminescence band suppression by DPC in PSII(-Mn,+Fe) samples suggests that there is relationship between the LA site and A_T band formation. The role of D1-His190 as an oxidant of exogenous electron donors at the LA site is discussed. In contrast to electrogenic electron transfer from Mn(II) at the HA site to Y_Z^•, photovoltage due to Mn(II) oxidation in iron-blocked PSII(-Mn) core particles was not detected.