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

Abstract

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 microM (corresponds to K(m) for Mn(II) oxidation at the HA site), but detected at Mn concentration 100 microM (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 pK(a) 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.

Photosysem II: where does the light-induced voltage come from?

“…• that is compensated by the ∆Ψ decay phase in the same time scale. It should be noted that a very similar amplitude was reported earlier, but this signal had faster kinetics in the presence of exogenous Mn for PS II (-Mn) samples (29). It can be assumed that the binding site of NH 2 OH can be close to the binding site of exogenous Mn.…”

“…• in Mn-depleted PS II complexes. The data recently obtained in the presence of synthetic trinuclear Mn-complex is also in favor of the latter assumption (29). The electrogenic phase (∼25% of the fast phase, τ∼160 ms) observed under these conditions was ascribed to the transfer of an electron from the synthetic complex attached to the protein-water interface to the manganese at the protein-embedded Mn-binding site.…”