Blocking of Electron Donation by Mn(II) to Y_Z^• following Incubation of Mn-Depleted Photosystem II Membranes with Fe(II) in the Light

Abstract

The donation of electrons by Mn(II) and Fe(II) to Y(Z*) through the high-affinity (HA(Z)) site in Mn-depleted photosystem II (PSII) membranes has been studied by flash-probe fluorescence yield measurements. Mn(II) and Fe(II) donate electrons to Y(Z*) with about the same efficiency, saturating this reaction at the same concentration (ca. 5 microM). However, following a short incubation of the membranes with 5 microM Fe(II), but not with Mn(II) in room light, added Mn(II) or Fe(II) can no longer be photooxidized by Y(Z)(*). This blocking effect is caused by specifically bound, photooxidized Fe [> or =Fe(III)] and is accompanied by a delay in the fluorescence yield decay kinetics attributed to the slowing down of the charge recombination rate between Q(a-) and Y(Z*). Exogenously added Fe(III), on the other hand, does not donate electrons to Y(Z*), does not block the donation of electrons by added Mn(II) and Fe(II), and does not change the kinetics of the decay of the fluorescence yield. These results demonstrate that the light-dependent oxidation of Fe(II) by Y(Z*) creates an Fe species that binds at the HA(Z) site and causes the blocking effect. The pH dependence of Mn(II) electron donation to Y(Z*) via the HA(Z) site and of the Fe-blocking effect is different. These results, together with sequence homologies between the C-terminal ends of the D1 and D2 polypeptides of the PSII reaction center and several diiron-oxo enzymes, suggest the involvement of two or perhaps more (to an upper limit of four to five) bound iron cations per reaction center of PSII in the blocking effect. Similarities in the interaction of Fe(II) and Mn(II) with the HA(Z) Mn site of PSII during the initial steps of the photoactivation process are discussed. The Fe-blocking effect was also used to investigate the relationship between the HA(Z) Mn site and the HA sites on PSII for diphenylcarbazide (DPC) and NH2OH oxidation. Blocking of the HA(Z) site with specifically bound Fe leads to the total inhibition of electron donation to Y(Z*) by DPC. Since DPC and Mn(II) donation to PSII is noncompetitive [Preston, C., and Seibert, M. (1991) Biochemistry 30, 9615-9624], the Fe bound to the HA(Z) site can also block the DPC donation site. On the other hand, electron donation by NH2OH to PSII still occurs in Fe-blocked membranes. Since hydroxylamine does not reduce the Fe [> or =Fe(III)] specifically bound to the HA(Z) site, NH2OH must donate to Y(Z*) through its own site or directly to P680+.

The extrinsic PsbO protein modulates the oxidation/reduction rate of the exogenous Mn cation at the high-affinity Mn-binding site of Mn-depleted PSII membranes

“…• reduction as demonstrated in two samples ( (Semin et al 2002). In this sample similar to sample 1 (Fig.…”

“…Blocking of the high-affinity Mn-binding site with iron cations PSII(−Mn/Tris) membranes (20 μg Chl/ml [0.08 μM]) were incubated in buffer A, containing 10 μM FeSO 4 under cool white fluorescent room light (4 μE m −2 s −1 , PAR; 1-cm optical pathlength) for 3 min at room temperature (Semin et al 2002). The membranes were then pelleted, washed once with buffer A, and subsequently resuspended in buffer A.…”

Decoupling of the processes of molecular oxygen synthesis and electron transport in Ca2+-depleted PSII membranes

“…Notably, the FYD of Cl --depleted PSII(-Ca/NaCl) membranes treated with one preflash (Fig. 4b) is not distinguishable from that observed in Mn-depleted PSII samples (Ghirardi et al 1996;Semin et al 2002), In the latter case, the fluorescence yield decreases very rapidly (t 1/2 = 20-30 ms) due to charge recombination within the Y Z • Q A -couple to a residual fluorescence level called F final that is typically 20-25% of F max (Metz et al 1989). This F final can be explained by the reduction of a portion of the Y Z • radical by an unknown endogenous electron donor (Ghirardi et al 1996;Metz et al 1989).…”

Characteristic features of the interaction between Fe(II) cations and the donor side of the manganese-depleted photosystem II

“…Only Y Z participates in Mn(II) oxidation essential for binding. As a whole, the first stage of the Mn cluster reconstruction and Y Z blockade by Fe cations have much in common: the reducing agents inhibit both processes [9,17], two processes are similarly dependent on pH [14,15], Y Z blockade results from binding at least two Fe cations [9,10,14], etc. Such similarity between the light-induced Mn cluster self-assemblage and the Y Z blockade by Fe cations provoked us to the study some characteristics of the blockade and to assess its effects on the putative alternative electron donors to P680 + .…”

“…Indeed, the C-terminal sites of D1 and D2 proteins in the PSII reaction center (RC) comprise all the specific amino acid motifs [11] essential for binding and stabilizing both Fe and Mn cations in such enzymes as ribonucleotide reductase and methane monooxygenase [12,13]. Due to the blockade [14], at least two and no more than five Fe cations bound to the high-affinity Mn-binding site (and probably to other Mn-binding sites as well) make Y Z unavailable for the exogenous electron donors, such as Mn(II), DPC, and Fe(II).…”