pH Dependence of the Efficiency of Binding of Iron Cations to the Donor Side of Photosystem II

B. K. Semin^1*, L. N. Davletshina¹, A. Yu. Aleksandrov¹, V. Yu. Lanchinskaya², A. A. Novakova², and I. I. Ivanov¹

¹Faculty of Biology, Lomonosov Moscow State University, Moscow 119899, Russia; fax: (7-095) 939-1115; E-mail: semin@biophys.msu.ru; davlet@biophys.msu.ru; ivanov@biophys.msu.ru

²Faculty of Physics, Lomonosov Moscow State University, Moscow 119899, Russia; fax: (7-095) 932-8820; E-mail: aan@runar.phys.msu.ru

^* To whom correspondence should be addressed.

Received July 1, 2003; Revision received September 12, 2003

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Light-induced interaction of Fe(II) cations with the donor side of Mn-depleted photosystem II (PS II(-Mn)) results in the binding of iron cations and blocking of the high-affinity (HA_Z) Mn-binding site. The pH dependence of the blocking was measured using the diphenylcarbazide/2,6-dichlorophenolindophenol test. The curve of the pH dependence is bell-shaped with pK₁ = 5.8 and pK₂ = 8.0. The pH dependence of the O₂-evolution mediated by PS II membranes is also bell-shaped (pK₂ = 7.6). The pH dependence of the process of electron donation from exogenous donors in PS II(-Mn) was studied to determine the location of the alkaline pH sensitive site of the electron transport chain. The data of the study showed that the decrease in the iron cation binding efficiency at pH > 7.0 during blocking was determined by the donor side of the PS II(-Mn). Mössbauer spectroscopy revealed that incubation of PS II(-Mn) membranes in a buffer solution containing ⁵⁷Fe(II) + ⁵⁷Fe(III) was accompanied by binding only Fe(III) cations. The pH dependence of the nonspecific Fe(III) cation binding is also described by the same bell-shaped curve with pK₂ = 8.1. The treatment of the PS II(-Mn) membranes with the histidine modifier diethylpyrocarbonate resulted in an increase in the iron binding strength at alkaline pH. It is suggested that blocking efficiency at alkaline pH is determined by competition between OH^- and histidine ligand for Fe(III). Because the high-affinity Mn-binding site contains no histidine residue, this fact can be regarded as evidence that histidine is located at another (other than high-affinity) Fe(III) binding site. In other words, this means that the blockage of the high-affinity Mn-binding site is determined by at least two iron cations. We assume that inactivation of oxygen-evolving complex and inhibition of photoactivation in the alkaline pH region are also determined by competition between OH^- and a histidine residue involved in coordination of manganese cation outside the high-affinity site.

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KEY WORDS: photosystem II, oxygen-evolving complex, manganese, iron, histidine, Mössbauer spectroscopy, histidine modification

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