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

Abstract

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 (HAZ) 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.

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

“…Having in mind that Fe(II) is not bound by the native PSII membranes (Table 1) and the depletion of Mn clusters makes the oxidants at the donor side of PSII available for the exogenous electron donors [1], we concluded that the diminished concentration in the light could resulted from Fe(II) oxidation and from binding Fe(III) cations thus produced to the PSII(-Mn) membranes. Indeed, the absence of mononuclear Fe(II)-binding centers in the PSII(-Mn) membranes was confirmed by moessbauer spectrometric data [23]. However, it is plausible that Fe(II) or cations of intermediate valence may be found in polynuclear Fe clusters, same as in iron-sulfur clusters [24].…”

The Role of Protonation States in Ligand-Receptor Recognition and Binding

“…In general, any pH-dependent binding involves proton uptake/release and therefore causes changes in the protonation states of ionizable groups. The literature is full of examples of experimental data demonstrating pH-dependent binding from ion binding to small peptides [20] or proteins [21], and acid-organic compounds [22], to protein-small molecule [23, 24] and protein-protein binding [25–27]. All of these examples demonstrate that protonation states frequently change due to the binding.…”