Iron Bound to the High-Affinity Mn-Binding Site of the Oxygen-Evolving Complex Shifts the pK of a Component Controlling Electron Transport via Yz

Boris K. Semin, Michael Seibert

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Abstract

Flash-probe fluorescence spectroscopy was used to compare the pH dependence of charge recombination between YZ. and Q a- in Mn-depleted, photosystem II membranes [PSII(-Mn)] and in membranes with the high-affinity (HAZ) Mn-binding site blocked by iron [PSII(-Mn,+Fe); Semin, B. K., Ghirardi, M. L., and Seibert, M. (2002) Biochemistry 41, 5854-5864]. The apparent half-time for fluorescence decay (t1/2) in PSII(-Mn) increased from 9 ms at pH 4.4 to 75 ms at pH 9.0 [with an apparent pK (pKapp) of 7.1]. The actual fluorescence decay kinetics can be fit to one exponential component at pH <6.0 (t 1/2 = 9.5 ms), but it requires an additional component at pH >6.0 (t1/2 = 385 ms). Similar measurements with PSII(-Mn,+Fe) membranes show that iron binding has little effect on the maximum and minimum t 1/2 values measured at alkaline and acidic pHs but that it does shift the pKapp from 7.1 to 6.1 toward the more acidic pK app value typical of intact membranes. Light-induced Fe(II) blocking of the PSII(-Mn) membrane is accompanied by a decrease in buffer Fe(II) concentration. This decrease was not the result of Fe(II) binding, but rather of its oxidation at two sites, the HAZ site and the low-affinity site. Mössbauer spectroscopy at 80 K on PSII(-Mn,+Fe) samples, prepared under conditions providing the maximal blocking effect but minimizing the amount of nonspecifically bound iron cations, supports this conclusion since this method detected only Fe(III) cations bound to the membranes. Correlation of the kinetics of Fe(II) oxidation with the blocking parameters showed that blocking occurs after four to five Fe(II) cations were oxidized at the HA Z site. In summary, the blocking of the HAZ Mn-binding site by iron in PSII(-Mn) membranes not only prevents the access of exogenous donors to YZ but also partially restores the properties of the hydrogen bond net found in intact PS(II), which in turn controls the rate of electron transport to YZ.

Original languageAmerican English
Pages (from-to)6772-6782
Number of pages11
JournalBiochemistry
Volume43
Issue number21
DOIs
StatePublished - 1 Jun 2004

NREL Publication Number

  • NREL/JA-590-36922

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