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Flash-induced blocking of the high-affinity manganese-binding site in photosystem II by iron cations: dependence on
Boris K Semin1, Michael Seibert
1Chemical and Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, USA.
Abstract:
Incubation of Fe(II) cations with Mn-depleted PSII membranes (PSII(-Mn)) under weak continuous light is accompanied by blocking of the high-affinity, Mn-binding (HAZ) site with ferric cations (Semin, B.K. et al. Biochemistry 2002, 41, 5854-5864). In this study we investigated the blocking yield under single-turnover flash conditions. The flash-probe fluorescence method was used to estimate the blocking efficiency. We found that the yield of blocking increases with flash number and reaches 50% after 7 flashes. When the dark interval between the flashes (Delta t) was varied, we found that the percentage of blocking decreases at Delta t < 100 ms (t 1/2, 4-10 ms). No inhibition of the blocking yield was found at longer time intervals (as with photoactivation). This result shows the necessity of a dark rearrangement during the blocking process (the dual-site hypothesis described in the text) and indicates the formation of a binuclear iron center. During the blocking experiments, we found a binary oscillation of the Fmax elicited during a train of flashes. The oscillations were observed only in the presence of Fe(II) cations or other electron donors (including Mn(II)) but not in the presence of Ca2+. Chelators had no effect on the oscillations. Our results indicate that the oscillations are due to processes on the acceptor side of PSII and to the appearance of "acceptor X" after odd flashes. Acceptor X is reduced by QA- at very high rate (<<2 ms), is not sensitive to DCMU, and is rather stable in the dark (t l/2 approximately 2 min). These properties are similar to those of nonheme Fe(III) (Fe(III)NHI). When Fe(II)NHI was oxidized with ferricyanide (Fe(CN)6), the fluorescence decay kinetics and yield of fluorescence were identical to those observed when the sample was exposed to 1 flash prior to the fluorescence measurement. We suggest that acceptor X is Fe(III)NHI, oxidized by the semiquinone form of QB-. This is similar to the mechanism of "reduction-induced oxidation of Fe(II)NHI" by exogenous quinones reported in the literature. We suggest that involvement of QB- in the oxidation of Fe(II)NHI in PSII(-Mn) membranes is due to the modification of the QB-binding site and increase of its redox potential resulting from extraction of the functional Mn cluster.
Insights
Iron cations block manganese-binding sites in photosystem II (PSII) membranes, with blocking efficiency increasing with flashes. This study reveals dark rearrangements and the formation of a binuclear iron center, suggesting a novel mechanism in photosynthesis.
Area of Science:
- Photosynthesis research
- Bioinorganic chemistry
- Membrane protein biochemistry
Background:
- Manganese-depleted photosystem II (PSII(-Mn)) membranes are susceptible to high-affinity, manganese-binding (HAZ) site blocking by ferric cations.
- Previous studies indicated blocking occurs under continuous light, but blocking dynamics under controlled flash conditions were unexplored.
Purpose of the Study:
- To investigate the blocking yield of the HAZ site by ferric cations in PSII(-Mn) membranes under single-turnover flash conditions.
- To elucidate the mechanism of HAZ site blocking and identify the nature of 'acceptor X' on the PSII acceptor side.
Main Methods:
- Flash-probe fluorescence method to quantify the blocking efficiency of the HAZ site.
- Variations in dark interval duration (Δt) between flashes to study blocking kinetics.
- Analysis of Fmax oscillations during flash trains in the presence of different cations and chelators.
Main Results:
- Blocking yield increased with flash number, reaching 50% after 7 flashes, and was inhibited at dark intervals < 100 ms, indicating a necessary dark rearrangement and binuclear iron center formation.
- Binary oscillations in Fmax were observed with Fe(II) or other electron donors, but not Ca2+, suggesting processes on the PSII acceptor side.
- Oscillations were linked to 'acceptor X', a species rapidly reduced by QA- and stable in the dark, identified as nonheme Fe(III) (Fe(III)NHI) oxidized by the QB- semiquinone.
Conclusions:
- The blocking of the HAZ site by Fe(II) involves a dual-site mechanism requiring dark rearrangement and formation of a binuclear iron center.
- 'Acceptor X' is likely Fe(III)NHI, oxidized by QB-, with its involvement suggesting a modified QB- binding site in PSII(-Mn) membranes.
- These findings provide insights into iron-cation interactions and electron transfer mechanisms in photosystem II.
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