Photosystem II of peas: effects of added divalent cations of Mn, Fe, Mg, and Ca on two kinetic components of P+680 reduction in Mn-depleted core particles

Abstract

The catalytic Mn cluster of the photosynthetic oxygen-evolving system is oxidized via a tyrosine, Y(Z), by a photooxidized chlorophyll a moiety, P(+)(680). The rapid reduction of P(+)(680) by Y(Z) in nanoseconds requires the intactness of an acid/base cluster around Y(Z) with an apparent functional pK of <5. The removal of Mn (together with bound Ca) shifts the pK of the acid/base cluster from the acid into the neutral pH range. At alkaline pH the electron transfer (ET) from Y(Z) to P(+)(680) is still rapid (<1 micros), whereas at acid pH the ET is much slower (10-100 micros) and steered by proton release. In the intermediate pH domain one observes a mix of these kinetic components (see R. Ahlbrink, M. Haumann, D. Cherepanov, O. Bögershausen, A. Mulkidjanian, W. Junge, Biochemistry 37 (1998)). The overall kinetics of P(680)(+) reduction by Y(Z) in Mn-depleted photosystem II (PS II) has been previously shown to be slowed down by divalent cations (added at >10 microM), namely: Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+) (C.W. Hoganson, P.A. Casey, O. Hansson, Biochim. Biophys. Acta 1057 (1991)). Using Mn-depleted PS II core particles from pea as starting material, we re-investigated this phenomenon at nanosecond resolution, aiming at the effect of divalent cations on the particular kinetic components of P(+)(680) reduction. To our surprise we found only the slower, proton steered component retarded by some added cations (namely Co(2+)/Zn(2+)>Fe(2+)>Mn(2+)). Neither the fast component nor the apparent pK of the acid/base cluster around Y(Z) was affected. Apparently, the divalent cations acted (electrostatically) on the proton release channel that connects the oxygen-evolving complex with the bulk water, but not on the ET between Y(Z) and P(+)(680), proper. Contrastingly, Ca(2+) and Mg(2+), when added at >5 mM, accelerated the slow component of P(+)(680) reduction by Y(Z) and shifted the apparent pK of Y(Z) from 7.4 to 6.6 and 6.7, respectively. It was evident that the binding site(s) for added Ca(2+) and Mg(2+) were close to Y(Z) proper. The data obtained are discussed in relation to the nature of the metal-binding sites in photosystem II.

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

“…Under those conditons, blocking of the HA Z site could not have occurred, because preincubation of Mn-depleted PSII core particles with Fe(II) for the length of time under weak light, necessary for blocking (49), did not occur. Nevertheless, some of the results from that work (74) support our suggestion that the decrease in the Y Z • reduction rate by iron blocking should be accompanied by an increase in the Y Z oxidation (P680 + reduction) rate. Ahlbrink et al (74) found that addition of Ca 2+ and Mg 2+ separately accelerated the reduction of P680 + , shifting the apparent pK app of base B from 7.4 to ∼6.7.…”

“…Nevertheless, some of the results from that work (74) support our suggestion that the decrease in the Y Z • reduction rate by iron blocking should be accompanied by an increase in the Y Z oxidation (P680 + reduction) rate. Ahlbrink et al (74) found that addition of Ca 2+ and Mg 2+ separately accelerated the reduction of P680 + , shifting the apparent pK app of base B from 7.4 to ∼6.7. Indeed, we observed a complementary effect of these cations in our flash-probe fluorescence experiments, where Ca 2+ and Mg 2+ decreased the rate of Y Z • reduction (unpublished results).…”

“…All these cations with the exception of Ca 2+ and Mg 2+ decreased the rate of P680 + reduction. Since Fe(II) decreased rather than increased the rate of electron transfer from Y Z to P680 + , we note that the Fe(II) in that study (74) was added just prior to the measurements. Under those conditons, blocking of the HA Z site could not have occurred, because preincubation of Mn-depleted PSII core particles with Fe(II) for the length of time under weak light, necessary for blocking (49), did not occur.…”

“…Thus, one would predict that blocking the HA Z site might be accompanied by an acceleration of Y Z oxidation (or P680 + reduction), corresponding to the restoration (at least partially) of the rate of electron transfer disrupted by Mn depletion. However, the effects of several metal cations (Co 2+ , Zn 2+ , Fe 2+ , Fe 3+ , Mn 2+ , Ca 2+ , and Mg 2+ ) on the rate of P680 + reduction were measured recently (74). All these cations with the exception of Ca 2+ and Mg 2+ decreased the rate of P680 + reduction.…”

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

“…Electron donation by added Mn(II) has been analyzed by the application of a number of spectroscopic methods such as monitoring the reduction of dichlorophenolindophenol (DCIP), a PSII electron acceptor (23,25,(28)(29)(30)(31)(32)35); measuring the yield and decay of flashinduced chlorophyll a fluorescence (24, 32-34, 39, 40); and observing Y Z • reduction by EPR (21,26,30). Several indirect methods of investigating Mn-binding sites have also been used, including inhibition of diphenylcarbazide (DPC) oxidation by micromolar concentrations of Mn(II) (the DPCinhibition assay) (28,31,(33)(34)(35)(36)(37)(38)(39), the effect of metal cations on the reduction of P680 + (30,41,42), and the application of amino acid modifiers (20,30,31,33,34,38).…”