Dataset Information

Structural Changes in the Acceptor Site of Photosystem II upon Ca2+/Sr2+ Exchange in the Mn4CaO5 Cluster Site and the Possible Long-Range Interactions.

ABSTRACT: The Mn4CaO5 cluster site in the oxygen-evolving complex (OEC) of photosystem II (PSII) undergoes structural perturbations, such as those induced by Ca2+/Sr2+ exchanges or Ca/Mn removal. These changes have been known to induce long-range positive shifts (between +30 and +150 mV) in the redox potential of the primary quinone electron acceptor plastoquinone A (QA), which is located 40 Å from the OEC. To further investigate these effects, we reanalyzed the crystal structure of Sr-PSII resolved at 2.1 Å and compared it with the native Ca-PSII resolved at 1.9 Å. Here, we focus on the acceptor site and report the possible long-range interactions between the donor, Mn4Ca(Sr)O5 cluster, and acceptor sites.

SUBMITTER: Koua FHM

PROVIDER: S-EPMC6722538 | biostudies-literature | 2019 Aug

REPOSITORIES: biostudies-literature

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Publications

Structural Changes in the Acceptor Site of Photosystem II upon Ca2+/Sr2+ Exchange in the Mn4CaO5 Cluster Site and the Possible Long-Range Interactions.

Koua Faisal Hammad Mekky FHM

Biomolecules 20190814 8

The Mn4CaO5 cluster site in the oxygen-evolving complex (OEC) of photosystem II (PSII) undergoes structural perturbations, such as those induced by Ca2+/Sr2+ exchanges or Ca/Mn removal. These changes have been known to induce long-range positive shifts (between +30 and +150 mV) in the redox potential of the primary quinone electron acceptor plastoquinone A (QA), which is located 40 Å from the OEC. To further investigate these effects, we rea ...[more]

PMID: 31416291

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Project description:The extracellular calcium-sensing receptor (CaSR) controls vital bone cell functions such as cell growth, differentiation and apoptosis. The binding of the native agonist (Ca2+) to CaSR activates the receptor, which undergoes structural changes that trigger a cascade of events along the cellular signaling pathways. Strontium (in the form of soluble salts) has been found to also be a CaSR agonist. The activation of the receptor by Sr2+ is considered to be the major mechanism through which strontium exerts its anti-osteoporosis effect, mostly in postmenopausal women. Strontium-activated CaSR initiates a series of signal transduction events resulting in both osteoclast apoptosis and osteoblast differentiation, thus strengthening the bone tissue. The intimate mechanism of Sr2+ activation of CaSR is still enigmatic. Herewith, by employing a combination of density functional theory (DFT) calculations and polarizable continuum model (PCM) computations, we have found that the Ca2+ binding sites 1, 3, and 4 in the activated CaSR, although possessing a different number and type of protein ligands, overall structure and charge state, are all selective for Ca2+ over Sr2+. The three binding sites, regardless of their structural differences, exhibit almost equal metal selectivity if they are flexible and have no geometrical constraints on the incoming Sr2+. In contrast to Ca2+ and Sr2+, Mg2+ constructs, when allowed to fully relax during the optimization process, adopt their stringent six-coordinated octahedral structure at the expense of detaching a one-backbone carbonyl ligand and shifting it to the second coordination layer of the metal. The binding of Mg2+ and Sr2+ to a rigid/inflexible calcium-designed binding pocket requires an additional energy penalty for the binding ion; however, the price for doing so (to be paid by Sr2+) is much less than that of Mg2+. The results obtained delineate the key factors controlling the competition between metal cations for the receptor and shed light on some aspects of strontium's therapeutic effects.

Project description:Highlights • We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system.• This model is used to interpret recently published data for Ca2+- and Sr2+-containing PSII in the S2 state, in which the g = 2.0 and g = 4.1 conformations coexist.• The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics.• Contrary to previous reports, there is no significant effect of substituting Sr2+ for Ca2+ on any of the exchange rate constants.• The exchange rate of the slowly-exchanging water (Ws) in the S2 state g = 4.1 conformation is much faster than that in the g = 2.0 conformation, consistent with the assignment of Ws to W1 or W2 bound as terminal ligands to Mn4. We derive a model that provides an exact solution to the substrate-water exchange kinetics in a double-conformation system and use this model to interpret recently published data for Ca2+- and Sr2+-containing PSII in the S2 state, in which the g = 2.0 and g = 4.1 conformations coexist. The component concentrations derived from the kinetic model provide an analytic description of the substrate-water exchange kinetics, allowing us to more accurately interpret the results. Based on this model and the previously reported data on the S2 state g = 2.0 conformation, we obtain the substrate-water exchange rates of the g = 4.1 conformation and the conformational change rates. Two conclusions are made from the analyses. First, contrary to previous reports, there is no significant effect of substituting Sr2+ for Ca2+ on any of the exchange rate constants. Second, the exchange rate of the slowly-exchanging water (Ws) in the S2 state g = 4.1 conformation is much faster than that in the S2 state g = 2.0 conformation. The second conclusion is consistent with the assignment of Ws to W1 or W2 bound as terminal ligands to Mn4; Mn4 has been proposed to undergo an oxidation state change from Mn(IV) in the g = 2.0 conformation to Mn(III) in the g = 4.1 conformation.

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Dataset Information

Structural Changes in the Acceptor Site of Photosystem II upon Ca2+/Sr2+ Exchange in the Mn4CaO5 Cluster Site and the Possible Long-Range Interactions.

Publications

Structural Changes in the Acceptor Site of Photosystem II upon Ca<sup>2+</sup>/Sr<sup>2+</sup> Exchange in the Mn<sub>4</sub>CaO<sub>5</sub> Cluster Site and the Possible Long-Range Interactions.

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