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 particlesстатья

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[1] 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 / R. Ahlbrink, B. K. Semin, A. Y. Mulkidjanian, W. Junge // Biochimica et Biophysica Acta - Bioenergetics. — 2001. — Vol. 1506, no. 2. — P. 117–126. The catalytic Mn cluster of the photosynthetic oxygen-evolving system is oxidized via a tyrosine, Yz, by a photooxidized chlorophyll a moiety, P-680(+). The rapid reduction of P-680(+) by Yz in nanoseconds requires the intactness of an acid/base cluster around Yz 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 Yz to P-680(+) is still rapid (< 1 <mu>s), whereas at acid pH the ET is much slower (10-100 mus) 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. Bogershausen, A. Mulkidjanian, W. Junge, Biochemistry 37 (1998)). The overall kinetics of P-680(+) reduction by Yz in Mn-depleted photosystem II (PS II) has been previously shown to be slowed down by divalent cations (added at > 10 muM), namely: Mn2+, Co2+, Ni2+, Cu2+, Zn2+ (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 Co2+/Zn2+ > Fe2+>Mn2+). Neither the fast component nor the apparent pK of the acid/base cluster around Yz was affected. Apparently, the divalent cations acted (electrostatically) on the proton release channel that connects the oxygen-evolving complex wit the bulk water, but non on the ET between Yz and P680+, proper. Contrastingly, Ca2+ and Mg2+, when added at >5 mM, accelerated the slow component of P-680(+) reduction by Yz and shifted the apparent pK of Yz from 7.4 to 6.6 and 6.7, respectively. It was evident that the binding site(s) for added Ca2+ and Mg2+ were close to Yz proper. The data obtained are discussed in relation to the nature of the metal-binding sites in photosystem II. (C) 2001 Elsevier Science B.V. All rights reserved. [ DOI ]

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