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

The catalytic Mn cluster of the photosynthetic oxygen-evolving system is ox idized via a tyrosine, Yz, by a photooxidized chlorophyll a moiety, P-680(). The rapid reduction of P-680(+) by Yz in nanoseconds requires the intact ness 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/bas e cluster from the acid into the neutral pH range. At alkaline pH the elect ron transfer (ET) from Yz to P-680(+) is still rapid (< 1 <mu>s), whereas a t 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 componen ts (see R. Ahlbrink, M. Haumann, D. Cherepanov, O. Bogershausen, A. Mulkidj anian, W. Junge, Biochemistry 37 (1998)). The overall kinetics of P-680(+) reduction by Yz in Mn-depleted photosystem II (PS II) has been previously s hown 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 res olution, aiming at the effect of divalent cations on the particular kinetic components of P-680(+). reduction. To our surprise we found only the slowe r, 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 t he slow component of P-680(+) reduction by Yz and shifted the apparent pK o f 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 phot osystem II. (C) 2001 Elsevier Science B.V. All rights reserved.