Evidence for impaired hydrogen-bonding of tyrosine YZ in calcium-depleted Photosystem II

Photosystem II(PS II) evolves oxygen from two bound water molecules in a four-stepped reaction that is driven by four quanta of light, each oxidizing the chlorophyll moiety P-680 to yield P-680(+). When starting from its dark equilibrium (mainly state Si), the catalytic center can be clocked through its redox states (S-0...S-4) by a series of short flashes of light. The center involves at least a Mn-4-cluster and a special tyrosine residue, named Y-Z, as redox cofactors plus two essential ionic cofactors, Cl- and Ca2+. Centers which have lost Ca2+ do not evolve oxygen. We investigated the stepped progression in dark-adapted PS II core particles after the removal of Ca2+. Y-Z was oxidized from the first flash on. The difference spectrum of Y-Z --> Y-Z(ox) differed from the one in competent centers, where it has been ascribed to a hydrogen-bonded tyrosinate. The rate of the electron transfer from Y-Z to P-680(+) was slowed down by three orders of magnitude and its kinetic isotope effect rose up from 1.1 to 2.5, Proton release into the bulk was now a prerequisite for the electron transfer from Y-Z to P-680(+). On the basis of these results and similar effects in Mn-(plus Ca2+-)depleted PS II (M. Haumann et al., Biochemistry, 38 (1999) 1258-1267) we conclude that the presence of Ca2+ in the catalytic center is required to tune the apparent pK of a base cluster? B, to which Y-Z is linked by hydrogen bonds. The deposition of a proton on B within close proximity of Y-Z (not its release into the bulk !) is a necessary condition for the reduction in nanoseconds of P-680(+) and for the functioning of water oxidation. The removal of Ca2+ rises the pK of B, thereby disturbing the hydrogen bonded structure of YZB. (C) 1999 Elsevier Science B.V. All rights reserved.