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 P680 to yield P+680. When starting from its dark equilibrium (mainly state S1), the catalytic center can be clocked through its redox states (S0ellipsisS4) by a series of short flashes of light. The center involves at least a Mn4-cluster and a special tyrosine residue, named YZ, 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+. YZ was oxidized from the first flash on. The difference spectrum of YZ-->YoxZ differed from the one in competent centers, where it has been ascribed to a hydrogen-bonded tyrosinate. The rate of the electron transfer from YZ 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 YZ 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 YZ is linked by hydrogen bonds. The deposition of a proton on B within close proximity of YZ (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.