P680+• reduction kinetics and redox transition probability of the water oxidizing complex as a function of pH and H D isotope exchange in spinach thylakoids

The rise of fluorescence as an indicator for P680(+.) reduction by Y-Z and the period-four oscillation of oxygen yield induced by a train of saturating flashes were measured in dark-adapted thylakoids as a function of pH in the absence of exogenous electron accepters. The results reveal that: (i) the average amplitude of the nanosecond kinetics and the average of the maximum fluorescence attained at 100 mu s after the flash in the acidic range decrease with decreasing pH; (ii) the oxygen yield exhibits a pronounced period-four oscillation at pH 6.5 and higher damping at both pH 5.0 and pH 8.0; (iii) the probability of misses in the S-i-state transitions of the water oxidizing complex is affected characteristically when exchangeable protons are replaced by deuterons [at pH <6.5, the ratio alpha(D)/alpha(H) is larger than 1 whereas at pH >7.0 values of <1 are observed]. The results are discussed within the framework of a combined mechanism for P680(+.) reduction where the nanosecond kinetics reflect an electron transfer coupled with a "rocket-type" proton shift within a hydrogen bridge from Y-Z to a nearby basic group, X [Eckert, H.-J., and Renger, G. (1988) FEBS Lett. 236, 425-431], and subsequent relaxations within a network of hydrogen bonds. It is concluded that in the acidic region the hydrogen bond between Y-Z and X (most likely His 190 of polypeptide D1) is interrupted either by direct protonation of X or by conformational changes due to acid-induced Ca2+ release. This gives rise to a decreased P680(+.) reduction by nanosecond kinetics and an increase of dissipative P680(+.)Q(A)(-.) recombination at low pH. A different mechanism is responsible for the almost invariant amplitude of nanosecond kinetics and increase of alpha in the alkaline region.