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

Abstract

The rise of fluorescence as an indicator for P680(+)* reduction by YZ 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 acceptors. The results reveal that: (i) the average amplitude of the nanosecond kinetics and the average of the maximum fluorescence attained at 100 micros 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 Si-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 YZ 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 YZ 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(+)* 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.

Effects of methanol on the S i -state transitions in photosynthetic water-splitting

“…An increase of a by about 10% is expected to give rise to a decrease of the average oxygen yield per single turnover flash of a similar extent (Messinger and Renger 1994;Christen et al 1999). Indeed a small decrease of oxygen yield is observed in the FIOPs displayed in Fig.…”

“…The miss parameter (probability that no S i state advancement occurs) maybe affected by all redoxequilibria and kinetics within PSII and is therefore a very sensitive probe for changes within PSII. The magnitude of a strongly depends on pH and temperature (Messinger and Renger 1994;, and is also affected by H/D isotope exchange (Christen et al 1999).…”

Role of Hydrogen Bonds in Photosynthetic Water Splitting

“…In competition with these kinetics, P680 þ * also becomes reduced by Q À A * , with half-lifetimes of 100-200 ms [70,71], thus giving rise to misses in the advancement of the Kok cycle. This back reaction does not exhibit a significant kinetic H/D exchange effect [72].…”

“…The possibility of an LBHB configuration for the Y Z -His 190 pair was first discussed in Ref. [72], butwas not favoured in spite of the striking coincidence of the calculated activation energy of about 10 kJ mol À1 [76] for an LBHB array with the experimental value for the 'fast' nanosecond kinetics of P680 þ * reduction by Y Z [68]. Later, however, the existence of an LBHB configuration was proposed on the basis of low-temperature EPR measurements [91], and this idea was further extended by the assumption that the nature of the hydrogen bond switches from an LBHB configuration in state Y Z His 190 to a normal (weak) hydrogen bond in state Y Z * His 190H þ [92].…”

“…A pK a value of <5.0 was estimated from titration experiments on PS II membrane fragments from spinach [72]. Measurements of the amplitude of the 'fast' nanosecond component as a function of pH in PS II core complexes from T. elongatus revealed that the decrease in the acidic region can be described by a single pK a of 4.6 [111].…”