The Role of Hydrogen Bonds for the Multiphasic P680^+• Reduction by Y_Z in Photosystem II with Intact Oxyen Evolution Capacity. Analysis of Kinetic H/D Isotope Exchange Effects

Abstract

The mechanism of multiphasic P680(+)* reduction by YZ has been analyzed by studying H/D isotope exchange effects on flash-induced changes of 830 nm absorption, DeltaA830(t), and normalized fluorescence yield, F(t)/F0, in dark-adapted thylakoids and PS II membrane fragments from spinach. It was found that (a) the characteristic period four oscillations of the normalized components of DeltaA830(t) relaxation and of F(t)/F0 rise in the nanosecond and microsecond time domain are significantly modified when exchangeable protons are replaced by deuterons; (b) in marked contrast to the normalized steady-state extent of the microsecond kinetics of 830 nm absorption changes which increases only slightly due to H/D exchange (about 10%) the Si state-dependent pattern exhibits marked effects that are most pronounced after the first, fourth, fifth, and eighth flashes; (c) regardless of data evaluation by different fit procedures the results lead to a consistent conclusion, that is, the relative extent of the back reaction between P680(+)*QA-* becomes enhanced in samples suspended in D2O; and (d) this enhancement is dependent on the Si state of the WOC and attains maximum values in S2 and S3, most likely due to a retardation of the "35 micros kinetics" of P680(+)* reduction. In an extension of our previous suggestion on the functional role of hydrogen bonding of YZ by a basic group X (Eckert, H.-J., and Renger, G. (1988) FEBS Lett. 236, 425-431), a model is proposed for the origin of the multiphasic P680(+)* reduction by YZ. Two types of different processes are involved: (a) electron transfer in the nanosecond time domain is determined by strength and geometry of the hydrogen bond between the O-H group of YZ and acceptor X, and (b) the microsecond kinetics reflect relaxation processes of a hydrogen bond network giving rise to a shift of the equilibrium P680(+)*YZ <==> P680YZ(OX) toward the right side. The implications of this model are discussed.

Environment of TyrZ in Photosystem II from Thermosynechococcus elongatus in which PsbA2 Is the D1 Protein

“…Refs. [43][44][45]. In this framework, the present data would thus point to a slower proton transfer process in PsbA2-PSII.…”

Biosynthetic Exchange of Bromide for Chloride and Strontium for Calcium in the Photosystem II Oxygen-evolving Enzymes

“…ϩ P 680 7 S n TyrZ ⅐ P 680 7 (S nϩ1 Tyr Z P 680 )Ј 7 S nϩ1 Tyr Z P 680 equilibria, where the (S nϩ1 Tyr Z P 680 )Ј denotes an unrelaxed state (82,83). A change in the protein relaxation processes is expected to modify one or more of these equilibria.…”

Inhibition of the oxygen-evolving complex of photosystem II and depletion of extrinsic polypeptides by nickel

“…8B). Thus, the presence of Ni 2+ may influence the specific orientation of Tyr Z or hinder its deprotonation that is needed for electron transfer to P680 + (Christen and Renger 1999).…”