Effect of Dehydration on Light-Induced Reactions in Photosystem II:  Photoreactions of Cytochrome b559

Abstract

The effect of dehydration on the reaction pattern of photosystem II (PS II) has been studied by measuring and analyzing spectral changes induced by continuous wavelength illumination in films of untreated and hydroxylamine-washed PS II membrane fragments dehydrated to different levels. The obtained data revealed (i) the extent of light-induced formation of about one Q(A)(-*)per 230 chlorophylls (Chl) remains virtually invariant to dehydration down to the lowest values of relative humidity (6-8% RH); (ii) a decrease of the RH to 30% leads to severe blockage of the electron transfer from Q(A)(-*) to Q(B) and the progressive replacement of water oxidation by photooxidation of high potential (HP) cytochrome (Cyt) b559 in untreated PS II samples or accessory Chl and carotenoid (Car) molecules in samples with preoxidized Cyt b559; (iii) photooxidation of Cyt b559 is followed by its photoreduction, concomitant with photooxidation of Chl and Car; (iv) in dry samples with preoxidized Cyt b559, not more than a half of total Cyt b559 can be photochemically reduced, independent of the extent of Cyt b559 in the HP form; (v) at low RH values, Cyt b559 photoreduction in samples with preoxidized heme groups and photoaccumulation of Q(A)(-*) take place with biphasic kinetics with similar rate constants for both processes; (vi) Cyt b559 photoreduction in dry samples is DCMU insensitive, while the dark rereduction of photooxidized Cyt b559 is inhibited by DCMU; (vii) fast and slow kinetic phases of Cyt b559 photoreduction dramatically differ in their dependencies on the intensity of CW illumination and are associated with electron donation to Cyt b559 from Q(A)(-*) and pheophytin(-*), respectively. The pathways of light-induced electron transfer in PS II involving Cyt b559 are discussed.

Carboxylate Shifts Steer Interquinone Electron Transfer in Photosynthesis

“…Our DFT results suggest that redistribution of charges in the Q A Ϫ Fe(histidine) 4 Likely, there are further changes in the extended H-bonded network around the Q A FeQ B site (52-54), which could not be explicitly included in this investigation because of limited structural information from crystallography. Removal of critical water molecules may explain the blockage of Q A Ϫ 3 Q B ET after dehydration of reaction centers (55,56). Further studies are required to clarify whether a spin state change of Fe(II) parallels the coordination change.…”

The functional role of protein dynamics in photosynthetic reaction centers investigated by elastic and quasielastic neutron scattering

“…A comparison with the QENS data of PS II membrane fragments suggests that there is a close correlation between the thermal activation of molecular dynamics and the electron transfer efficiency from Q A to Q B [10,13]. At the same time, the suppression of molecular dynamics upon dehydration [10,11,13] seems to be responsible for the inhibition of Q A to Q B electron transfer below 45% r.h. [7].…”

“…For example, the light-induced primary charge separation remains fully functional even at very low temperatures, while the Q A → Q B electron transfer is completely inhibited below about 200 K [6] and below a hydration of 45% relative humidity (r.h.) [7]. Likewise, individual reaction steps of the water oxidizing complex possess a Corresponding author: pieper@ut.ee a characteristic dependence on temperature (see [1] and references therein).…”

Recovery of photosystem�I and II activities during re-hydration of lichen Hypogymnia physodes thalli

“…Recently, it was shown that the optical properties of PSII membrane fragments remain unaltered if dehydrated as thin films (Kaminskaya et al 2003). Hence the change in the optical path length of light is unlikely to affect the absorbance changes at 830 nm in measurements with dried thalli.…”