The major source of energy in the biosphere is sun- light. However, only oxygenic photosynthetic organisms are able to convert the energy of photons emitted by the Sun into the energy of chemical bonds of carbohydrates by splitting water into molecular oxygen and reducing equivalents (electrons and protons). The pigment–pro- tein complex of photosystem 2 (PS 2) embedded into the thylakoid membranes of cyanobacteria and chloroplasts functions as a light-driven water-plastoquinone oxidore- ductase [1, 2]. All the organic and inorganic redox-active cofactors involved in charge transfer reactions are believed to reside on the D1 and D2 subunits of the reac- tion center (RC). A Mn 4 CaO 5 cluster together with its coordinating amino acids and four water molecules form a catalytic site where oxidation of water molecules occurs [3-7]. It should be noted that the water-oxidizing com- plex (WOC), which is located on the donor side of the enzyme, is the most fragile site within PS 2 and is easily susceptible to oxidative damage. The use of osmolytes in the stabilization of biomole- cules is an old trick of Nature ([8-13] and references therein). In the presence of these small molecules, photo- synthetic organisms counteract various stress conditions that they encounter. The osmolytes range from sugars to polyols, amino acids and their derivatives, and so forth. Among these, in recent years trehalose, which is natural- ly produced by several species of eubacteria, archaea, some fungi, certain invertebrates, and lower plants, has received considerable attention [14, 15]. The specific physical and chemical characteristics of trehalose such as relative inertness of the glycosidic link- age, the existence of both crystalline and amorphous states, thermostability (melting point, 203°C), high glass transition temperature, high stability over a wide pH range (3.5-10.0), and high hydrophilicity (water solubili- ISSN 0006-2979, Biochemistry (Moscow), 2015, Vol. 80, No. 1, pp. 61-66. © Pleiades Publishing, Ltd., 2015. Published in Russian in Biokhimiya, 2015, Vol. 80, No. 1, pp. 79-86. Originally published in Biochemistry (Moscow) On-Line Papers in Press, as Manuscript BM14-208, December 28, 2014. 61 Abbreviations: DCBQ, 2,6-dichloro-p-benzoquinone; DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea; P680, primary elec- tron donor; PS 2, photosystem 2; Q A and Q B , primary and sec- ondary quinone acceptors; RC, reaction center; WOC, water- oxidizing complex; Y Z , redox-active tyrosine 161 of D1 protein. * To whom correspondence should be addressed. Effect of Trehalose on Oxygen Evolution and Electron Transfer in Photosystem 2 Complexes M. D. Mamedov 1,2 *, I. O. Petrova 1 , D. V. Yanykin 2 , A. A. Zaspa 1 , and A. Yu. Semenov 1,3 1 Belozersky Institute of Physical-Chemical Biology, Lomonosov Moscow State University, 119991 Moscow, Russia; fax: +7 (495) 939-3181; E-mail: mahirmamedov@yandex.ru 2 Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia; fax: +7 (496) 733-0532; E-mail: ya-d-ozh@rambler.ru 3 Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, 117977 Moscow, Russia; fax: +7 (495) 651-2191; E-mail: semenov@genebee.msu.ru Received July 14, 2014 Revision received September 2, 2014 Abstract—The pigment–protein complex of photosystem 2 (PS 2) catalyzes the light-driven oxidation of water molecule and the reduction of plastoquinone. In this work, we studied the effect of the disaccharide trehalose, which is unique in its physicochemical properties, on isolated PS 2 complex. It was found that trehalose significantly stimulated the steady-state rate of oxygen evolution. The study of single flash-induced fluorescence decay kinetics demonstrated that trehalose did not affect the rate of Q A – oxidation, although it led to an increase in the relative fractions of PS 2 reaction centers capable of Q A – oxidation. Trehalose also prevented PS 2 complexes from being inactivated on prolonged storage. We propose that in the presence of trehalose, which affects the extent of hydration, the protein can preferentially exist in a more optimal confor- mation for effective functioning. DOI: 10.1134/S0006297915010071 Key words: photosystem 2, water-oxidizing complex, oxygen evolution, trehalose, chlorophyll fluorescence, plastoquinone oxidation