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Biophysical Journal 85:1317-1325 (2003)
© 2003 The Biophysical Society

Sucrose and Glycerol Effects on Photosystem II

Kelly M. Halverson and Bridgette A. Barry

Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, St. Paul, Minnesota 55108-1022

Correspondence: Address reprint requests to Bridgette A. Barry, 140 Gortner Laboratory, 1479 Gortner Ave., BMBB, University of Minnesota, St. Paul, MN 55108-1022. Tel.: 612-624-6732; Fax: 612-625-5780; E-mail: barry{at}cbs.umn.edu.

Photosystem II catalyzes the oxidation of water and the reduction of plastoquinone. The active site cycles among five oxidation states, which are called the Sn states. PSII purification procedures include the use of the cosolvents, sucrose and/or glycerol, to stabilize water splitting activity and for cryoprotection. In this study, the effects of sucrose and glycerol on PSII were investigated. Sucrose addition was observed to stimulate the steady-state rate of oxygen evolution in the range from 0 to 1.35 M. Glycerol addition was observed to stimulate oxygen evolution in the range from 0 to 30%. Both cosolvents were observed to be inhibitory at higher concentrations. Sucrose addition was shown to have no effect on the rate of QA- oxidation or on the KM for exogenous acceptor. PSII was then treated to remove extrinsic proteins. In these samples, sucrose addition stimulated activity, but glycerol addition was inhibitory at concentrations higher than ~0.5 M. This inhibitory effect of glycerol at relatively low concentrations is attributed to glycerol binding to the active site, when extrinsic subunits are not present. Reaction induced FTIR spectra, associated with the S1 to S2 transition of the water-oxidizing complex, exhibited significant differences throughout the 1,800–1,200 cm-1 region, when glycerol- and sucrose-containing samples were compared. These measurements suggest a cosolvent-induced shift in the pKA of an aspartic or glutamic acid side chain, as well as structural changes at the active site. These structural alterations are attributed to a change in preferential hydration of the oxygen-evolving complex.




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