PARENT SESSION
Posters P1A Proton coupled electron transport and ATPase. Abstracts (172-180)

Alpha- and beta-cyclodextrins, two new uncouplers of electron transfer and ATP synthesis in the thylakoid membrane. Mario Fragata^*,1, Subhan Dudekula², ¹ Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada² Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada

ABSTRACT- Cyclodextrins (CD) are cyclic oligomers of D-glucose (Gluc) linked by alpha-1,4 glycosidic bonds. Alpha- and beta-CD, constituted respectively of 6 and 7 Gluc units, alter the lipid composition of the thylakoid membrane [Rawyler A, Siegenthaler PA (1996) Biochim. Biophys. Acta 1278: 89-97] and likely also its protein structure [Sridharan G, Gaudreau S, Dalstein L, Huiban C, Lejeune A, Fragata M (2001) Z. Naturforsch. 56c: 792-802]. We found (Dudekula S, Alvarez D, Sridharan G, Fragata M, submitted) that alpha- and beta-CD enhance the oxygen-evolving activity (OE) of photosystem II in thylakoid membranes (TM) from barley. While the OE increase is not greater than about 7 % up to 6 mM CD, at higher concentrations one observes a sharp OE enhancement: e.g., at 14-16 mM CD the OE increase is 80 % and 178 % in respectively alpha-CD- and beta-CD-treated TM. Moreover, the relationship between OE and CD concentration (OEvsCD) is represented by a S-shaped curve displaying a transition at 8-10 mM CD. In the present work, we studied the effect of the cyclodextrins on the ATP synthesis in isolated thylakoids using the luciferin-luciferase luminescence assay. The data disclose a 50 % inhibition of the ATP synthesis activity at CD concentrations of about 6 mM, i.e., below the onset concentration for the sharp OE enhancement transition seen in the OEvsCD curves. First, this work shows clearly that low concentrations of alpha- and beta-CD uncouple the electron transfer and the ATP synthesis in isolated thylakoid membranes. Secondly, at high concentration the cyclodextrins cause a sharp OE increase that may have its origin in structural rearrangements in TM. Thirdly, the twofold aspect of the CD effect has its counterpart in the activity pattern of other uncouplers [see McCarty RE (1980) Meth. Enz. 69: 719-728]. Work supported by a grant to MF from NSERC Canada.

KEY WORDS: oxygen evolution, uncoupling, cyclodextrins, ATP synthesis