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Electron transport proton transfer and their relation to photophosphorylation in chloroplasts iii. effect of metabolic state on proton transport in chloroplasts


Electron transport proton transfer and their relation to photophosphorylation in chloroplasts iii. effect of metabolic state on proton transport in chloroplasts



Biologicheskie Membrany 2(8): 776-788



ISSN/ISBN: 0233-4755

The effects of buffer capacity of the thylakoid inner bulk phase on the photoinduced uptake of protons and the energy coupling were studied in isolated bean chloroplasts. The light induced binding of protons to buffering groups of thylakoid membrane is 1,5-2-fold lower under phosphorylation conditions (state 3) than that in the state of photosynthetic control (state 4). The number of protons accumulated in the bulk phase of matrix is virtually the same in teh states 3 and 4. The transmembrane pH difference (.DELTA. pH) was evaluated using various buffers capable of accumulating inside the thylakoids. The .delta. pH across the swollen thylakoids (chloroplasts in hypotonic medium) was estimated as ca. 2-3. Stimulation of photophosphorylation produced no considerable drop in .DELTA.pH. These findings are interpreted as the evidence for nonequilibrium distribution of protons between buffering groups of the matrix and membrane in the state 3. The kinetics of electron transport, rate of ATP synthesis and efficacy of energy coupling show no dependence on the buffer capacity of the thylakoid inner bulk phase. The obtained results suggest the possibility of alternative pathways for protons which are released during plastoquinol oxidation: release of protons into the inner bulk phase (state 4), and intramembrane proton transport via the ATP-synthetase complex (state 3), bypassing the aqueous inner bulk phase of thylakoids.

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Accession: 005347665

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