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Proton channel of the chloroplast ATP synthase, CF0: Its time-averaged single-channel conductance as function of pH, temperature, isotopic and ionic medium composition

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Summary

The proton-driven ATP synthase of chloroplasts is composed of two elements, CF0 and CF1. The membrane bound CF0 conducts protons and the peripheral CF1 interacts with nucleotides. By flash spectrophotometric techniques applied to thylakoid membranes from which about 50% of total CF1 was removed, we have previously determined the protonic (timeaveraged) single-channel conductance of CF0. Being in the order of 1 pS, it was sufficiently large to support the proposed role of CF0 as a low-impedance access for protons to the coupling site in CF0CF1. On the other hand, it was too large to be readily reconciled with current concepts of proton supply to and proton conduction through the channel.

We studied the time-averaged single-channel conductance of CF0 under variation of pH, pD, ionic composition, temperature, and water/membrane structure with the following results: (i) CF0 was proton-specific even against a background of 300mm monovalent or 30mm divalent catins. (ii) While the conductance of CF0 was pH/pD-independent in the range from 5.6–8.0, in D2O it was lower by a constant factor of 1.7 than in H2O (iii) Addition of glycerol diminished the conductance and abolished the isotope effect. (iv) The Arrhenius activation energy was 42 kJ/mol and thus intermediate between the ones found for the water-filled pore, gramicidin (30 kJ/mol), and the mobile carrier, valinomycin (65 kJ/mol).

The results implied that CF0 is endowed with an extremely proton-specific (107-fold) selectivity filter. Its conductance is very high, and its conduction cycle is not necessarily rate limited by a protolytic reaction. The mechanisms of rapid proton supply to the channel mouth and of proton conduction remained enigmatic.

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  1. Biophysik, Fachbereich Biologie/Chemie, Universität Osnabrück, 4500, Osnabrück, West Germany

    Gerd Althoff, Holger Lill & Wolfgang Junge

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  1. Gerd Althoff
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  2. Holger Lill
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  3. Wolfgang Junge
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Althoff, G., Lill, H. & Junge, W. Proton channel of the chloroplast ATP synthase, CF0: Its time-averaged single-channel conductance as function of pH, temperature, isotopic and ionic medium composition. J. Membrain Biol. 108, 263–271 (1989). https://doi.org/10.1007/BF01871741

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