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ATP Synthase Activation and Photosynthesis

The chloroplast ATP synthase complex catalyzes the synthesis of ATP from ADP and free phosphate by coupling anhydride bond formation with the proton transmembrane electrochemical potential (µ_H⁺) across the thylakoid membrane. The light-dependent regulation of this complex depends on an intricate but ill-defined interplay between the proton electrochemical potential across the thylakoid membrane and thioredoxin-mediated redox modulation of a cysteine/disulfide bridge located on the ATP synthase -subunit.

cfq mutants experience compromised photosynthesis under low light conditions.

To investigate the physiological significance of the thiol regulation, Guosheng Wu and colleagues screened mutants of ATP synthase in Arabidopsis thaliana for candidates whose redox responses differed from that of wild type. One very interesting mutant, cfq (coupling factor quick recovery), had a point mutation in the atpC1 gene and caused an amino acid substitution (E244K) in the -subunit near the location of the regulatory disulfide. The authors found that the mutant disulfide bond was energetically more difficult to reduce resulting in a significantly lower ATP synthase activity in isolated chloroplasts containing the mutant. Subsequently, the overall rate of intact leaf photosynthesis in the cfq mutant is compromised under low light. The evidence that this mutation in the -subunit alters the redox potential of the ATP synthase was further demonstrated by atpC1 rescue experiments. The importance of this study lies in these results providing direct evidence that the energy requirement (via the electrochemical potential) for activation of the chloroplast ATP synthase is modulated by the thioredoxin-dependent reduction of the -subunit disulfide bond. The results also demonstrate for the first time that the activation state of the ATP synthase can limit leaf level photosynthesis.

FOOTNOTES

See referenced article, J. Biol. Chem. 2007, 282, 36782-36789

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