![]() ![]() The structure is divided into the membrane-integral F o moiety, comprising subunits ab 2c 9–12, and the peripheral F 1 part, composed of subunits α 3β 3γδε. The Na + (Li + or H +)-translocating ATP synthase ofįig. This torque is required to liberate, via rotation of the γ subunit, tightly bound ATP from catalytic sites in F 1. Here, we will give a detailed account of the essential importance of the membrane potential in generating rotational torque by the F o motor. Remarkably, the ATP synthase of chloroplasts or Escherichia coli is dependent on the ΔΨ component, because no ATP synthesis can be measured in its absence in spite of considerable ΔpH values. However, these results have recently been challenged: it was shown that the ‘acid bath procedure’ used in all these experiments not only generates the desired ΔpH but also a ΔΨ of significant size ( Kaim and Dimroth, 1998a, 1999). These results were corroborated and extended with the reconstituted ATP synthases of chloroplasts or other sources, leading to the conclusion that ΔpH and ΔΨ are not only thermodynamically but also kinetically equivalent driving forces for the synthesis of ATP (Junesch and Gräber, 1991 Turina et al., 1991). ![]() ![]() Important early studies reported on ATP synthesis by thylakoid membranes energised with either ΔpH or ΔΨ ( Jagendorf and Uribe, 1966 Uribe, 1973), and similar results were obtained using submitochondrial particles ( Thayer and Hinkle, 1975). However, this equilibrium equation says nothing about how the two components contribute to the kinetics of ATP formation during normal operation this depends on the mechanism of energy conversion. Hence, thermodynamically, both parameters of the proton-motive force undoubtedly contribute driving forces for the synthesis of ATP according to equation 1. the / ratios, in the various organisms ( Hoffmann and Dimroth, 1991 Krulwich, 1995). In spite of these variations, there is a remarkable similarity in the phosphorylation potentials, i.e. For the alkaliphiles, which keep their internal pH at around 8.5, the membrane potential not only serves as the exclusive thermodynamic driving force for ATP synthesis, but it also has to compensate for the counteracting inverse ΔpH. Extreme species are the alkaliphiles and acidophiles, which have specialised to grow at pH values above 10 or below 4, respectively. Bacteria are remarkably flexible and are able grow in environments with a wide range of pH values. In contrast, chloroplasts appear to operate mainly on ΔpH which, at approximately 3 pH units in the steady state, far exceeds the ΔΨ of :::50 mV (Gräber and Witt, 1976). In mitochondria, the ΔΨ component significantly exceeds the ΔpH component and is therefore regarded as the main driving force for ATP production. The contribution of either parameter to the proton-motive force varies widely in different organisms. ![]()
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