Important subunit interactions in the chloroplast ATP synthase

摘要

General structural features of the chloroplast ATP synthase are summarized highlighting differences between the chloroplast enzyme and other ATP synthases. Much of the review is focused on the important interactions between the ε and γ subunits of the chloroplast coupling factor 1 (CE_1) which are involved in regulating the ATP hydrolytic activity of the enzyme and also in transferring energy from the membrane segment, chloroplast coupling factor 0 (CF_0), to the catalytic sites on CF_1. A simple model is presented which summarizes properties of three known states of activation of the membrane-bound form of CF_1. The three states can be explained in terms of three different bound conformational states of the ε subunit. One of the three states, the fully active state, is only found in the membrane-bound form of CF_1. The lack of this state in the isolated form of CF_1, together with the confirmed presence of permanent asymmetry among the α, β and γ subunits of isolated CF_1, indicate that ATP hydrolysis by isolated CF_1 may involve only two of the three potential catalytic sites on the enzyme. Thus isolated CF_1 may be different from other F_1 enzymes in that it only operates on 'two cylinders' whereby the γ subunit does not rotate through a full 360 °during the catalytic cycle. On the membrane in the presence of a light-induced proton gradient the enzyme assumes a conformation which may involve all three catalytic sites and a full 360 °rotation of γ during catalysis.