Important subunit interactions in the chloroplast ATP synthase

General structural features of the chloroplast ATP synthase are summarized highlighting differences between the chloroplast enzyme and other ATP syntheses. Much of the review is focused on the important interactions between the epsilon and gamma subunits of the chloroplast coupling factor 1 (CF1) 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 (CF0). to the catalytic sites on CF1. A simple model is presented which summarizes propel ties of three known states of activation of the membrane bound form of CF1. The three states can be explained in terms of three different bound conformational states of the epsilon Subunit. One of the three slates, the fully active state, is only found in the membrane-bound form of CF1. The lack of this state in the isolated form of CF1, together with the confirmed presence of permanent asymmetry among the alpha, beta and gamma subunits of isolated CF1, indicate that ATP hydrolysis by isolated CF1 may involve only two of the three potential catalytic sires on the enzyme. Thus isolated CF1 may be different from other Fl enzymes in that it only operates on two cylinders whereby the gamma subunit does not relate through a full 360 degrees 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 degrees rotation of gamma during catalysis. (C) 2000 Elsevier Science B.V. All rights reserved.