| Abstract: | This review concerns the catalytic sector of F1 factor of the H+-dependent ATPases in mitochondria (MF1), bacteria (BF1) and chloroplasts (CF1). The three types of F1 have many similarities with respect to the structural parameters, subunit composition and catalytic mechanism. An alpha 3 beta 3 gamma delta epsilon stoichiometry is now accepted for MF1 and BF1; the alpha 2 beta 2 gamma 2 delta 2 epsilon 2 stoichiometry for CF1 remains as matter of debate. The major subunits alpha, beta and gamma are equivalent in MF1, BF1 and CF1; this is not the case for the minor subunits delta and epsilon. The delta subunit of MF1 corresponds to the epsilon subunit of BF1 and CF1, whereas the mitochondrial subunit equivalent to the delta subunit of BF1 and CF1 is probably the oligomycin sensitivity conferring protein (OSCP). The alpha beta gamma assembly is endowed with ATPase activity, beta being considered as the catalytic subunit and gamma as a proton gate. On the other hand, the delta and epsilon subunits of BF1 and CF1 most probably act as links between the F1 and F0 sectors of the ATPase complex. The natural mitochondrial ATPase inhibitor, which is a separate protein loosely attached to MF1, could have its counterpart in the epsilon subunit of BF1 and CF1. The generally accepted view that the catalytic subunit in the different F1 species is beta comes from a number of approaches, including chemical modification, specific photolabeling and, in the case of BF1, use of mutants. The alpha subunit also plays a central role in catalysis, since structural alteration of alpha by chemical modification or mutation results in loss of activity of the whole molecule of F1. The notion that the proton motive force generated by respiration is required for conformational changes of the F1 sector of the H+-ATPase complex has gained acceptance. During the course of ATP synthesis, conversion of bound ADP and Pi into bound ATP probably requires little energy input; only the release of the F1-bound ATP would consume energy. ADP and Pi most likely bind at one catalytic site of F1, while ATP is released at another site. This mechanism, which underlines the alternating cooperativity of subunits in F1, is supported by kinetic data and also by the demonstration of partial site reactivity in inactivation experiments performed with selective chemical modifiers. One obvious advantage of the alternating site mechanism is that the released ATP cannot bind to its original site.(ABSTRACT TRUNCATED AT 400 WORDS) |
