Recent developments on structural and functional aspects of the F1 sector of H+-linked ATPases

Publisher Website
Google Scholar
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 Ft have many similarities with respect to the structural parameters, subunit composition and catalytic mechanism. An α3β3γ2δ2ε2 stoichiometry is now accepted for MF1 and BF1; the α2β2γ2δ2ɛ2 stoichiometry for CFI remains as matter of debate. The major subunits α, β and γ are equivalent in MF1, BF1 and CF1; this is not the case for the minor subunits δ and ε. The δ subunit of MFI corresponds to the ε subunit of BF1 and CF1, whereas the mitochondria) subunit equivalent to the δ subunit of BF1 and CF1 is probably the oligomycin sensitivity conferring protein (OSCP). The a β γ assembly is endowed with ATPase activity, β being considered as the catalytic subunit and y as a proton gate. On the other hand, the 6 and E subunits of BFI and CFI most probably act as links between the F1 and F0 sectors of the ATPase complex. The natural mitochondria) ATPase inhibitor, which is a separate protein loosely attached to MF1, could have its counterpart in the E subunit of BF1 and CF1. The generally accepted view that the catalytic subunit in the different F1 species is β comes from a number of approaches, including chemical modification, specific photolabeling and, in the case of BF1, use of mutants. The a subunit also plays a central role in catalysis, since structural alteration of a 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. The chemistry of the condensation reaction of ADP and Pi to form ATP has not yet been elucidated. Although implicitly admitted, definite evidence that the condensation reaction does not involve a phosphorylated intermediate has been acquired recently by analysis of the stereochemical course of the phosphoric residue transfer in ATP synthesis or hydrolysis. Whereas the catalytic events of ATP synthesis are well understood, the regulatory mechanism, and particularly the role of the so-called inhibitory peptides, remain enigmatic.

This publication has 0 references indexed in Scilit: