The Structure of ATPsynthases in Photosynthesis and Respiration

Abstract

A large number of biochemical reactions in cells are coupled with the hydrolysis of ATP, e.g. biosynthesis, ion translocation, muscle contraction. The largest amount of ATP is generated by F-type H+-ATPases, which are membrane integrated enzymes occurring in the cytoplasma membranes of bacteria, inner mitochondrial membranes and thylakoid membranes. Together with A-type and V-type ATPases they form a specific protein family, which couples proton (Na+) translocation across the membrane with the synthesis or hydrolysis of ATP. ATPases of this family are found in all taxonomic kingdoms and comprise F-ATPases, A-ATPases in Archaea and some bacteria and V-ATPases in the inner membranes of eukaryotes. A- and F-ATPases can function in ATP hydrolysis and ATP synthesis, although their main physiological role is ATP synthesis in most organisms. In contrast, V-ATPases are dedicated proton pumps, which work only in the direction of ATP hydrolysis. The predecessor of these ATPases was already present in the last universal common ancestor and probably might have evolved from RNA transporting translocases, which were located in the primordial membranes. Due to this evolutionary relationship, A-, V- and F-ATPases have a common architecture, which comprises a membrane-embedded part (F0, A0 or V0) and an extrinsic part (F1, A1 or V1) separated by a connecting region. This architecture supports a rotary mechanism, in which the rotation of the membrane- embedded part is concomitant with the transport of H+ (or Na+) across the membrane. The rotation of the membrane-embedded rotor is conveyed via a central stalk in the connecting region to the extrinsic part, where it is coupled to conformational changes in the catalytic nucleotide binding sites that support ATP synthesis/hydrolysis. An additional peripheral connection between the extrinsic part and the membrane bound part prevents the co-rotation of the extrinsic part and thus supports efficient coupling. Although, atomic structures of the holoenzymes are still missing, atomic models of sub-complexes and subunits give detailed insights into how the rotary mechanism couples ion-translocation with ATP synthesis/hydrolysis in this protein family.