ATP-Driven Proton Pumps


ATP-Driven Proton Pumps
F-Type ATPases
The F-type ATPase active transporters play a central role in energy-conserving reactions in mitochondria, bacteria, and chloroplasts; .The F-type ATPases catalyze the uphill transmembrane passage of protons driven by ATP hydrolysis (“F-type” originated in the identification of these ATPases as energy-coupling factors). The Fo integral membrane protein complex (subscript o denoting its inhibition by the drug oligomycin) provides a transmembrane pore for protons, and the peripheral protein F1 (subscript 1 indicating that it was the first of several factors isolated from mitochondria) is a molecular machine that uses the energy of ATP to drive protons uphill (into a region of higher H_ concentration).

       
  F-Type ATPases

The FoF1 organization of proton pumping transporters must have developed very early in evolution. Eubacteria such as E. coli use an FoF1 ATPase complex in their plasma membrane to pump protons outward, and archaebacteria have a closely homologous proton pump, the AoA1 ATPase. The reaction catalyzed by F-type ATPases is reversible, so a proton gradient can supply the energy to drive the reverse reaction, ATP synthesis . When functioning in this direction, the F-type ATPases are more appropriately named ATP synthases. ATP synthases are central to ATP production in mitochondria during oxidative phosphorylation and in chloroplasts during photophosphorylation, as well as in eubacteria and archaebacteria. The proton gradient needed to drive ATP synthesis is produced by other types of proton pumps powered by substrate oxidation or sunlight.

V-type ATPases
A class of proton-transporting ATPases structurally (and possibly mechanistically) related to the F-type ATPases, are responsible for acidifying intracellular compartments in many organisms (thus V for vacuolar). Proton pumps of this type maintain the vacuoles of fungi and higher plants at a pH between 3 and 6, well below that of the surrounding cytosol (pH 7.5). V-type ATPases are also responsible for the acidification of lysosomes, endosomes, the Golgi complex, and secretory vesicles in animal cells. All V type ATPases have a similar complex structure, with an integral (transmembrane) domain (Vo) that serves as a proton channel and a peripheral domain (V1) that contains the ATP-binding site and the ATPase activity. The mechanism by which V-type ATPases couple ATP hydrolysis to the uphill transport of protons is not understood in detail.
                         

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