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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