المضخات III المحفزة بال ATP: مضخات من النوع V

V-type pumps are ATP-driven pumps found on membranes of eukaryotic subcellular compartments including plant vacuoles, lysosomes, and endosomes. They also occur on plasma membranes of specialized eukaryotic cells and some prokaryotes. These pumps are essential for maintaining proton gradients across these membrane systems.

V-type pumps consist of two turbine-like domains: the cytosolic V1 domain, which contains eight subunits including a hexamer of alternating A and B subunits and performs ATP hydrolysis, and the transmembrane V0 domain, which contains at least five subunits and transports protons across the membrane.

When ATP enters the hexameric subunit of the V1 domain, it hydrolyzes into ADP and inorganic phosphate. The energy released rotates the central stalk and c-ring subunits of the V0 domain. This rotation allows protons from the cytosol to bind c-ring subunits and be released into the organelle lumen against the concentration gradient.

V-type pump proton translocation activity is vital for pH homeostasis, endocytosis, protein trafficking, urine acidification, and neurotransmitter release. These pumps maintain proton gradients necessary for proper organellar function and cellular signaling. Dysfunction of these pumps can impair multiple critical cellular processes.

Cells regulate V-type pumps through reversible dissociation of V0 and V1 domains triggered by nutrients and growth factors. Disulfide bond formation at cysteine residues of the A-subunit can prevent ATP hydrolysis. In epithelial cells, pump density modulation occurs through reversible fusion of vesicles containing high densities of V-type pumps with the apical membrane.

Mutations in V-type pump subunits are associated with renal tubular acidosis, osteoporosis, and neurodegenerative diseases. Complete loss of pump function can be lethal, making these pumps critical for survival. Understanding V-type pump dysfunction has identified them as potential drug targets for treating these conditions.

The a-subunit of the V0 domain contains two channels: one facing the cytosol where protons enter and bind to c-ring subunits, and another facing the organelle lumen where protons are released. As protons are released into the lumen, space becomes available for new protons to bind from the cytosol, creating a continuous pumping cycle.