13.7
Pompe azionate ATP III: pompe di tipo V
Le pompe di tipo V sono pompe guidate da ATP che si trovano nelle membrane vacuolari delle piante, nei lieviti, nelle membrane endosomiali e lisosomia…
Le pompe vacuolari o di tipo V sono un tipo di pompe azionate da ATP presenti principalmente sulle membrane dei compartimenti subcellulari eucariotici come i vacuoli delle piante, i lisosomi e gli endosomi.
Lepompe a V sono strutture simili a turbine con due domini: V1 e V0.
Il dominio transmembrana V0 comprende più subunità, tra cui una subunità a e un anello di subunità c che attraversano la membrana.
Il dominio citosolico V1 è costituito anche da numerose subunità, tra cui un esamero di subunità A e B alternate, un rotore e statori periferici.
Si consideri una pompa di tipo V presente sulla membrana lisosomiale di una cellula eucariotica.
Quando una molecola di ATP entra nella subunità esamerica del dominio V1, viene idrolizzata in ADP e fosfato inorganico.
L'energia rilasciata dall'idrolisi dell'ATP fa ruotare il gambo centrale e le subunità dell'anello c del dominio V0.
Un protone dal citosol entra attraverso un canale nella subunità a e si lega a una subunità dell'anello c.
Contemporaneamente, un canale rivolto verso il lume della subunità a rilascia il protone nel lisosoma facendo spazio a un altro protone per legare la subunità c dal citosol.
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Q1: Where are V-type pumps located in eukaryotic cells?
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.
Q2: What are the two main structural domains of V-type pumps?
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.
Q3: How does ATP hydrolysis drive proton transport in V-type pumps?
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.
Q4: What cellular processes depend on V-type pump function?
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.
Q5: How do cells regulate V-type pump activity?
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.
Q6: What diseases are associated with V-type pump mutations?
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.
Q7: How does the a-subunit facilitate proton movement in V-type pumps?
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.