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#ATPases

3 public questions tagged with this topic.

Membrane Transportation

Which of the following describes V-class ATPases?

V-class ATPases, vacuolar type H+-ATPases, are multi-protein rotary pumps distinct from glucose carriers SGLT or drug exporters ABCB1. They hydrolyze ATP in cytosolic V1 sector comprising A3B3 catalytic hexamer and use released energy to rotate central stalk and translocate protons through membrane-embedded Vo sector containing c-ring proteolipids and subunit a. Cellular localization includes lysosomes, endosomes, trans-Golgi network, synaptic vesicles, secretory granules, osteoclast ruffled border and kidney intercalated cell apical membrane. By acidifying compartments to pH 4.5-5.5 they create environment for acid hydrolase activation, protein processing, neurotransmitter loading via proton-coupled secondary transporters, and iron release from transferrin. In osteoclasts and kidney they pump H+ outward contributing to bone resorption and urine acidification. They do not transport glucose, which uses SGLT and GLUT families, nor export lipophilic drugs, which uses ABC transporters, and they are not restricted to bacteria but broadly expressed in eukaryotes for organellar pH regulation vital for autophagy and infection defense.

Ref: Forgac, Nat Rev Mol Cell Biol 2007, V-ATPase function; Nelson & Harvey, Annu Rev Cell Biol 1999.

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Membrane Transportation

F-type ATPases differ from other ATPases because they:

F-type ATPases, historically termed FoF1 ATP synthases, represent an evolutionary unique class of rotary translocases that normally synthesize rather than consume ATP. While P-type and V-type pumps hydrolyze ATP to build H+ or Ca2+ gradients, F-type complexes harness pre-existing proton motive force created by electron transport chains in mitochondria, chloroplast thylakoids, and bacterial plasma membranes. The membrane-embedded Fo sector contains an oligomeric c-ring that binds protons via conserved carboxylate, rotating against subunit a as protons move down gradient. This rotation drives the central gamma-epsilon stalk inside the alpha3beta3 F1 hexamer, forcing beta subunits through open, loose and tight conformations that bind ADP and inorganic phosphate and condense them into ATP via binding-change mechanism. Isolated enzymes reversibly hydrolyze ATP to pump protons when proton motive force collapses, but physiological role is ATP production, supplying bulk of cellular ATP for biosynthesis, motility and transport work. Such chemiosmotic coupling explains oxidative phosphorylation and photophosphorylation efficiency.

Ref: Stock et al., Curr Opin Struct Biol 2000, ATP synthase rotary mechanism; Alberts, Chapter 14.

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Membrane Transportation

V-class ATPases differ from P-class ATPases in that:

Distinguishing pump families centers on mechanism of coupling ATP hydrolysis to ion translocation. P-class pumps such as Na+/K+ ATPase, SERCA, PMCA and H+/K+ ATPase share formation of covalent phosphoenzyme intermediate on conserved aspartate within DKTGT motif, inhibited by vanadate mimicking phosphate. They typically transport monovalent or divalent cations like sodium, potassium, calcium, heavy metals with fixed stoichiometry. V-class proton pumps acidifying vacuoles, lysosomes, endosomes and Golgi represent rotary ATPases composed of cytosolic V1 domain peripheral stalk and membrane Vo proteolipid ring. They hydrolyze ATP in V1 to drive rotation of central stalk and c-ring translocating protons without ever forming phosphoprotein, mechanism analogous to F-type synthase but operating exclusively as proton ATPase. Importantly substrate specificity limited to protons, not sodium, potassium or calcium. Claim they transport Na+ and K+ misassigns function; Na+/K+ exchange belongs to P-class. Therefore difference lies in absence of phosphorylated intermediate and proton specific rotary mechanism versus phosphoenzyme mediated cation exchange.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 11: V-Type vs P-Type ATPases - Phosphorylation Difference.

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