Lecture 7 - ATPases Flashcards
(13 cards)
P-type ATPases
All pump cations (positively charged ions like H⁺, Na⁺, K⁺, Ca²⁺).
Use energy from ATP
All inhibited by micromolar concentrations of orthovanadate, a phosphate analog that blocks their phosphorylation cycle.
Named “P-type” because they form a phosphorylated intermediate during their pump cycle.
p-type atpase: Na⁺/K⁺ ATPase (Animal Cells)
Stoichiometry: Pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed.
Functions:
Maintains high intracellular K⁺ and low intracellular Na⁺ — vital for action potential generation in neurons and muscles.
Creates a Na⁺ electrochemical gradient used to drive secondary active transport (e.g., glucose uptake).
Inhibitor: Ouabain — a cardiac glycoside that binds to and inhibits the pump.
p-type atpase: Fungal and Plant H⁺ ATPase
Location: Plasma membrane of plants and fungi.
Stoichiometry: Pumps 1 H⁺ out per ATP hydrolyzed.
Functions:
Expel excess H⁺ produced by metabolism.
Generate a H⁺ electrochemical gradient to drive H⁺-coupled transport.
Maintain strong negative membrane potential (>-200 mV).
Regulate cytosolic pH by removing protons.
Acidify the extracellular environment, which helps to loosen plant cell walls during growth.
P-type atpase: Sarcoplasmic Endoreticulum Ca²⁺ ATPase (SERCA)
Location: Sarcoplasmic reticulum (SR) in muscle cells.
Stoichiometry: Pumps 2 Ca²⁺ into the SR per ATP hydrolyzed.
Functions:
Restores low cytosolic Ca²⁺ levels after muscle contraction.
Allows muscle relaxation by resequestering Ca²⁺ into the SR.
Structure: Single α-subunit (with 3 isoforms).
Inhibitor: Thapsigargin — blocks Ca²⁺ uptake by SERCA.
P-type ATPase: Plasma Membrane Ca²⁺ ATPase (PMCA ATPase)
Location: Plasma membranes of fungi, plants, and animals.
Stoichiometry: Exports 1-2 Ca²⁺ ions in exchange for H⁺, per ATP hydrolyzed.
Functions:
Maintains low cytosolic Ca²⁺, preventing Ca²⁺ toxicity.
Crucial for cell signalling (Ca²⁺ is a second messenger).
Structure: Single α-subunit.
P-type ATPase: Gastric Mucosal H⁺/K⁺ ATPase
Location: Plasma membrane of gastric epithelial cells (lining of the stomach).
Stoichiometry: Exchanges 2 K⁺ for 2 H⁺ per ATP hydrolyzed (electroneutral).
Functions:
Secretes H⁺ into the stomach lumen, creating the highly acidic environment (about 0.16 M HCl).
Essential for digestion and protection against pathogens.
CPx-ATPases:
Special subclass of P-type ATPases.
Transport heavy metals (e.g., Cu⁺, Zn²⁺, Pb²⁺, Cd²⁺), not just common cations.
Present in: Plants, fungi, animals.
Human Diseases:
Menkes disease: Systemic copper deficiency due to faulty Cu⁺ transport.
Wilson’s disease: Copper accumulation in the liver.
Evolution of ATPases
From an ancestral ATPase, two major P-type ATPase classes evolved early:
One specialized for heavy metal transport (CPx-type).
One for general cation transport (P-type).
General features of V-type ATPases
Found mainly on intracellular organelle membranes (e.g., endosomes, lysosomes, vacuoles).
Exclusively H⁺ ATPases (pump protons across membranes).
Mechanism and function of V-type ATPases
Mechanism: Rotational Catalysis:
ATP hydrolysis occurs at the A₃B₃ hexamer (top part).
Hydrolysis generates torque in the D subunit, causing rotation.
Rotation drives movement of H⁺ through a ring of 6 ‘c’ subunits in the membrane.
Function:
Acidify intracellular compartments (important for protein degradation, receptor recycling, etc.).
ABC Transporters
ABC = ATP-Binding Cassette transporters.
Function: Move a wide range of solutes into or out of the cell using energy from ATP hydrolysis.
Clinical Importance of ABC Transporters
Cystic fibrosis:
Caused by mutations in the CFTR protein, a specialized ABC transporter that normally moves Cl⁻ ions.
Multidrug resistance (MDR):
Certain cancer cells and pathogens use ABC transporters to pump out drugs, causing drug resitance
Flippase Model for MDR Transporters:
Substrate (often lipid-soluble drug) dissolves into the inner leaflet of the plasma membrane.
Binding to the MDR1 protein (forms a protected internal chamber).
ATP hydrolysis powers a conformational change.
Substrate is “flipped” to the outer leaflet.
Substrate diffuses away into the extracellular space.
explains how hydrophobic drugs are efficiently pumped out of cells, contributing to drug resistance.
