Structure, Function and Pharmacology of the Na+/K+ ATPase pump Flashcards
- Describe what is the pump-leak hhypothesis - Recall evidence that the Na pump is an ATPase - Describe what energises the pump - Describe the nature of the cation transport system - Describe the pump reaction mechanism - Define the five operational modes of the pump - Recall the molecular structure of the Na pump, tissue distribution of pump isoforms and pharmacological interactions with the alpha1 subunit
What is the Na-K ATPase pump
- member of the P-type ATPases
- pumps Na into the cell and K out of the cell
What does an electrochemical gradient do
critical for maintaining osmotic balance and a stable membrane potential, and the excitable properties of muscle/nerve cells
What does the Na+ gradient do
provides the energy that fuels Na-coupled transporters mediating the translocation of ions and substrates across the plasma membrane
Role of Na-K-ATPase pump in the kidney
- drives the reabsorption of Na+ and water
- essential in maintenance of body fluid and electrolyte homeostasis
How can cation gradients be abolished
- cell death
- withholding glucose
- use of specific inhibitors
What is the pump-leak hypothesis
- active extrusion of Na and uptake of K ions is balanced by a passive leak of these ions in opposing directions
- transmembrane potential exists -> Na takes place against chemical and potential gradients
Proof that the movement of Na and K across the membrane requires metabolic energy
cation transport is still active in red blood cells without oxidative metabolism -> suggests ATP rather than glycolysis/Kerb cycle events energise pump
How does the Na/K-ATPase pump work
- can be stopped at any time and is reversible
STEP 1: affinity of sodium for three binding sites
STEP 2: phosphorylation occurs
STEP 3: conformational change alters protein shape and diminishes affinity of sodium for binding site
STEP 4: potassium binding sites exposed -> high affinity for potassium
STEP 5: hydrolysis, detachment of phosphate group from residue
What is the transition step
transport step -> ratio of Na release and K binding corresponds to the ratio of 3 Na ions expelled for 2 K ions taken up
What does the Na pump do
- helps maintain normal trans-membrane electrochemical gradients of membrane permeability properties
- maintains stable membrane potential
- energy in gradient used to drive secondary active transport systems for sugars and amino acids
What does the catalytic subunit do
- possesses binding sites for Na, K, ATP and Mg and cardiac glycosides
- intrinsic ATPase activity
- 4 isoforms
- molecular szie ~ 112kDa
- developmentally regulated and expressed in tissue-specific manner
What does the regulatory subunit do
- three isoforms
- heavily glycosylated (28% w/w)
- crucially required for full enzyme activity, but also for enzyme assembly, intraacellular transport, and stability of alpha subunit
What is the small auxillary protein
- belongs to the FXYD protein family - named after invariant motif
- single-spanning membrane peptide
Evidence for multiple isoforms of the Na,K-ATPase
- different curve in brain and kidney, as sensitivity to cardiac glycoside are different when comparing tissues
- tissue specific antibodies against purified enzyme
- biphasic curve indicates drug binding to receptor
Expression on alpha 1 subunit
ubiquitous expression -> contains binding site for drugs such as digoxin
