W2&3 Membrane Transports + Na/K ATP-ase (Dustin) Flashcards
The phospholipid bilayer is most permeable to _______ and least permeable to ______
The phospholipid bilayer is most permeable to hydrophobic molecules (O2, H2O, H2) which freely diffuse. Least permeable to ions (Cl-, K+, etc)
Simple diffusion is what?
Transport through a membrane driven by concentration gradient, no transporters.
Must be lipid-soluble
Facilitated diffusion is what?
Transport driven by concentration gradient or electrochemical gradient, but a transporter molecule assists. Still, it doesn’t require energy
Primary active transport is what?
primary active transport needs energy to go against concentration or electrochemical gradient
as in Na/K-ATPase
What is secondary active transport?
secondary active transport uses concentration gradient made by primary active transport to drive further movement
usually involves sodium gradient created by the Na/K-ATPase
P-type ATPases are what?
A transport ATPase (active transporter),
“P” for “phosphorylation, they are reversibly phosphorylated by ATP (on an Asp residue) as part of the transport cycle
Most ATPases are P type
These are integral plasma membrane proteins
What are V-type ATPases?
A transport ATPase (active transporter),
located within the cell in the membranes of vacuoles/vesicles
Function is to create a proton gradient (proton pumps),
which may serve some other kind of transport that is done secondarily or acidify lysosomes etc.
What are F-type ATPases?
A transport ATPase (active transporter),
Active in mitochondrial inner membrane
Also a proton pump. Includes ATP synthase.
What does the ABC in ABC transporters stand for?
What are they?
A transport ATPase (active transporter),
ATP-binding casette transporter
Make specific transporters in the membranes responsible for the transport of cholesterol, bile acids, and many other things against a concentration gradient. Responsible for extruding some of the foreign molecules from cell, i.e. some drugs
The multidrug transporter MDR1 aka P glycoprotein is responsible for resistance against anti-tumor drugs
With P-type ATPases there are 4 domains mentioned in the alpha subunit
What are the functions of these domains:
T
N
P
A
- T: transport. Could also see it as “t” for transmembrane; its conformation enables some molecule to be transported through the protein
- N: ATP/ADP binding
- P: phosphorylation. Contains the Asp residue that is phosphorylated
- A: phosphatase activity
Note that the beta subunit projects extracellularly (not seen in image)
SERCA ATPase and K, H ATPase are examples of what type of ATPase?
P-type
Where is the K, H ATPase?
What does it do?
Stomach parietal cells
Makes for potassium reasborption, H excretion
Helps create the acidity of the stomach
What does SERCA stand for?
There are 3 isoforms of it, where are they?
(I doubt the test will be detailed enough to require you to correctly match them)
SERCA = Smooth Endoplasmic Reticulum Calcium ATPase,
Fills the sarcoplasmic reticulum with cytosolic calcium
- SERCA1: striated muscle
- SERCA2: smooth muscle, striated muscle, heart muscle (all types) - phospholamban inhibits it
- SERCA3: platelets, endothelial cells, other non-muscle
What does PMCA stand for?
What are PMCA ATPases?
PMCA = Plasma Membrane Calcium ATPase
These are P-type ATPases that remove calcium from the cell
What is the ratio of sodium-potassium transported by Na/K ATPase, and in what direction are the ions transported?
This pump is responsible for some of the negative resting membrane potential of a cell, what is that feature of the pump called?
3 Na+ out, 2 K+ in
This makes it an “electrogenic” pump
What are the approximate values of
intracellular and extracellular Na+, K+, and Ca2+
- K+: i.c. = 140 mM, e.c. = 5 mM
- Na+: i.c. = 10 mM, e.c. = 140 mM
- Ca2+: i.c. = 100nM, e.c. = 2 mM
What is the function of intracellular calcium level being so low?
Calcium is used for signal transduction, where a small increase in calcium can lead to muscle contraction, neurotransmission, etc.
Describe the postulated mechanism for the Na+/K ATPase
- Starts in E1 position. First the 3 Na+ bind intracellularly, then ATP is hydrolyzed
- Conformational change from E1 to E2 form
- the 3 sodium molecules now face the extracellular side, then diffuse away
- 2 Potassium from the outside can now fit inside the pump, which then loses its phosphate
- Return to original E1 conformation as K+ is released in the cytosol
In which position is Na+/K+ ATPase when inhibitors bind to it?
Which position binds better to ATP?
E2 position (open facing extracellularly for potassium) binds to the inhibitor Ouabain
E1 position has high affinity for ATP
With the SERCA pump, how does it work in comparison to the Na+/K+ ATPase?
What are the important domains?
Mostly similar
- There is a transmembrane domain with 2 Ca2+ binding sites internally
- Has a nucleotide binding domain with the ATP-binding site
- Phosphorylation domain with similar Asp binding site
- Actuator domain
What is the function of the beta subunit of Na+/K+ ATPase?
The beta subunit is mostly extracellular with only one transmembrane segment (there are 10 TM segments in alpha subunit, which is also where all the action of Na/K ATPase takes place)
The beta subunit stabilizes the alpha subunit, which involves the highly glycosylated structure of the beta subunit that is not able to be pushed around the membrane as easily as the alpha subunit.
What is the function of the FXYD subunit of Na+/K+ ATPase (sometimes called the gamma subunit)
Regulates the transport kinetics of the alpha subunit, adding to the diversity of how the different isoforms of Na+/K+ ATPases throughout the body function (Essentially, creates tissue-specific sodium potassium pumps)
The N-terminal segment has a conserved Phe-X-Tyr-Asp motif (hence the shorter amino acid abbreviations of FXYD)
Particularly important in heart and kidney
What are the therapeutic effects of Ouabain/ Strophantine/ digitalis?
(many names to be familiar with for this compound from the foxglove plant)
This is a cardiac glycoside and the most important inhibitor of Na+/K+ ATPase
- Slight inhibition of sodium potassium pump in the heart muscle -> increase in intracellular sodium (bc it isn’t pumped out)
- Increase in intracellular sodium means the sodium-calcium exchanger reverses direction, bringing calcium into the cell
- Increased intracellular calcium makes for more forceful contractions (increased ionotropy)
The heart’s sodium-potassium pump contains alpha 2 subunits, which are more sensitive to Oubain/ Strophantine than the other types of alpha subunits, which are not expressed as much in other tissues
Why does the structure of Ouabain look so similar to steroid hormones when it is produced by plants?
Because the body produces an endogenous form of ouabain, which the plant has mimicked because it works as a protective toxin. The endogenous form is released by the zona fasciculata and increases vascular tone -> increased blood pressure
