LEC 49 Transporters & Pumps Flashcards
What are transporters?
catalysts that work in repeated cycles with the protein alternating between inward-facing and outward facing conformations
Slide 4
Transporters function very similarly with respect to kinetics to what other class of molecules?
Enzymes
Slide 5
Direction of net transport through a facilitated diffusion transporter is always in what direction?
Down the gradient
High to low concentration
Slide 7
In facilitated diffusion, is ATP required?
No
Slide 3
Which glucose transporters are insulin-dependent?
GLUT4
Slide 8
What are the insulin-independent glucose transporters?
GLUT1, GLUT2 (bidirectional), GLUT3, & GLUT5 (fructose)
BRICKLIPS
Brain, RBCs, Intestine, Cornea, Kidney, Liver, Islet, Placenta, Sperms
Slide 8
How does GLUT1 also function bidirectionally?
think blood-brain barrier
GLUT1 is on both sides of the endothelial cell. On the blood side, transport is inward; on the brain side, transport is outward. In both cases transport is from higher concentration to lower concentration
Slide 8
What are some of the main points regarding facilitated diffusion?
- Requires a membrane protein that can be in either inward-facing or outward-facing conformation
- One substrate at a time; no ATP hydrolysis
- Substrate saturation
- Competition between substrates
- Direction of transport can be either inward or outward, depending on the substrate gradient; transport is always downhill, from high to low concentration.
- Both empty and substrate-bound forms of the transporter
can reorient between inward- and outward- facing states
Slide 9
What is primary active transport?
Energy of ATP hydrolysis drives uphill transport of one or more substrates, including cations, drugs, lipids (flippase), but not anions
Slide 11
What is the most important example of primary active transport?
Na, K - ATPase
Sodium/Potassium Pump
Slide 11
How does the sodium-potassium pump work?
What does it pump and where? What’s required?
Pumps 3 Na+ out and 2 K+ in hydrolyzes ATP in the process
Slide 12
Why is the ATP required in the Na/K ATPase?
the energy of the ATP is required to overcome the energetically unfavorable conformational changes in order to bind/release the sodium and potassium
Slide 12
Explain the phosphorylation/dephosphorylation of the sodium-potassium pump.
2 K+ go into the cell (pump dephosphorylated)
3 Na+ go out of the cell (pump phosphorylated)
Slide 13
Explain the Na/K-ATPase catalytic cycle?
- 3 cytosolic Na+ bind to E1 conformation (high Na affinity)
- E1 phosphorylated by ATP forming E1-P
- Conformational change to E2-P which releases the Na+ (low Na affinity) and binds 2 extracellular K+ (high K affinity)
- E2-P dephosphorylated and conformational change to E2 which releases to K into the cytosol and returns it to E1
Slide 15
How do ATP-Binding Cassettes work?
- two membrane domains with 6 TM segments each and two nucleotide binding domains
- Drug, from either cytosol or inner half of lipid bilayer, binds to pocket between the two membrane domains
- ATP binds to Nucleotide Binding Domains and induces conformational change that expels drug into extracellular medium
Slide 17
Whats another name for a coupled cotransporter?
Symporter
Symporter = SAME DIRECTION
Slide 18
What is an example of a coupled cotransporter?
2 Na-Glucose (SGLT)
Slide 19
How does SGLT (sodium-glucose cotransporter) work?
- In glucose-absorbing epithelial cells, the inward Na+ gradient across the luminal (apical) membrane drives downhill Na+ influx via SGLT.
- SGLT couples Na+ influx to glucose influx, and the Na+ gradient drives glucose influx against a concentration gradient, with no ATP hydrolysis.
Slide 19
Which cotransporter is a target of Furosemide (Lasix)?
Na-K-2Cl Cotransporter
Slide 21
How does the Na-K-2Cl cotransporter work?
- Couples the transport of 1 Na+, 1 K+, and 2 Cl- ions in the same direction
- No ATP hydrolysis, but regulation by phosphorylation
- Driving force is normally inward because the inward Na+ and Cl- gradients overcome the outward K+ gradient
Slide 21
How does Na-K-2Cl- Cotransporter couple the ion fluxes?
By constraining the translocation event to all sites filled or all sites empty (same idea as 2 Na-Glucose cotransporter, but now there are 4 substrates)
Slide 21
What is another name for coupled exchangers?
Antiporters
Slide 22
What are coupled exchangers?
Substrates are required to move in opposite directions
Slide 22
What makes antiporters different from symporters in terms of their conformational changes (translocation event)?
translocation event requires bound substrate; empty transporter cannot re-orient between inward-facing and outward-facing
Slide 22
