W2: Lesson 2.2 Transmembrane Transpor 1 Flashcards
Explain why passive diffusion is often faster than either facilitated or active diffusion.
Channels are basically holes in the membrane,
created by a tube of protein and small molecules move through them driven by concentration gradient. This can be fast since there is really nothing that limits the movement of the molecule
Carrier proteins are 10thousand times slower b/c molecules have to wait for the proteins to transfer a noun molecule across the membrane and back before another one can start.
Pumps are the slowest b/c the speed is first reduced b/c a protein carrier is involved and then further reduced b/c the pump needs to convert energy into the motion of molecules against its concentration gradient.
What are the four different types of pumps discussed in our lectures?
P-class pump (Na+/K+ pump) V-class proton pumps (H+ pump in vesicles, e.g., lysosomes, plant vacuoles) F- class proton pumps (FOF1 ATP synthase in mitochondria) ABC (ATP-binding cassette) transporter (transports phospholipids, cholesterol, lipophilic drugs)
Which class of pump would be involved in moving a) sodium, b) hydrogen ions and c) macromolecules?
V type or vacuolar pumps : sodium and hydrogen
F type: mostly hydrogen
Mostly pump hydrogen ions exclusively but can also move sodium and other macromolecules such as sucrose
What is a unique characteristic of a P-type pump
multipass transmembrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. This class includes many of the ion pumps that are responsible for setting up and maintaining gradients of Na + , K + , H+ , and Ca 2+ across cell membranes.
The nucleotide binding domain phosphorylates the phosphorylation domain
Phosphorylation causes a very large conformation change in the regulatory domains, which in turn reshapes the arrangement of the eleven transmembrane helices
As a result, the side channel facing the cytoplasm closes and the side facing the exterior opens.
What is the overall structure of the V/F class pumps?
Large, mushroom-shaped clusters of proteins known as the V1 or F1 domain and it interacts with the nucleotide ATP and ADP
Rod-shaped structure, called a stalk and it’s attached to a ring of hydrophobic subunits that is embedded in the membrane bilayer
Subunits together with the stalk are called a rotor
The F1 and V1 domains are held in place by an arm of protein subunits that is knows as the stator and the whole complex is a nanomachine.
What is the general structure of an ABC pump, and how does it work?
Can move large and complex molecules.
Two domains that bind ATP
Association of ATP w/these domains bring them together, which closes an entry channel into the proteins interior.
At the same time an exit port on the opposite side of the membrane is opened.
The hydrolysis of ATP and release of ADP and inorganic phosphate, then returns the protein to its initial state
What is the multidrug resistance protein and what does it do?
MDR-1protein is actually the first eukaryoticABC transporter that was identified.
It’s a single protein with two functional domains. Each domain has a single ATP-binding site
Human MDR protein is also called p-glycoprotein is highly expressed in cells of the digestive and excretory systems and its thought to be involved in moving dietary toxins out of cells lining these organs. It’s also capable of excreting drug molecules from cells and over-expression of this protein in tumor cells makes the cells resistant to a variety of anti-cancer drugs, a condition known as multi drug resistance.
Which side of the cell membrane has the initial ligand binding site of an ABC protein in a typical prokaryote, and what does this mean for the general function of prokaryotic ABC protein pump?
Prokaryotes: Outer membrane contains pore-forming proteins which allow the entry of the solute molecules into the periplasmic space.
A system of substrate binding proteins then facilitates the association of molecules in the periplasmic space with the ABC transporters which are found on the inner membrane
Which side of the cell membrane has the ATP binding domain of a prokaryotic and a eukaryotic ABC class protein pump?
Bacteria and other prokaryotes: mostly the side opposite the ATP binding domains
Eukaryotes: the entry port and ATPase domains are on the same side of the membrane.
Discuss the mechanism underlying cystic fibrosis
Caused by a defective ABC chloride transporter
Normal CTFR protein transports allows chloride ion to leave in a regulated manner, sodium, and water follows
In the airway, mucus-secreting cells produce a layer of mucus, which lines the space of the airway. It protects the cells from dehydration and also traps inhaled debris and pathogens. The water leaving the airway epithelial cells as a consequence of the chloride ion channel, dilutes the mucus and makes it fluid enough that it can be moved out of the airway, carrying the trapped particles with it. makes an overlying mucus layer fluid, so airway cells can clear debris and pathogens
Mutant CTFR is non-functional - the movement of chloride ion is blocked and water fails to leave the cell and dilute the mucus layer. Foreign particles become trapped and build up in the lung tissue.
Airways of CTFR patients have thick, non-fluid mucus that traps bacteria and debris in lung tissue
Why do carrier proteins transfer molecules much more slowly than channel protein?
Channels are basically holes driven by concentration gradient nothing really limits movement of molecule whereas carrier proteins depend on the availability of the protein, e.g., protein has to take loaded molecule to the other side of the membrane before returning to get a new molecule/material. Molecules still move down concentration gradient
Active transport is the slowest b/c it has to wait for carrier proteins to do it’s round and in addition it requires energy (convert energy to motion)to move molecules against its concentration gradient
What are the major sources of energy that can be used to drive the activity of pump proteins?
ATP (universal energy-carrying molecule)
Light (for plants and bacteria)
Which pumps use ATP as an energy source?
P type pumps Calcium Pump Sodium/potassium pump F and V class pumps ABC protein pumps
What kind of pump do plants and bacteria use as energy sources?
light driven pump
Explain the purpose & structure of of the p-type calcium pump:
Maintain low levels of calcium in cytoplasm and maintains calcium concentration outside the cell and w/in some membrane bound compartments like the endoplasmic reticulum
Structure:
Has 11 alpha helical transmembrane domains that form a transmembrane channel
Cystolic side: has 3 globular domains known as:
1) Nucleotide-binding domain 2)Phosphorylation domain 3) Activator domain
Function:
1) At start of pump cycle, calcium channel is closed on the exoplasmic side and open towards the cytoplasm. Within the channel are two binding sites for calcium ions
3) Calcium binds to these domains and ATP then binds to the nucleotide-binding domain
4) Nucleotide binding domain phosphorylates the phosphorylation domain (as all p-pumps do)
5)Phosphorylation causes
a conformational change in the regulatory domains, which reshapes 11 transmembrane helices
6) As a result, the side of the channel facing the cytoplasm then closes and the side facing the exterior opens and release calcium from binding pocket
7) once calcium is released from the binding pocket, the structure also release ADP (inorganic phosphate), and becomes dephosphorylated to complete the cycle
Explain function and steps of the p-type sodium/potassium exchange pump:
Function: Major source of the ionic gradient across the animal cell’s plasma membrane
Pump transports 3 positively charged sodium ions OUT of the cell (nah, get out) and moves in 2 positively charged potassium ions INTO the cell (k, come in)
This happens for every molecule of ATP that’s consumed
STEPS:
1) Binding of 3 Na+ ions and ATP to the pump on the cytoplasmic side of the membrane
2) Phosphorylation of the pump occurs from the binding of Na+ and ATP and moves the pump across the membrane where it can release 3Na+
3) The binding of potassium on the outside of the cell and dephosphorylation of the pump then transfers the pump back across to the cytoplasmic surface
4) the release of potassium brings the cycle back to it’s starting point
How does the sodium/potassium pump create a net negative charge on the inside of the cell? Why is this important?
Sodium and potassium have a single positive charges. A net electrical imbalance of one positive ion is also produced for every cycle of the pump, resulting in a net negative charge on the inside of the cell.
This negative charge on the inside of the cell is important for many biological processes.
Which drugs affect the sodium-potassium pump? How do they affect the pump?
Ouabain
Digitalis
These drugs bind to and block the potassium binding sites and have the overall effect of increasing sodium concentrations inside the cell.
This indirectly affects other concentrations of positive ions, like Ca2+ in the cell, which is involved in regulation the concentration of cardiac muscle tissue,
This is why these drugs are used to treat heart conditions
