Chapter 10 Flashcards
How do membrane transporters lower the activation energy for crossing the membrane?
Replacing the hydration shell of the substrated with polar groups along the transfer path in the protein interior
What are the 3 points of channels?
- channels are membrane pores that can only transport molecules and ions down the electrochemical gradient
- they can have very high conductance rates because they bind the substrate very weakly
- they do not show saturation behaviour. Rate of transport through channel is proportional to substrate concentration
What are the three points of carriers?
- they have kinetically distinct step where substrate is bound to protein
- they generally have lower rate of transport than channels
- they can catalyze both active (against Electrochemical gradient) and passive (down the EG) transport
What is the equation for finding the initial rate of transport (Vo)?
Vo= Vmax [S]out / Kt + [S]out
[S]out- substrate concentration outside the cell
Vmax- is maximum transport rate
Kt- Michaelis constant in enzyme kinetics
Page 4
How do polar sidechains surround glucose in transport interior?
Polar sidechains form hydrogen bond with glucose and the residues with polar sidechains are located on the side of the helix that faces inside the pore
Picture on slide 5
What is the difference between symport, uniport, and antiport?
Symport- two substrates in one direction
Uniport- one substrate in one direction
Antiport- two substrates in opposite directions
Why is ATP a perfect universal energy currency?
The hydrolysis is a highly exergonic reaction and the reaction is reversible under the physiological conditions so ATP can be easily regenerated in enzymatic reactions
What are the two features of ATPase as a primary active transporters powered by ATP hydrolysis?
- Enzyme cycles between 2 conformational states (Enz 1 and Enz 2)
- When they catalyze ATP hydrolysis the phosphate group on the end does not go straight to water, it has an intermediate stop
Diagram on slide 8
What are α-helices used in all active transporters membranes instead of β-barrels?
α-helices allows greater freedom of movement than the rigid β-barrel where strands are connected by hydrogen bonds
When transferring from Enz 1 to Enz 2 some of the middle helices in protein move
Picture on slide 9
How does ATP synthase and it’s rotation work?
The flow of protons through the embedded part of atp synthase in the membrane rotates it which also rotates the cylindrical turbine, this rotating forces the release of newly synthesized ATP
Picture on slide 10 and 11
What are the two functions of ATP synthase?
Reversibility of direction of proton flow
ATP synthase reaction
How do secondary active transporters operate?
A rocker switch mechanism
V -> Λ -> V -> Λ diagram on slide 12
These transitions are triggered by protonation and deprotonation of charged residues in the protein interior
What is a glucose example of interplay between several transporters?
Glucose uptake in the intestine
How is glucose uptaked in the intestine?
What is the ΔG formula?
Na+ and glucose transport are coupled
2 Na are transported for each glucose molecule
High concentration of Na outside and low inside
ΔGtotal= 2ΔGNa + ΔGgluc
What is ΔG at equilibrium?
0
What is required to ensure high specificity and provide gating?
What is gating?
More complex structure of biological channels is needed
Gating is a trigger that closes and opens the channel in response to an electric or chemical signal
What are the three basic properties of protein ion channels?
- High conduction rates- K+ ions flow across K+ channels at 10^7or8 per sec
- selectivity- the smaller Na+ ion is essentially excluded
- Gating- conduction is turned on and off by regulated conformational changes that open and close the pore
Why can’t sodium get across K+ channels?
Because of the geometry of the oxygen stage
If sodium tries to get between oxygens the distance between them will be bigger since sodium is smaller than K+ which will cause it to wobble and the energy of the electrostatic interactions is different
What are the steps in K+ entering K+ channels and getting through?
Each K+ ion is coordinated by 8 oxygen atoms in the selectivity filter, the geometry of the oxygen matches perfectly with K, two K+ ions can occupy selectivity filter at a given time each positioned one spot apart (1,3 or 2,4)
Picture on slide 17
Why do the K+ ions go in the selectivity filter one spot apart?
So they don’t get stuck together
Why does the Na+ channel work in neurons?
The Na+ channel is a single polypeptide that has four domains surrounding the central pore and selectivity filter. The channels opens in response to the drop in electric potential
This opening is triggered by the movement of the voltage sensor
How do the voltage gates of Na+ channels open?
Their are two positively charged stuck like poles that when approached by Na+ will spring up which slides the channel open and let’s the Na+ ion through
Picture on slide 18
What are toxins that target ion channels?
They are deadly to our bodies
Tetrodotoxin is found in puffer fish that binds to the voltage gated Na+ channels in nerve endings in muscle, this causes paralysis and death
