Chapter 10

Question 1

How do membrane transporters lower the activation energy for crossing the membrane?

Answer 1

Replacing the hydration shell of the substrated with polar groups along the transfer path in the protein interior

Question 2

What are the 3 points of channels?

Answer 2

• 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

Question 3

What are the three points of carriers?

Answer 3

• 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

Question 4

What is the equation for finding the initial rate of transport (Vo)?

Answer 4

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

Question 5

How do polar sidechains surround glucose in transport interior?

Answer 5

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

Question 6

What is the difference between symport, uniport, and antiport?

Answer 6

Symport- two substrates in one direction
Uniport- one substrate in one direction
Antiport- two substrates in opposite directions

Question 7

Why is ATP a perfect universal energy currency?

Answer 7

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

Question 8

What are the two features of ATPase as a primary active transporters powered by ATP hydrolysis?

Answer 8

1. Enzyme cycles between 2 conformational states (Enz 1 and Enz 2)
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

Question 9

What are α-helices used in all active transporters membranes instead of β-barrels?

Answer 9

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

Question 10

How does ATP synthase and it’s rotation work?

Answer 10

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

Question 11

What are the two functions of ATP synthase?

Answer 11

Reversibility of direction of proton flow

ATP synthase reaction

Question 12

How do secondary active transporters operate?

Answer 12

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

Question 13

What is a glucose example of interplay between several transporters?

Answer 13

Glucose uptake in the intestine

Question 14

How is glucose uptaked in the intestine?

What is the ΔG formula?

Answer 14

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

Question 15

What is ΔG at equilibrium?

Answer 15

0

Question 16

What is required to ensure high specificity and provide gating?
What is gating?

Answer 16

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

Question 17

What are the three basic properties of protein ion channels?

Answer 17

1. High conduction rates- K+ ions flow across K+ channels at 10^7or8 per sec
2. selectivity- the smaller Na+ ion is essentially excluded
3. Gating- conduction is turned on and off by regulated conformational changes that open and close the pore

Question 18

Why can’t sodium get across K+ channels?

Answer 18

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

Question 19

What are the steps in K+ entering K+ channels and getting through?

Answer 19

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

Question 20

Why do the K+ ions go in the selectivity filter one spot apart?

Answer 20

So they don’t get stuck together

Question 21

Why does the Na+ channel work in neurons?

Answer 21

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

Question 22

How do the voltage gates of Na+ channels open?

Answer 22

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

Question 23

What are toxins that target ion channels?

Answer 23

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