The Erythrocyte Glucose Transporter and Anion Exchange Protein are Examples of Carrier Proteins

Question 1

The glucose transporter is an example of what kind of carrier? (uniport, symport, antiport)

Answer 1

The glucose transporter is a uniport carrier

Question 2

Movement of glucose into erythrocyte is done by?

Answer 2

facilitated diffusion mediated by a uniport carrier protein

Question 3

How can erythrocytes take up glucose?

Answer 3

the erythrocyte is capable of glucose uptake by facilitated diffusion b/c of its low intracellular glucose concentration and the presence in its plasmic membrane of a glucose carrier protein, or glucose transporter GLUT; GLUT1 in erythrocytes.

Question 4

What are erythrocytes?

Answer 4

Red blood cells

Question 5

How does GLUT1 mediated uptake of glucose display features of facilitated diffusion? (4 ways)

Answer 5

1. it is specific for glucose
2. it proceeds down a concentration gradient without energy input
3. it exhibits saturation kinetics
4. it is susceptible to competitive inhibition by related monosaccharides

Question 6

GLUT1 is what kind of protein?

Answer 6

Integral membrane protein with 12 hydrophobic transmembrane segments

Question 7

GLUT1 is thought to transport glucose by?

Answer 7

altering conformational states

Question 8

What are the conformational states of the GLUT1 transporter?

Answer 8

• one conformational state, T1, has the binding site for glucose open on the outside of the cell
• other conformational state, T2, has the binding site open to the inside of the cell

Question 9

Process of GLUT1 mediated uptake of glucose?

Answer 9

1. process begins when a D-glucose molecule collides with and binds to a GLUT1 molecule that is in its T1 conformation
2. with glucose bound, GLUT1 now shifts to its T2 conformation
3. the conformational change allows the release of the glucose molecule to the interior of the cell
4. after which the GLUT1 molecule now returns to its original conformation, with the binding site again facing outward

Question 10

How is transport by GLUT1 reversible?

Answer 10

a carrier protein can facilitate transport in either direction which is determined by the relative solute concentrations outside and inside the cell (glucose concentration is kept low in most animal cells)

Question 11

The immediate phosphorylation of glucose upon entering the cell does what?

Answer 11

keeps the concentration of glucose low b/c phosphorylated glucose cannot bind to carrier proteins

Question 12

another example of facilitated diffusion involves?

Answer 12

the transport of chloride and bicarbonate ions by anion exchange protein of the erythrocyte plasma membrane

Question 13

What kind of carrier is the erythrocyte anion exchange protein? (uniport, symport, antiport)

Answer 13

The erythrocyte anion exchange protein is an antiport carrier

Question 14

What does the anion exchange protein facilitate?

Answer 14

facilitates reciprocal exchange of Cl- and HCO3- ions only in opposite directions across the plasma membrane( exchanged in a strict 1:1 ratio)

Question 15

How is the anion exchange protein similar to the glucose transporter? how is it different?

Answer 15

like the glucose transporter. the anion exchange protein is thought to function by alternating between two conformational states. However the anion exchange protein requires the simultaneous binding of two solutes,one on each side of the membrane

Question 16

What is the “ping-pong” mechanism

Answer 16

the anion exchange protein alternating between two conformational states

Question 17

Steps of the “ping-pong” mechanism for the anion exchange protein?

Answer 17

1. in the first state, it binds to a Cl- ion on one side of the membrane, which causes a change to the second state
2. in the second state, the Cl- ion is moved across the membrane and is released
3. the release of the Cl- ion causes the protein to bind to HCO3-
4. the binding of HCO3- causes a shift back to the first conformation
5. in this conformation, HCO3- is moved out of the cell, allowing the carrier to bind chloride again

Question 18

What is the biological relevance of the anion exchange?

Answer 18

in tissues, waste CO2 diffuses into erythrocytes where it is converted to HCO3- by the enzyme carbonic anhydrase. As the concentration of HCO3- rises it moves out of the cell, coupled with uptake of Cl- to prevent a net charge imbalance - in the lungs the entire process is reversed.
( in tissues, concentration of HCO3- is high, in lungs, concentration of CO2 is high)