Question

3. Cell membranes contain a variety of integral and peripheral membrane proteins.

a. Explain the differences between symport, antiport and uniport proteins compared to ion channels.

b. There are a variety of glucose transport proteins in the cell membrane with some relying heavily on the sodium/potassium ATPase pump. (1) Explain this relationship and indicate how this is important to the translocation of glucose across the intestinal lumen into the blood. (2) CHALLENGE: Suggest an in vitro experiment that would allow you to test this relationship and determine in which direction the glucose is transported (e.g. apical to basal vs basal to apical). Hint: See Fig. 11-30 in your text and associated text.

c. CHALLENGE: You purify an integral membrane protein and perform a hydropathy plot. Analysis of this protein shows 7 substantial peaks in the 0 to + range. You presume that this profile suggests that you may be working with a GPCR. Explain the basis for your reasoning and explain how you might be able to test this hypothesis.

Answer 1

3.

a.Uniport is a protein which transports only one molecule in a specific direction.

Symport is a molecule which transports two molecules at the same time and in the same direction.

Antiport is a molecule which transports two molecules at the same time but in opposite directions.

b. Glucose is absorbed from the intestinal lumen to capillary blood in the intestinal wall. There are glucose transporters called SGLT which transport glucose in the the epithelial cells from the lumen. These transporters use the down hill transport of the Na ions from the  lumen for transporting glucose in to the epithelial cells. There are Na/K-ATPase pumps on the basolateral side of the epithelial cells which will carry 3 Na ions out of the epithelial cells and bring 2 K ions in to the epithelial cells by using 1 ATP.
This transporter in the basolateral side is reducing the Na ion concentration in the epithelial cells. Taking this lower Na ion concetration SGLT brings more Na down the gradient along with glucose. This is secondary active transport. Glucose enters in to the blood by diffusion through another transporter called GLUT-2, as the concentration in the epithelial cells is increased by the inflow of more glucose from the lumen.

You can perform invitro experiments by blocking ATP availability to test the movement of the molecules. One of the transporters also can be blocked to test the direction of flow of the molecules through the membrane.

c. Hydropathy plots show hydrophobicity lengths in the peptides. 7 peaks indicate that we are working with GPCR because, this receptor has seven transmembrane helices crossing the plasma membrane which is hydrophobic internally.