Question

What are the three mechanism used to close/open channels? What is the difference between channels made of proteins with beta-pleated sheets versus alpha helix structures? Give an example of each.

What type of amino acid must produce the helix or sheet that spans the lipid bilayer?

Describe how the glucose transporter is regulated.

Answer 1

Ion channels are the posre forming proteins that pass through the plasma membrane and allows the transport of ions through it. Different ion channels allow specific ion throigh the pores depending upon their chemical natures. The opening and closing of the channels are not random rather much regulated by two main mechanism. On the basis of that the channels could be classified into 2 groups.

1. Voltage gateed ion channels - As the name suggests this channels open and close on the basis of the voltage gardient across the plasma membrane. This group of channels play a major role in nerve stimulus transmission. When they are in resting potential i.e no stimulus is present the the outer side of the membrane is positively charged whereas the inner side is negatively charged and this causes the closing of the sodium channels.when there is atimulus it causes the opening of the sodium channels leading to the entry of sodium ions into the cells. This causes a shift of positive charge inside and negative charge outside. So Na channels in neurons are examples of voltage gated ion channels.

2. Ligand gated iron channels - These channels open upon binding of ligands to the external lignand binding region of the receptor. This binding consequently causes a change in the conformation in the internal part of the channels that leads to the opening of the channels and consequent movement of the specific ions. Example nicotinic acetyle choline receptor.

3. Other type of opening and closing of channels can be done by light mediated.

Most of the membrane are made up of alpha helices. THough there are a number of channels proteins that are majorly made up of trnsmembrane beta sheets. Within the membrane the anti paralles betasheets pass and are joined by the hydrogen bonds. Exmple in Porin which is completely made up of beta sheets. Whereas alpha helices are also found in the membrane proteins. VOltage gated K and Ca channels are examples of alpha helical transmembrane proteins. Biochemical studies showed that they are mostly composed of hydrophobic residues and very little hydrophilic residues.

The transmembrane domain of the alpha helices contain mainly the hydrophobic residues. like glycine, Leucine, Valine etc. Mostly the aromatic amino acids are found in the beta pleated sheets.

There are a no of glucose transporters expressed in the cell membranes of different tissues. The expression of GLUTs are tightly regulated by the glucose level in the blood as well as in the cell. Which in turn is regulated by the hormone insuline. So Insuline regulates the expression of Glucose transporters.