Question

5. Explain role of the α helix in transmembrane proteins. Relate to types of amino acid residues.

6. Explain the role of the β sheet in transmembrane proteins.

7. Explain how detergents solubilize membrane proteins; give the 2 common detergents.

8. Explain the function and basic structure of the cell cortex.

9. Explain the experiments of Frye and Edidin on mouse-human hybrid cells and how it relates to types of restrictions for membrane proteins.

Answer 1

5) The two kinds of transmembrane proteins are alpha-helical and beta-barrels. α-Helices are the most common protein structure element, which crosses transmembrane protein. They are present in the inner membranes of bacterial cells or the plasma membrane of eukaryotes and the outer membranes. They consist hydrophobic amino acid residues and small amount of hydrophilic residues. The polar carbonyl oxygen facilitates and strengthens the hydrogen bonds within the helix. This hydrogen bonding confines polar side chains in the hydrophobic surroundings, like as a lipid bilayer. The polar side chains in turn hydrogen bond to other helices. Two residues, glycine and proline, which are helix breakers in water make twists in the helix which play significant roles in functional mechanisms.

6) Beta-barrels present in the outer membranes of Gram-negative bacteria, cell wall of Gram-positive bacteria, outer membrane of mitochondria and chloroplasts. They cross cellular membrane and acts as a pore through which molecules can diffuse. Transmembrane proteins are classified as Type I and Type II. In Type I, the N-terminal is present on the exterior of the membrane. In Type II, the C-terminal present on the exterior of the membrane.

The beta sheet curls up to form a hollow cylinder, which creates a channel in the membrane.

All beta-barrel transmembrane proteins have simplest up and down topology, which reflect their common evolutionary origin and similar folding mechanism.