Question

What would be the expected orientation of an amphipathic alpha helix occurring in a globular protein in aqueous solution?

Select one:

a. Hydrophilic amino acids are complementary to hydrophobic amino acids, so the hydrophilic side of the alpha helix should be facing toward the hydrophobic core of the protein.

b. A soluble protein will have mostly hydrophilic amino acids in its core, so the helix will be arranged to have the hydrophilic side facing toward the core of the protein.

c. A soluble protein will have lots of hydrophilic amino acids on its surface, so the helix will be arranged to have the hydrophilic side facing toward the outside of the protein.

d. A soluble protein will have a hydrophobic core and a hydrophilic surface, so hydrophilic and hydrophobic amino acids will be equally distributed in the helix.

Answer 1

An amphipathic alpha-helix contains two types of residues in the structure. The hydrophilic residues or water-loving and the hydrophobic residues or water-hating. If this residue is placed in water or any other polar solvent, the hydrophobic regions of the amphipathic alpha-helix will reorient itself towards the inside of the core as the residues are non-polar (hydrophobic) and will not interact with water. Similarly, the hydrophilic/polar residues will reorient themselves towards the water or polar solvent side.

Therefore, the correct option is C. A soluble protein will have lots of hydrophilic amino acids on its surface, so the helix will be arranged to have the hydrophilic side facing toward the outside of the protein.

The other options are incorrect because hydrophilic amino acids will not face towards the core, they will be on the outer surface so that they can interact with the aqueous solvent. They will also not be equally distributed meaning that the hydrophobic residues will cluster together and hydrophilic will cluster together in stretches, depending upon their conformation.