Question

Transmembrane proteins:

             I. are predominantly constructed of a single hydrophobic α-helix.

            II. are often constructed from bundles of hydrophobic α -helices.

            III. are sometimes constructed from β-barrels with hydrophobic interiors and exteriors.

            IV. may have transmembrane helices identified by the free energy of transfer to water.

a. II, III

b. II, III, IIV

c. I, IV

d. I, III, IV

Answer 1

Transmembrane proteins or integral membrane proteins are the proteins that span the entire cell membrane in order to perform certain functions. For instance, Transporter proteins, Membrane receptors etc.

Topology of the transmembrane proteins is governed by the specific amino acid composition of the different segments.

These are present in the lipid bilayer in three different forms- single alpha helix, multiple alpha helixes or a rolled-up beta sheet (beta barrel). The stability of each of these topologies is determined by the chemical denaturation studies.

• Transmembrane proteins are mostly found in the helical form constructed from bundles of hydrophobic α-helix because of the stability of this particular conformation identified by the denaturation studies. So, the presence of single-spanning α-helix is not predominant.
• Because of the higher thermodynamic stability of the folding of the beta-barrels, they are sometimes constructed form β-barrels posessing hydrophobic interiors and exteriors.
• The scale of the transfer of free energy to water is also known as 'hydrophobicity'. The topology or the transmembrane helices are determined by the hydropathy plots drawn from the known hydrophobicity of individual amino acids and the physico-chemical constraints imposed by the lipid environment.

Hence out of the given options, II, III and IV are correct.