Question

Which of the following statements is true?

a. Due to the strength of covalent bonds, the tertiary structures of most proteins are stabilized by one or more disulfide bridges.

b. Salt bridging interactions usually provide the greatest degree of stabilization of the native conformation of the protein when they are located on surfaces exposed to water.

c. Ramachandran plots are used in determining the hydrophobicity of amino acid side chains in a protein.

d. Detergents cause the denaturation of proteins primarily via disruption of hydrophobic interactions between amino acid side chains.

e. All of the above statements are true.

Answer 1

Ans.

Option. A. Incorrect. The overall 3D conformation of a polypeptide is called its tertiary structure. It is stabilized by H-bonds between side chains and van der Waal’s forces and hydrophobic interaction forming the hydrophobic core. Tertiary structure may also be stabilized by disulfide bridge (-S-S-) between cysteine residues brought together in close proximity during folding.

Many proteins, like hemoglobin and myoglobin, don’t have disulfide bonds, but they have stable tertiary structures. So, presence of disulfide bond is NOT the crucial stabilizing factor for most of proteins.

Option. B. Salt bridges are weak, non-covalent, attractive forces between oppositely charged side chains. They are very few in number when compared to H-bonds (weak, non-covalent) or van der wall’s interactions. So, salt bridging interaction can’t have more stabilizing effect than H-bonds or hydrophobic interactions.

Option. C. Incorrect. Ramachandran plot is used to determine the possible secondary structures based on dihedral angles.

Option. D. correct. Detergents are amphipathic molecules. The hydrophobic part interacts with the non-polar residues at the hydrophobic core and disrupts it. Disruption of hydrophobic core exposes the non-polar residues to polar aqueous environment. The interaction between hydrophobic residues and polar aqueous environment is not favorable and the protein can no longer be in solvated state. Thus, disruption of hydrophobic core denatures simultaneous with its precipitation.