Question

If a globular protein is solubilized in an aqueous medium then the amino acids which may found on the surface of that protein in that solution are
(A) Glutamic acid
(B) Aspartic acid
(C) Lysine
(D) Phenyl alanine
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A), (B) and (C) only.
• C. (A), (B), (C) and (D).
• D. (B), (C) and (D) only.

Hint:

"Like dissolves like."
Charged amino acids (Glu, Asp, Lys, Arg) are extremely polar and reside on the surface of cytosolic proteins to make favorable electrostatic contacts with water.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
When a globular protein folds in an aqueous environment, it organizes its structure to minimize thermodynamic free energy.
According to the hydrophobic effect, hydrophobic (nonpolar) amino acid residues are sequestered away from water into the interior core of the protein.
In contrast, hydrophilic (polar and charged) amino acid residues remain exposed on the outer surface to interact with water molecules through hydrogen bonding and electrostatic interactions.
Detailed Explanation:
Let us classify the side chain properties of the given amino acids:
- (A) Glutamic acid: An acidic amino acid with a negatively charged carboxylate side chain ($-\text{COO}^-$) at physiological pH. It is highly hydrophilic and is found on the protein surface.
- (B) Aspartic acid: An acidic amino acid with a negatively charged carboxylate side chain ($-\text{COO}^-$) at physiological pH. It is highly hydrophilic and is found on the protein surface.
- (C) Lysine: A basic amino acid with a positively charged protonated amino group ($-\text{NH}_3^+$) at physiological pH. It is highly hydrophilic and is found on the protein surface.
- (D) Phenylalanine: An aromatic amino acid with a large hydrophobic benzene-ring side chain. It is nonpolar and hydrophobic, so it is sequestered into the interior core of the protein.
Therefore, the amino acids likely to be found on the surface of the globular protein are Glutamic acid, Aspartic acid, and Lysine.

Step 2: Final Answer:
The correct option is (B), containing (A), (B), and (C) only.