(D) Loss of quaternary structure

The molten globule state features native-like secondary structure but disrupted tertiary contacts, a key protein folding intermediate. The correct answer is (A) State with high degree of secondary structure and loss of tertiary structure.

Correct Answer

(A) State with high degree of secondary structure and loss of tertiary structure.

Molten globule (MG) retains compact α-helices/β-sheets (secondary) but side-chain packing fluctuates, lacking fixed tertiary structure. Observed under mild denaturation (pH 2-4, low urea), it’s more compact than unfolded but dynamic.

Protein Folding Context

MG acts as a kinetic intermediate between unfolded (random coil) and native states during folding. Native-like radius of gyration (Rg ~10% larger), ANS dye binding to exposed hydrophobics, and spectroscopic signals (CD for secondary, no Trp burial) define it.

Option Breakdown

(A) State with high degree of secondary structure and loss of tertiary structure

Correct. Classic MG hallmark: preserved H-bonded secondary elements, molten/non-specific tertiary interactions.

(B) State with complete loss of secondary structure

Incorrect. Describes fully denatured/unfolded state (no CD signal at 222 nm); MG shows strong secondary CD.​

(C) Completely unfolded state

Incorrect. Unfolded is extended coil (high Rg, no structure); MG is globular/compact with residual order.

(D) Loss of quaternary structure

Incorrect. Quaternary involves multi-subunit dissociation (e.g., hemoglobin tetramer); MG applies to single-chain globular proteins.​

MG Characteristics Table