Question 36:

When a protein has two or more polypeptide subunits, their folding arrangement is called:

(A) Primary structure
(B) Secondary structure
(C) Tertiary structure
(D) Quaternary structure

Answer: (D) Quaternary structure. This describes the spatial arrangement of multiple polypeptide subunits in a multi-subunit protein.

Option Explanations

Protein structure levels build hierarchically; the question targets multi-subunit proteins specifically.

• (A) Primary structure: Linear amino acid sequence linked by peptide bonds; ignores 3D folding entirely.

• (B) Secondary structure: Local folding patterns like α-helices or β-sheets via H-bonds; single-chain feature.​

• (C) Tertiary structure: Overall 3D fold of one polypeptide chain from R-group interactions; single subunit only.​

• (D) Quaternary structure: Correct—arrangement of two+ folded subunits (e.g., hemoglobin’s tetramer).

Introduction
Protein two more polypeptide subunits folding arrangement defines quaternary structure in biochemistry. Essential for GATE Life Sciences, it distinguishes multi-subunit proteins like enzymes.​

Protein Structure Hierarchy

Proteins fold from primary sequence to quaternary complexes.

• Primary: Amino acid order.

• Secondary: Helices/sheets.

• Tertiary: Single-chain 3D shape.

• Quaternary: Subunit assembly via noncovalent bonds.​

Examples: Hemoglobin (4 subunits), DNA polymerase (multi-subunit).​

GATE MCQ Breakdown

Question 36: Protein with two+ polypeptide subunits folding?
(A) Primary (B) Secondary (C) Tertiary (D) Quaternary

(D)—Only level for multi-subunit arrangement.