24. The following statements were made to describe a typical collagen structure.
    A. Collagen has a triple-helical domain structure which consists of three distinct α-chains.
    B. The collagen triple helix is stabilized by isoprenyl bonds.
    C. Each α-chain has a left-handed polyproline ll-type helix.
    D. Each α-chain is composed of multiple triplet sequences of Gly-Y-Z in which Y is commonly proline and Z is usually hydroxyproline.
    Which one of the following options has all correct statements?
    (1) A, C and D
    (2) A, B and C
    (3) A and B only
    (4) B and D only

Collagen Structure: Triple Helix, Amino Acid Composition, and Stability

Introduction

Collagen is the most abundant structural protein in the extracellular matrix of animals, providing tensile strength and structural integrity to tissues such as skin, tendons, and bones. The unique triple-helical structure of collagen plays a crucial role in its function. This article explores the key structural features of collagen and evaluates the correctness of the given statements.

Understanding the Structure of Collagen

Collagen consists of three polypeptide chains, known as α-chains, that form a unique triple-helical structure. These chains are stabilized through hydrogen bonding and specific amino acid sequences that allow for tight packing and structural integrity.

Analysis of the Given Statements

Let’s examine each statement carefully:

Statement -A: Collagen has a triple-helical domain structure which consists of three distinct α-chains.

• Correct. Collagen is composed of three polypeptide α-chains that intertwine to form a right-handed triple helix. Each chain is a left-handed polyproline II-type helix, contributing to the stability of the overall structure.

Statement -B: The collagen triple helix is stabilized by isoprenyl bonds.

• Incorrect. Collagen is stabilized primarily by hydrogen bonds between the backbone amide groups and hydroxylated proline residues. Covalent cross-links (such as lysine-derived cross-links) also contribute to stability, but isoprenyl bonds are not involved in collagen stabilization.

Statement- C: Each α-chain has a left-handed polyproline II-type helix.

• Correct. The individual α-chains of collagen adopt a left-handed polyproline II (PPII) helix, which allows them to twist together into a right-handed triple helix.

Statement -D: Each α-chain is composed of multiple triplet sequences of Gly-Y-Z, in which Y is commonly proline and Z is usually hydroxyproline.

• Correct. The characteristic repeating sequence of collagen is Gly-X-Y, where:
  • Glycine (Gly) is necessary for the tight packing of the helix.
  • X (Y position) is often proline (Pro).
  • Y (Z position) is commonly hydroxyproline (Hyp), which provides additional stability through hydrogen bonding.

Correct Answer: (1) A, C, and D

Key Features of Collagen Structure

1. Triple Helix Formation

• Each of the three α-chains adopts a left-handed polyproline II-type helix.
• These three chains then form a right-handed triple helix.

2. Stabilization Mechanisms

• Hydrogen bonding between peptide bonds and hydroxylated residues stabilizes the structure.
• Covalent cross-links (e.g., lysine-derived cross-links) contribute to mechanical strength.
• Hydroxyproline plays a critical role in maintaining the helical stability.

3. Importance of Glycine in Collagen

• Glycine, the smallest amino acid, is required at every third position in the sequence (Gly-X-Y) to allow tight packing of the triple helix.
• Without glycine, the helical structure would not be possible.

Types and Functions of Collagen

Collagen is classified into different types, with Type I collagen being the most abundant. Other types include:

• Type II: Found in cartilage.
• Type III: Found in skin, blood vessels, and internal organs.
• Type IV: Forms the basement membrane.

Experimental Methods to Study Collagen Structure

1. X-ray Crystallography: Determines high-resolution collagen structures.
2. Fourier Transform Infrared (FTIR) Spectroscopy: Analyzes collagen secondary structures.
3. Circular Dichroism (CD) Spectroscopy: Assesses collagen stability and triple-helix formation.

Conclusion

Collagen is a fundamental structural protein with a unique triple-helical structure composed of three left-handed polyproline II α-chains. The correct statements about collagen structure are A, C, and D, making option (1) the correct answer. Understanding collagen’s molecular structure is crucial for biomedical applications, including tissue engineering, wound healing, and biomaterial development.