39. Collagen is the most prevalent extracellular matrix protein. Which of the following is NOT true for collagen?
(1) Collagen is composed of triple helix consisting of two α polypeptide chain and one β

polypeptide chain wound around one another in a rope-like structure.
(2) Glycine accounts for almost one third of the amino acids within collagen molecule.
(3) Ascorbate is essential for collagen formation required for hydroxylation of proline.
(4) Individual collagen polypeptide chains are synthesized on membrane-bound ribosomes with N-terminal signal sequences for directing them to ER lumen.

 

The statement that is NOT true for collagen is:

(1) Collagen is composed of a triple helix consisting of two α polypeptide chains and one β polypeptide chain wound around one another in a rope-like structure.

Explanation:

• Collagen’s triple helix is composed of three α polypeptide chains, not two α chains and one β chain. These three left-handed helices twist together into a right-handed triple helix structure, forming the unique rope-like superhelix of collagen.

• Glycine is indeed about one-third of the amino acid residues in collagen, allowing tight packing in the triple helix due to its small size.

• Ascorbate (vitamin C) is essential for proline hydroxylation, a post-translational modification required for stabilizing collagen’s triple helix, without which collagen synthesis is impaired.

• Collagen polypeptides (pro-α chains) are synthesized on membrane-bound ribosomes with an N-terminal signal peptide targeting them to the endoplasmic reticulum (ER) lumen, where further processing and folding occur.

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Summary Table

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Conclusion

The incorrect statement is (1) because collagen’s triple helix is formed by three α polypeptide chains rather than two α chains and one β chain. This basic structural fact, along with the importance of glycine, ascorbate, and proper biosynthesis pathways, defines collagen’s molecular biology.