Discussion
The triple helix domain of collagen consists of repeats of the X-Y-Z amino acids. The amino acid present at 'X' position is required in every third position in order for the polypeptide chain is :
Answer: D
The residue required at every third position is glycine.
Collagen's triple helix is built from repeating triplets, usually written Gly-X-Y, in which this question's 'X' is the glycine position. Every third residue lies at the centre of the tightly wound triple helix, where there is room only for glycine, whose side chain is a single hydrogen atom.
Proline and hydroxyproline are common at the other two positions, where they rigidify the chains, and lysine (with hydroxylysine) is used for cross-linking. None of them fits at the centre of the helix, where a glycine substitution causes diseases such as osteogenesis imperfecta.
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