In collagenous regions of proteins, what fraction of residues are glycine?

Introduction:
Collagen is the most abundant protein in mammals and a hallmark of connective tissues. Its unique triple-helix architecture relies on a highly repetitive sequence pattern. This question targets recognition of the Gly-X-Y motif and the specific frequency of glycine needed to pack three chains tightly.

Given Data / Assumptions:

• Collagen chains feature a repeating Gly-X-Y pattern.
• Glycine is the smallest amino acid with a single hydrogen side chain.
• Tight packing near the helix axis requires minimal steric bulk.

Concept / Approach:
Every third residue must be glycine to fit into the crowded center of the triple helix. The X and Y positions are often proline and hydroxyproline, which contribute to rigidity. Therefore, glycine constitutes one-third of residues in collagenous segments.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical and structural studies confirm glycine at every third position; substitution with larger residues destabilizes the triple helix and causes disease phenotypes (e.g., osteogenesis imperfecta).

Why Other Options Are Wrong:

• One-fourth, one-tenth, two-thirds, half: do not match the strict Gly-X-Y pattern required for triple-helix packing.

Common Pitfalls:
Confusing overall glycine content of a whole protein with the strict periodicity in collagenous regions.

Final Answer:
one-third