Mutational studies on collagen proteins demonstrate that substitution of one particular amino acid significantly affects the normal structure of the collagen molecules. Which of the following amino acids is absolutely required for the stable formation of the collagen triple helix?
A) alanine
B) cysteine
C) glycine
D) phenylalanine
E) tryptophan

Question

Quizplus · Accepted Answer

Glycine is absolutely required for the stable formation of the collagen triple helix because it is the smallest amino acid and fits into the tight spaces in the helix, allowing the three strands to pack closely together.  All collagens contain 3-stranded helical segments of similar structure. The unique properties of each type of collagen are due mainly to segments that interrupt the triple helix and that fold into other kinds of 3-dimensional structures. The triple-helical structure of collagen arises from an unusual abundance of 3 amino acids: glycine, proline, and hydroxyproline. These amino acids make up the characteristic repeating motif Gly-Pro-X, where X can be any amino acid. Each amino acid has a precise function. The side chain of glycine, an H atom, is the only one that can fit into the crowded center of a 3-stranded helix. Hydrogen bonds linking the peptide bond nitrogen of a glycine residue with a peptide carbonyl group in an adjacent polypeptide help hold the 3 chains together. It is because of this role of glycine in collagen that it is indispensable for normal collagen structure and function.

Mutational Studies on Collagen Proteins Demonstrate That Substitution of One