Which of the following amino acids is NOT commonly found in collagen?

Collagen is a structural protein that is abundant in connective tissues, and its unique composition plays a crucial role in its function and stability. It is characterized by a repeating sequence, primarily made up of glycine, proline, and hydroxyproline. Glycine is the most abundant amino acid in collagen and is crucial for the formation of the triple helix structure due to its small size, allowing for tight packing of the helical structure. Proline and its derivative, hydroxyproline, are also integral to the stability and structure of collagen, contributing to the rigidity of the triple helix.

In contrast, tyrosine is not commonly found in collagen. While tyrosine plays important roles in other proteins and biochemical processes, it is not part of the typical repeating sequences or structure of collagen. Its larger side chain does not fit well in the tightly packed structure that is characteristic of collagen fibers. Therefore, identifying tyrosine as not commonly found in collagen reflects an understanding of collagen's specific amino acid composition and the structural requirements of this essential protein.