The amino acid that constitutes the highest proportion in the structure of collagen is Glycine.

- Collagen is the most abundant protein in the human body and is characterized by its unique triple helix structure.
- The primary structure of collagen consists of repeating amino acid sequences, where every third amino acid is glycine.
- This specific pattern is crucial because glycine, being the smallest amino acid with a hydrogen as its side chain, allows the tight packing of the three polypeptide chains in the triple helix.

- Glycine accounts for approximately one-third of the total amino acid content in collagen, making it the most abundant.
- Proline and hydroxyproline are also abundant and important for the stability of collagen due to their role in forming rigid structures and hydrogen bonding, but they are present in smaller amounts than glycine.
- Although lysine and its hydroxylated form contribute to crosslinking that strengthens collagen fibers, their proportion in collagen is significantly less compared to glycine.

Reference: Gray’s Anatomy, 41st Edition, Volume 1, Chapter on Connective Tissue / Page 300