Collagen fibre, molecular model. Collagen is a long structural protein, which usually takes the form of a triple helix known as tropocollagen. The three amino acids (protein subunits) glycine, hydroxyproline and proline together make up the polypeptide strands that are twisted around each other, as seen here. Collagen is the most abundant protein in the human body and has a tough fibrous tensile strength. It is found throughout the body's connective tissues (skin, cartilage, bone, ligaments and tendons) providing structural and metabolic support.