Collagen is the main protein of connective tissue. It has great tensile strength, and is the main component of ligaments and tendons. It is responsible for skin elasticity, and its degradation leads to wrinkles that accompany aging. Collagen also fills out the cornea where it is present in crystalline form.

Collagen has very interesting amino acid composition. It contains a lot of glycine and proline, as well as two amino acids that are not inserted directly by ribosomes: hydroxyproline and hydroxylysine, the former as a rather large percentage of the total amino acids. They are changed from proline and lysine in enzymatic processes, for which vitamin C is required.

Another rare feature of collagen is its regular arrangement of amino acids. Its sequence generally follows the pattern Gly-X-Y, where X is proline, and Y is proline or hydroxyproline. There are very few other proteins with such regularity. The resulting structure is called a collagen helix.

Collagen occurs in many places throughout the body, and occurs in different forms known as types:

• Type I collagen - This is the most abundant collagen of the human body. It is present in scar tissue, the end product when tissue heals by repair. It is also found in tendons.
• Type II collagen - Articular cartilage
• Type III collagen - This is the collagen of granulation tissue, and is produced quickly by young fibroblasts before the tougher type I collagen is synthesised.
• Type IV collagen - Basement membrane