Collagen is the main protein of connective tissue. 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 and two amino acids that are not inserted directly in ribosomes: hydroxyproline and hydroxylysine, former of them in quite big quantities. They are changed from proline and lisyne in enzymatic process, but details of this process still aren't known.
Another rare feature of collagen is its regular arrangement of amino acids: usually every third amino acid is glycine. There are very few other proteins which also have such a regularity.
Collagen forms structure collagen helix.