Final answer:
Collagen proteins become insoluble after maturation to enhance their stability, which is important for their structural role in the body.
Step-by-step explanation:
After step 2 of collagen protein maturation, the proteins become insoluble to enhance their stability. This is particularly important for fibrous proteins like collagen, which are meant to provide structural support and face various physical and chemical stresses. The insolubility is a result of strong stabilizing interactions such as disulfide linkages, hydrogen bonding, hydrophobic interactions, and ionic bonds that make the mature protein less reactive and prevent unwanted folding or denaturation. Collagen's stability and insolubility are crucial for its role in tissues such as skin, tendons, and bones.