This protein is insoluble in water, salt, acid and alkaline solutions. It is called ‘animal skeletal protein’. This protein is not present in the plant body. Lives only in animal bodies. It has no action on enzymes. It is found in animal bones, hair, nails, horns, skin, hoofs etc. Eg- keratin (skin, nails, hair), collagen (skin), tendon (bone), elastin (ligament), fibroin (silk) etc.