Structural Protein Measured in terms of their total mass, the largest class is the structural one. Structural proteins are fibrous proteins.The most familiar of the fibrous proteins are probably the keratins, which form the protective covering of all land vertebrates: skin, fur, hair, wool, claws, nails, hooves, horns, scales, beaks and feathers.tructural Protein Measured in terms of their total mass, the largest class is the structural one. Structural proteins are fibrous proteins.