Classification and Functions of Proteins

Beyond the four structural levels examined in the previous section, proteins are also classified in two complementary ways -- by their overall molecular shape and by the biological function they perform.

On the basis of shape, proteins fall into two broad groups. Fibrous proteins are long, thread-like or rod-shaped molecules built largely around extended secondary structures such as the alpha-helix or beta-pleated sheet; they are typically insoluble in water and serve mechanical, structural roles -- keratin in hair, nails and horns, collagen in skin, bone and connective tissue, and fibrin in blood clots are familiar examples. Globular proteins, in contrast, are compact, roughly spherical molecules formed when a polypeptide chain folds back on itself into a tightly packed tertiary structure; they are generally soluble in water and carry out dynamic, functional roles rather than purely structural ones -- enzymes, antibodies, many hormones and carrier proteins are globular. …