1. As enzymes. Many proteins function as enzymes to catalyse specific chemical reactions. For eg trypsin, pepsin etc.
2. As carriers. Some proteins act as carriers which bind and transport specific molecules across a membrane or in a body fluid. GLUT-4 enables glucose transport into cells.
3. Nutrient and storage proteins. Seeds of many plants store nutrient proteins required for the growth of embryonic plant as in wheat, corn. Ovalbumin is the protein of egg and casein is the milk protein.
4. Contractile or motile proteins. Proteins actin and myosin are filamentous proteins found in the contractile system of skeletal muscles which help in the movement.
5. Structural proteins. Some fibrous proteins form supporting filaments. Tendons are formed of collagen proteins and ligaments of elastin; keratin is found in hair, nails and feathers.
6. Defence proteins. Immunoglobins or antibodies form immune system of the body. These recognize bacteria, viruses or foreign proteins.
7. Regulatory proteins. Certain hormones or antibodies are globular proteins (such as insulin). They regulate cellular or physiological functions.
8. Receptor proteins. Receoptors proteins help in the reception of smell, taste. hormone etc.
9. Blood clotting. Proteins like fibrinogens help in blood clotting.