Based on the function they perform, proteins are classified into the following groups:

1. Structural proteins—keratin of hair and nails and collagen of bone
2. Enzymes or catalytic proteins—hexokinase and pepsin
3. Transport proteins—haemoglobin and serum albumin
4. Hormonal proteins—insulin and growth hormone
5. Contractile proteins—actin and myosin
6. Storage proteins—ovalbumin and glutelin
7. Genetic proteins—nucleoproteins
8. Defence proteins—snake venoms and immunoglobulins
9. Receptor proteins—hormones and viruses