Based on composition, there are two types of proteins as follows:
- Simple Proteins: These proteins are made up of only amino acids e.g., albumins, globulins, glutelin.
- Conjugated proteins: These are formed by the binding of a simple protein with a non-protein called the prosthetic group. Depending upon the type of prosthetic group, conjugated proteins are of several types few of which are mentioned below:
Based on shape, proteins are classified into two types:
1.Fibrous proteins: They are thread-like structural proteins that may occur singly or in groups. Fibrous proteins generally possess secondary structures and are insoluble in water. The common example of fibrous proteins is collagen of connective tissue, actin and myosin of muscles, keratin of scales, feathers, hair, claws, nails, horns and hoofs, the silk of spider web. Fibrinogen is also a fibrous protein but soluble in blood plasma. It forms insoluble fibrin during the clotting of blood.
2.Globular proteins: They are spherical in shape and are involved in metabolic activities. They have a tertiary or quaternary structure. Smaller globular proteins are usually soluble in water and are not coagulated by heat, e.g., histones. Egg albumin, serum globulins and glutelin (wheat, rice) are examples of large globular proteins, which get coagulated by heat.
Role/ Functions of proteins
Proteins perform several functions in organisms. The main functions of proteins are as follows:
1.Structural Proteins: Many proteins serve as a building material of cells and tissues. They take part in the formation of a colloidal complex of protoplast, cell membranes, organelles, extracellular matrices and fibres. Some proteins form supporting structures, e.g., elastin of ligaments, collagen of tendons, cartilages, bone and connective tissue. Keratin is the major constituent of the external protective structure of animals like hair, feathers, horny layer of skin, nails, claws, hoofs etc.
2.Enzymes: Many proteins function as enzymes to catalyse biochemical reactions that occur in the living world. Enzymes play a key role in metabolism. Every enzyme is specific in action. E.g., Amylase, protease, RUBISCO etc. RUBISCO (Ribulose Bisphosphate Carboxylase Oxygenase) is the most abundant enzyme found in plants. It the principal enzyme involved in photosynthesis.
3.Carrier Proteins (Transport proteins): Some proteins act as carriers that bind and transport specific molecules across a membrane or in a body fluid. Hemoglobin of RBCs transports oxygen in the body. RBCs contain haemoglobin (Hb) which is made up of 4 polypeptide chains and contains an iron ion. Iron has a strong affinity for oxygen. In the lungs, oxygen combines with haemoglobin to produce oxyhemoglobins. In body cells, oxygen is released and can diffuse into a body cell. Other examples include proteins such as the myoglobin of muscles that store oxygen. α-globulin of blood carries thyroxine and bilirubin, β-globulin transport vitamins A, D and K, cholesterol and ions in the blood.
4.Receptor Proteins: Several proteins present on the external surface of the cell membrane act as receptor molecules. A receptor protein molecule receives chemical signals from outside a cell. When such chemical signals bind to a receptor, they cause some form of cellular/tissue response. For example, when insulin is released in the blood, the receptor proteins present on the cell membrane bind with this insulin. This binding changes the permeability of the cell membrane and glucose can enter the cell through glucose channels.
5.Hormones: Some hormones are proteinaceous, e.g., insulin(sugar metabolism regulating hormone} parathyroid (calcium and phosphate transport regulating hormones). Hormones play a key role in the regulation of metabolism.
6.Contractile Proteins: Myosin and actin make the muscle fibres contractile to bring about movements and locomotion. The contractile system is made up of protein actin but association with myosin is essential for contraction.
7.Defensive proteins: Some proteins act as antibodies (immunoglobulins) that participate in the defence mechanism of the body.
8.Storage Proteins: These occur in milk, eggs and seeds to nourish the young ones. They include casein of milk, albumin of egg white and glutelin in cereals.