Properties
The proteins have the following important properties –
1) Macromolecules. Proteins are very large molecules. Their molecular weight varies from the minimum of 4500 for adrenocorticotrophin to the maximum of 4.600.000 for pyruvate dehydrogenase.
2) Solubility. The proteins, due to the large size of their mo;ecules, form colloidal solutions and do not diffuse through cell membranes. They are, therefore, hydrolysed during digestion to their constituent amino acids by breakdown of the peptide bonds. The amino acids have relatively small moleculesw and can diffuse through the cell membranes.
3) Permeation. Proteins cannot pass through cell membranes. They enter or leave the cells by endocytosis and exocytosis. Normally, each cell prepares its own proteins by polymerisation of amino acids.
4) Amphotery. The proteins are amphoteric. The amino group is basic and can combine with acids, the carboxyl group is acidic and can combine with bases. Because of having both the groups, the proteins are very important buffers in the cells.
5) Reactivity. Due to the presence of different reactive groups on the side chains of their amino acids, the proteins may bind to a variety of inorganic ions, chemical groups and organic molecules in the cells.
6) Denaturation and Renaturation. Extreme changes in temperature or pH disrupt the bonds that maintain tertiary protein structure, leading to loss of the functional activity of proteins. This change is called denaturation. This is why no organism can survive temperatures above 45 – 55 C. Colour and appearance of an egg change on cooking due to denaturation of its proteins. Egg albumin becomes opaque during cooking because the denatured proteins are insoluble and solidify.
A denatured protein may spontaneously refold into its original structure provided conditions are suitable. This is called renaturation. This shows that tertiary structure is determined by primary structure.
Types. The proteins may be classified on the basis of the shape, components or nature of their molecules.
Shape of Molecules
Proteins are of 2 types according to the shape of their molecules : fibrous and globular.
1) Fibrous Proteins. The fibrous proteins have polypeptide chains spirally wound to form fibres. They have secondary structure. They are insoluble in water, are tough and are contractile. They are nonenzymatic and structural in role. The common examples are collagen of connective tissue, actin and myosin of muscles, keratin of scales, feathers, hair, claws, nails, horns and hoofs, silk in spider web.
Fibrinogen is a fibrous but soluble protein of blood plasma. It forms insoluble fibrin when blood clots.
2) Globular Proteins. The globular proteins have polypeptide chains coiled about themselves to form spherical molecules. They are noncontractile and have tertiary structure. They may be enzymatic or nonenzymatic. Small globular proteins, such as histones, are usually soluble in water and are not coagulated by heat. Large globular proteins are insoluble and are coagulated by heat. These include egg albumen, serum globulins, haemoglobin of
erythrocytes, glutens of wheat and rice, most of the enzymes and many hormones.