Protein Properties





The followings are the important protein properties:

Protein Properties: Molecular weight

Proteins are macromolecules and have high molecular weights (e.g., serum albumin 69000, serum globulin 150000-1300000, fibrinogen 400000, and hemoglobin 68000). The molecular weight of a protein depends on the number of amino acids chains present in the protein molecule.




Protein properties: Amphoteric nature and isoelectric pH

Like the amino acids, proteins are also ampholytes i.e., they have the capability to donate as well as accept H+ that is the proton. This is due to the presence of free -COOH group at one end and -NH2 group at the other end of the peptide chains and also due to the presence of many ionisable groups in the side chains of the amino acid residues. Since proteins have electric charges, they migrate in an electric field, the direction of migration depending on the net charge on the molecule.
For each protein, there is a specific pH at which the positive and negative charges on its molecules become equal so that the net charge comes to zero. This pH is called isoelectric pH (pi) or isoelectric point at which the molecules of the protein remain in the form of dipolar ions or zwitterions which do not move in an electric field. If a protein solution is maintained at its isoelectric pH, since the molecules of the protein form zwitterions, they exert no electrostatic repulsion and they tend to aggregate and precipitate. If the pH of a protein solution is less than its pi, the protein molecules remain as cations by accepting proton form the solution. But if the pH > pi, the protein molecules remain as anions by donating proton.




Protein properties: Solubility

Every protein has a characteristic solubility. In general, globular proteins (e.g., albumins) have greater solubility than fibrous proteins (e.g., keratin, collagen). Smaller protein molecules are more soluble than larger molecules. Solubility of a protein also depends on salt concentration and pH of the solution. For example, globulins are soluble in neutral sodium chloride solution but are almost insoluble in water. Some proteins like casein are soluble in alkaline pH. In separation of proteins from the nurture the difference in solubility helps a great.

Protein properties: Colloidal nature

Since proteins are macromolecules, their solutions are colloidal in nature is another protein properties.. Hence protein solutions exhibit colloidal properties like hyclnition and osmotic pressure, viscosity, electrophoretic movement, precipitation by sailing out, coagulation' etc. Most of the properties of protoplasm are attributed lo the proteins present in it. Proteins do not pass through semipermeable membranes ; this properly of proteins is of great importance on physiological aspect.




Protein properties: Denaturation

In animal and plant tissue the proteins possess many characteristic properties such as solubility, optical rotation, viscosity, electrophoretic mobility, sedimentation rate, osmotic pressure etc. A change of a protein molecule from its natural three dimensional form to an altered form leading to u change in its properties is called denatnration of tlie protein. However, denaluration is not associated with rupture of the covalent peptide bonds. Hence, due to denatnration of a protein molecule, its primary structure (amino acid sequence), molecular weight and osmotic pressure do not change but other properties are altered and biological activities are lost. . As a result of denaturation, the protein molecules may aggregate and precipitate. The denaturation of a protein may be reversible or irreversible.