Physical and chemical properties of protein

Physical properties of protein

  • Pure proteins are generally tasteless, though the predominant taste of protein hydrolysates is bitter.
  • Pure proteins are odourless. Because of the large size of the molecules, proteins exhibit many properties that are colloidal in nature.
  • Proteins, like amino acids, are amphoteric and contain both acidic and basic groups.
  • They possess electrically charged groups and hence migrate in an electric field.
  • Many proteins are labile and readily modified by alterations in pH, UV radiation, heat and by many organic solvents.
  • The absorption spectrum of protein is maximum at 280 nm due to the presence of tyrosine and tryptophan, which are the strongest chromophores in that region.
  • Hence the absorbance of the protein at this wavelength is adapted for its determination

Denaturation of protein

  • The comparatively weak forces responsible for maintaining secondary, tertiary and quaternary structure of proteins are readily disrupted with resulting loss of biological activity
  • This disruption of native structure is termed denaturation
  • Physically, denaturation is viewed as randomizing the conformation of a polypeptide chain without affecting its primary structure.
  • Physical and chemical factors are involved in the denaturation of protein.
  • Heat and UV radiation supply kinetic energy to protein molecules causing the atoms to vibrate rapidly, thus disrupting the relatively weak hydrogen bonds and salt linkages.
  • This results in denaturation of protein leading to coagulation. Enzymes easily digest denatured or coagulated proteins.
  • Organic solvents such as ethyl alcohol and acetone are capable of forming intermolecular hydrogen bonds with protein disrupting the intramolecular hydrogen bonding.
  • This causes precipitation of protein. Acidic and basic reagents cause changes in pH, which alter the charges present on the side chain of protein disrupting the salt linkages.
  • Salts of heavy metal ions (Hg2+, Pb2+) form very strong bonds with carboxylate anions of aspartate and glutamate thus disturbing the salt linkages.
  • This property makes some of the heavy metal salts suitable for use as antiseptics.

Renaturation

  • Renaturation refers to the attainment of an original, regular three-dimensional functional protein after its denaturation.
  • When active pancreatic ribonuclease A is treated with 8M urea or ß mercaptoethanol, it is converted to an inactive, denatured molecule.
  • When urea or mercaptoethanol is removed, it attains its native (active) conformation.

Chemical properties of proteins

Colour reactions of proteins

  • The colour reactions of proteins are of importance in the qualitative detection and quantitative estimation of proteins and their constituent amino acids.
  • Biuret test is extensively used as a test to detect proteins in biological materials.

Biuret reaction

  • A compound, which is having more than one peptide bond when treated with Biuret reagent, produces a violet colour.
  • This is due to the formation of coordination complex between four nitrogen atoms of two polypeptide chains and one copper atom

Xanthoproteic reaction

  • Addition of concentrated nitric acid to protein produces yellow colour on heating, the colour changes to orange when the solution is made alkaline.
  • The yellow stains upon the skin caused by nitric acid are the result of this xanthoproteic reaction.
  • This is due to the nitration of the phenyl rings of aromatic amino acids.

Hopkins-Cole reaction

  • Indole ring of tryptophan reacts with glacial acetic acid in the presence of concentrated sulphuric acid and forms a purple coloured product.
  • Glacial acetic acid reacts with concentrated sulphuric acid and forms glyoxalic acid, which in turn reacts with indole ring of tryptophan in the presence of sulphuric acid forming a purple coloured product.

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