act3

• Polypeptide chains of proteins


  • They may fold into pleated sheet structures or coil helically
  • Folding is determined mainly by their amino acid sequence
  • Folding is stabilized by interchain hydrogen bonding
• Interactions stabilizing protein structure


  • Interchain hydrogen bonding
  • Ionic interaction or salt linkages
  • Hydrophobic forces of attraction
  • Dipole-dipole interaction
  • Disulfide linkage
• Changes in binding forces that stabilize the native protein in its natural conformation may lead to modifications in structure and composition
• Denaturation


  Results in changes in physical and chemical properties and loss of biological properties
• One distinctive consequence of denaturation is the decrease in solubility at the isoelectric point of the original protein
• Denaturation may also cause changes in particle size, increase in surface tension, increased reactivity of several side-chain groups, increased susceptibility to hydrolysis by proteases, or change in optical rotation in the direction of increased levorotation
• Precipitation of proteins is involved in many normal functions in the body, like blood clotting and casein precipitation during digestion
• Substances in the environment that alter protein structure and function will have a marked toxic effect if introduced into the body since animal tissues are chiefly protein
• Bacteria are effectively destroyed under certain conditions due to their protein content
• Precipitation of proteins is also employed in their identification and isolation
• Causes of denaturation


  • Heat
  • Violent shaking
  • Very high pressures
  • Ultraviolet radiation
  • Extremes of pH
  • High salt concentrations
  • Heavy metal ions
  • Organic solvents
  • Surface active agents
  • High concentrations of urea
  • Guanidine salts
  • Formamide
• Isoelectric point (pl)


  The pH at which the net charge of all a protein's dissociable groups is equal to zero, i.e. the sum of the positive charges equals the sum of the negative charges
• Isoelectric point (experimentally)


  The pH at which a protein does not migrate in an electrical field
• The solubility of most proteins is at a minimum at their isoelectric point
• The solubility increases at a pH slightly higher or lower than the isoelectric point when the protein molecules exist as anions or cations, respectively, and repulsion hinders aggregation
• The presence of other ions in the solution will alter the charge distribution and the net charge of the protein molecules so that the isoelectric point will usually differ from the isolonic point, which is the pH of a salt-free solution
• Proteins precipitate most readily at their isoelectric point regardless of the precipitating agent used
• Isoelectric precipitation of proteins is illustrated in the curdling of milk as it turns sour, as in the bacterial spoilage of milk and the preparation of cottage cheese
• Concentrated mineral acids and alkalis alter the ionization of carboxyl and amino groups, respectively, and thus disrupt salt linkages and hydrogen bonds
• Casein in milk, when acted on by the HCl of gastric juice, is precipitated as a curd
• Heller's ring test, a clinical test for protein in the urine, depends on the precipitation of any protein present when concentrated HNO3 is added
• Organic solvent precipitation of proteins


  Adding an organic solvent miscible with water such as methanol, ethanol, or acetone lowers the dielectric constant of the medium and decreases the activity of water, reducing the shielding of charges on the protein surface and favoring protein-protein interaction and precipitation
• The coagulating action of alcohol on proteins is used to fix tissues for histological examination and sterilize the skin with 70% alcohol solution
• Specific metallic ions combine with proteins at a pH higher than their isoelectric point when these proteins are in the anionic form, forming metal proteinates or coordination complexes