MODULE 7: PROPERTIES OF PROTEINS

• Casein


  Name for a family of related Phosphoproteins
• Casein


  • Makes up 80% of the proteins in cow's milk and between 20%-45% in human milk
  • The most important protein in milk which supplies essential amino acids as well as carbohydrates, Ca and P
  • Functions as storage protein which consists high number of non-interacting proline peptides
  • With relatively weaker tertiary structure, it can not denature
  • Relatively hydrophobic
  • Has a negative charge in milk (milk's pH=6.6)
• Isolation of Casein by Acidification

  1. Precipitation with 10% acetic acid

  2. Addition of ethyl alcohol-ethyl acetate mixture and filtration

  3. Drying

  4. Addition of NaOH & H2O
• Proteins are least soluble in water at their isoelectric points
• Biuret Test


  General test for proteins, when protein reacts with copper(II) sulfate, a positive test is the formation of a copper complex which has a violet color
• Ninhydrin Test


  Amino acids with free NH2 group and proteins containing free amino groups react with ninhydrin to give a purple-blue complex
• Ninhydrin


  Most commonly used to detect fingerprints, as the terminal amines of lysine residues in peptides and proteins sloughed off in fingerprints react with ninhydrin
• Isoelectric point (pI)


  The pH at which the majority of molecules of a compound in solution have no net charge
• Zwitterion


  Molecule has positive charge on one atom and negative charge on another atom, but which, no net charge
• Equilibrium shifts with change in pH

  1. Upon addition of an acid, H3O+ protonated the -COO-

  2. Upon addition of a base, the -NH3+ is deprotonated to donate to the -OH- from the base
• pH of milk: 6.6
• pH of milk is above the pI of casein


  Casein has a net negative charge
• Casein has a pI lower than the pH of milk

  Casein has no net charge
• pH at which no precipitation occurs: ~ pH 4 - 5
• pI of casein: pH 4.6
• Uses of Casein


  • Paint
  • Glue
  • Cheesemaking
  • Protein supplements
  • Medical and Dental Uses
• Paint

  • casein paint is a fast drying, water-soluble medium used by ancient artists until the late ‘60s and became less popular with the advent of acrylic paint
•  Glue 

  • casein based-glues were popular for woodworks
• Cheesemaking 

  • cheese consists of proteins and fats from milk and is produced by coagulation of casein
• Protein supplements

  • can form a gel and clot in the stomach which provides a sustained slow release of amino acids into the bloodstream
•  Medical and Dental Uses 

  • casein – derived compounds are used in tooth remineralization products to stabilize amorphous calcium phosphate
• pH of milk : 6.6

  • This pH is above the pI of casein, thus, at this pH, casein has a net negative charge
  • This results to an electrostatic attraction between casein proteins in milk
  • The aggregates of proteins is observed as precipitate
  • Addition of acetic acid lowers the pH of the milk
  • Casein has a pI lower than the pH of milk, where it has no net charged
  • At neutral charge, no electrostatic attraction occurs between proteins, thus, no precipitate is formed
  •  pH at which no precipitation occurs : ~ pH 4 – 5
  •  pI of casein : pH 4.6