BIOCHEM PRACTICAL EXAM

• Proteins


  The repeating unit is the amino acid. They function as biological catalysts or enzymes, transporters of oxygen, and hormones.
• Levels of protein structure

  • Primary
  • Secondary
  • Tertiary
  • Quaternary
• Primary structure


  • Composed of single covalently bonded amino acids
• Secondary structure


  • Two types: alpha helix and beta-sheets
• Alpha helix


  • One strand of amino acid chain bonded by intramolecular hydrogen bonds
• Beta-sheet


  • Two chains linked by hydrogen bonds
• Tertiary structure


  • Combination of alpha helix, beta-sheets, or both. Stabilized by salt linkages, hydrogen bonds, disulfide linkages, and hydrophobic interactions.
• Denaturation


  Unfolding of the complex secondary, tertiary, and quaternary structure of proteins
• Causes of denaturation


  • Heat, strong acids, organic solvents (e.g. alcohol)
• Heat causes denaturation


  Atoms vibrate more rapidly, breaking hydrogen bonds and hydrophobic interactions
• Strong acids cause denaturation


  Split salt linkages by ionizing the carboxylic group
• Alcohol causes denaturation


  Disrupts hydrogen bonds
• Heavy metal ions


  Denature proteins by combining with free carboxylate anions, causing precipitation
• Proteins (e.g. egg white) used as antidote for heavy metal poisoning
• Organic acids (e.g. picric, tannic)


  Used to precipitate alkaloids, disrupt salt linkages in proteins
• Leather is manufactured by coating animal skins with tannic acid (tanning)
• Biuret test


  Positive for compounds with two or more peptide linkages, gives purple color
• Ninhydrin test


  Gives blue to violet color with amino groups, except proline and hydroxyproline which give yellow
• Xanthoproteic test


  Nitration of amino acids with benzene rings gives yellow to orange color
• Millon's test


  Detects presence of phenol group in amino acids like tyrosine, gives white precipitate turning brick pink or red on heating
• Hopkins-Cole test


  Forms blue or violet color complex with indole ring of tryptophan
• Sakaguchi test


  Reacts with guanido group to give red to orange color, sensitive test for proteins
• Lead acetate test


  Specific for sulfur-containing amino acids like cysteine and methionine, forms brownish-black precipitate
• Coagulation by heat


  Heat causes unfolding and precipitation of proteins
• Coagulation by inorganic acids


  Concentrated HCl or H2SO4 cause precipitation of proteins, excess acid can redissolve the precipitate
• Coagulation by alcohol


  95% ethanol causes precipitation of proteins like albumin, gelatin, and peptone