Proteins

• Amino acids are monomer units which combine to make a polypeptide.
• Every amino acid has a central carbon atom to which are attached four different chemical groups

  • An amino group (NH2) 
  • Carboxyl group (COOH)
  • Hydrogen atom
  • R group ( each amino acid has a different R group)
• Forming polypeptides

  Formation of a peptide bond via a condensation reaction, water is made from combining an -OH from carboxyl group of one amino acid with a hydrogen atom from the amine group from another amino acid. These two amino acids are joined via a peptide bond.
• Primary structure of proteins:

  • The sequence of amino acids in a polypeptide chain. This sequence is determined by DNA, it is the primary structure that determines its ultimate shape and function. A change in a single amino acid in the primary structure may stop it carrying out its function.
• Secondary structure 

  • Hydrogen on the amine group has an overall positive charge and the O on the carboxyl group has a negative charge, these readily form hydrogen bonds. This causes the long polypeptide chain to be twisten in a 3D shape such as the coil known as the alpha helix
• Tertiary structure:

  • The alpha helix can be twisted and folded even more to give the complex and often specific 3D structure of each protein. This structure is maintained by a number of bonds, where the bonds occur depending on the primary structure of the protein.

  Bonds include: ionic bonds, hydrogen bonds, Disulfide bridges: when two molecules of the amino acid cysteine come close together, the sulphur atom in one cysteine bonds to the sulphur atom in the other.
• Quaternary structure: 

  • Combination of a number of different polypeptide chains and associated non-protein (prosthetic group) into a large complex protein molecule.
• Test for proteins (biuret test) 

  1. Place a sample of the solution in a test tube and add an equal volume of sodium hydroxide at room temp.
  2. Add a few drops of very dilute copper sulphate solution and mix gently
  3. A purple coloration indicates presence of peptide bonds and hence a protein, no protein = solution remains blue.