Proteins

• There are 20 amino acids, 9 of which are essential. The only difference between them is the R group.
• An amino acid consist of a carbon atom bonded to an amino group, a carboxyl group, an R group and a hydrogen atom.
• Polypeptides are formed by the condensation of 3 or more amino acids. A peptide bond forms when OH from carboxyl group and H from amino group is lost, releasing a water molecule.
• Primary protein structure: the sequence of amino acids in the polypeptide chain.
• Secondary protein structure: hydrogen bonds form between NH and C=O, causing the chain to coil into an alpha helix or fold into a beta pleated sheet.
• Tertiary protein structure: the chain is coiled and folded further. Hydrogen bonds, ionic bonds and disulphide bridges form between R groups.
• Quaternary protein structure: multiple polypeptide chains working together as a large molecule. Each polypeptide chain is called a subunit.
• Hydrogen bonds:

  • form between amino and carboxyl group in secondary structures, between polar R groups in tertiary structures.
  • weak bonds that are easily broken by high temperatures or pH changes.
• Ionic bonds:

  • forms between oppositely charged R-groups.
  • stronger than hydrogen bonds, but can be broken by pH changes.
• Disulphide bridges:

  • covalent bonds that form between sulphur atoms in cysteine bonds.
  • strong, not affected by high temperatures or pH changes.
• Protein functions:

  • Enzymes: biological catalysts that speed up chemical reactions (eg. amylase)
  • Transport: carry substances throughout the body (eg. haemoglobin)
  • Hormone: coordinate activities of different body systems (eg. insulin)
  • Defense: protect the body from foreign pathogens (eg. antibodies)
  • Structure: physically strong, builds different structures (eg. keratin, collagen)
• Biuret test:

  1. Add a few drops of NaOH solution to the sample to make it alkaline.
  2. Add copper(II) sulphate solution, then mix gently.
  3. If protein is present, the solution turns from blue to purple.
• Similarities of dipeptides

  • 2 amino acids joined by peptide bonds
  • amino group at one end, carboxyl group at the other
  • 2 R groups
  • all contain C, H, N, O

  Differences of dipeptides

  • different amino acids/R groups