Proteins

• Proteins are sequences of amino acids joined by peptide links.

  • When 2 amino acids join together, it is a dipeptide.
  • When 3 amino acids join together, it is a tripeptide.
  • When many amino acids join together, it is a protein or polypeptide.
• Formation of a protein= Condensation reaction= Two amino acids are joined together, linked through a peptide link, one molecule of water is released.

  Destruction of a protein= Hydrolysis reaction= Peptide link is broken between two amino acids, requires one molecule of water.

  A) Amino acid

  B) Amino acid

  C) Condensation

  D) Hydrolysis

  E) Dipeptide

  F) Peptide link

  
• 2 different amino acids can join to form 2 different dipeptides:

  
• Proteins, polypeptides, dipeptides and tripeptides can all be hydrolysed back into their  constituent amino acids. This can be done using an acid or alkaline catalyst.
• Acidic hydrolysis:

  • Conditions - 6 mol dm-3 HCl, reflux 24 hours:
  • Produces an alkyl ammonium salt.

  
• Alkali hydrolysis:

  • Conditions - Solution of aqueous NaOH, heated above 100 degrees.
  • Produces an carboxylate salt.

  
• Structure of proteins:

  • Primary structure - Sequence of amino acids joined by peptide links.
  • Secondary structure - Hydrogen bonds form between peptide links. This causes the amino acid sequence of coil, into an alpha helix, or fold, into a beta-pleated sheet.
  • Tertiary structure- The secondary structure folds and coils on itself to give a 3-dimensional structure. Additional bonds and intermolecular forces hold the polypeptide or protein in this shape.
• Additional bonds and intermolecular forces:

  • Van der Waals’ forces of attraction, these form between the alkyl R side groups
  • Hydrogen bonds, these form between OH / NH / C=O in the R side groups and the main chain. They stabilise and provide support for the secondary structure.
  • Ionic bonds, form between COO- and NH3+ in the R side group.
  • Disulphide bonds (sulphur – sulphur bonds), formed between cysteine residues. They stabilise the protein.