Proteins

• A dipeptide is formed when two amino acids join together
• a polypeptide is formed when more than two amino acids join together
• amino acids have an amino group (-NH2) a carboxyl group (-COOH) and a R group attached to a carbon atom
• all living things share only 20 amino acids
• amino acids join by condensation reactions and a peptide bond is formed
• di/polypeptides can be broken down by hydrolysis reactions
• levels of protein structure

  • primary
  • secondary
  • tertiary
  • quaternary
• the primary protein structure is the sequence of amino acids
• the secondary protein structure is when hydrogen bonds form between amino acids creating alpha helix or beta pleated sheets
• the tertiary protein structure forms when the polypeptide chain is folded further as hydrogen bonds, ionic bonds and disulphide bridges form
• disulphide bridges form when two molecules of the amino acid cysteine come close together
• the quaternary structure forms when multiple polypeptides join together - not all proteins have a quaternary structure
• the biuret test for proteins

  • add sodium hydroxide to make alkaline
  • add copper (II) sulphate

  positive result = blue -> purple
• When two amino acids join, the OH from one carboxyl (-COOH) group joins the H on the adjacent amino acids amino (-NH2) group to realease a molecule of water