Proteins

• • The monomers of proteins are amino acids
  • A dipeptide is formed when two amino acids join together
  • A polypeptide is formed when more than two amino acids join together
  • Proteins are made up of one or more polypeptides
• • Amino acids have the same general structure - a carboxyl group (-COOH), an amine group (-NH2) and an r group (also known as a variable side group) attached to a carbon atom
  • R groups generally contain carbon. The only exception to this rule is glycine - its R group consists of one hydrogen atom
• All living things share a bank of only 20 amino acids

  The only difference between them is what makes up their R group
• • Amino acids are linked together by condensation reactions to form dipeptides and polypeptides
  • A molecule of water is released during the reaction
  • The bonds formed between the amino acids are called peptide bonds
  • The reverse reaction happens when dipeptides and polypeptides are broken down
• Primary structure

  • This is the sequence of amino acids in the polypeptide chain
• Secondary structure:

  • The polypeptide chain doesn't remain flat and straight
  • Hydrogen bonds form between the amino acids in the chain
  • This makes it automatically coil up into an alpha helix or fold into a beta pleated sheet - this is the secondary structure
• Tertiary structure

  • The coiled or folded chain of amino acids is often coiled and folded further
  • More bonds form between different parts of the polypeptide chain, including hydrogen bonds and ionic bonds
  • Disulphide bridges also form whenever two molecules of the amino acid cytesine come close together - the sulfur atom in the other
  • For proteins made from a single polypeptide chain, the tertiary structure forms their final 3D structure
• Quaternary structure:

  • Some proteins are made of several different polypeptide chains held together by bonds
  • The quaternary structure is the way these polypeptide chains are assembled together
  • For proteins made from more than one polypeptide chain, the quaternary structure is the proteins final 3D structure
• The biuret test for proteins

  1. The test solution needs to be alkaline, so first you add a few drops of sodium hydroxide solution
  2. Then you add some copper(II) sulfate solution
  3. If protein is present, the solution turns purple. If there's no protein the solution will stay blue.
• Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end

  • One amine/NH2 group joins to a carboxyl/COOH group to form a peptide bond;
  • (So in chain) there is a free amine/NH2 group at one end and a free carboxyl/COOH group at the other
  • Each amino acid is orientated in the same direction in the chain