1.4 Proteins

• There are about 20 different amino acids that all have a similar chemical structure but behave in very different ways because they have different side groups.
• Each amino acid has an amine group (NH2) and a carboxylic acid group (COOH).
• When 2 amino acids are joined together in a condensation reaction, the amine group from one and the carboxylic acid group from another form a peptide bond, producing one molecule of water.
• Hydrolysis is the opposite of condensation and is the breaking of a peptide bond using a molecule of water.
• Proteins are polymers. Describe why this is.


  They are made up of long chains of smaller units called monomers - in this case, these are amino acids joined by peptide bonds.
• The R group is the variable group.
• The R group determines the properties of the protein
• Two amino acids joined in a condensation reaction and connected by a peptide bond form a dipeptide.
• Amino acids contain Carbon, Hydrogen, Oxygen, Nitrogen and a variable side chain.
• Amino acids are zwitterions as they have opposite charges at each end, though the overall charge is neutral.
• Variable R groups can contain sulfur.
• The primary structure of a protein is the sequence of amino acids in a polypeptide chain. They have peptide bonds.
• The secondary structure of a protein is the first folding of the polypeptide chain. It is made up of alpha helices (spirals of polypeptide chains) and beta pleated sheets (sheets of polypeptide chains). It has hydrogen bonds which form between the amine and carboxyl group of amino acids.
• The tertiary structure of a protein is the final folding of a single polypeptide chain. Interactions and bonds are between the variable R groups. The types of bonds are hydrogen bonds, ionic bonds, and disulphide bridges. Tertiary does not contain prosthetic groups.
• The quaternary structure of a protein is the 3D structure of a protein that is formed by the interaction of multiple polypeptide chains. May have a prosthetic group attached. Not all proteins have a quaternary structure.
• Name a protein that has quaternary structure and a prosthetic group.

  Haemoglobin.
• What would happen to the overall protein structure if the amino acid sequence in the primary structure changed?

  The order of the R groups would change. The tertiary structure would change because the hydrogen bonds, ionic bonds, and disulphide bridges would form in different places. The protein may no longer be functional.
• The two main types of proteins are fibrous and globular.
• Fibrous proteins:

  • little/no tertiary structure
  • long parallel polypeptide chains
  • strong cross linkages at intervals
  • long fibres and sheets formed
  • mostly insoluble
  • most have a structural role
• Globular Proteins:

  • complex tertiary structure
  • folded into spherical/globular shape
  • soluble
  • some have quaternary
  • roles in metabolic reactions - enzymes
• The main role of fibrous proteins is structure.
• Fibrous proteins are stable because of strong cross linkages.
• The main role of globular proteins is in metabolic reactions.
• Haemoglobin and enzymes are two proteins that have a globular shape to enable them to bind with other molecules and carry out their functions.
• Keratin in hair and nails is a fibrous protein as it needs to be strong.
• Ion channel proteins control brain signalling by allowing small molecules into and out of nerve cells.
• Enzymes in your saliva, stomach, and small intestine are proteins that help you digest food.
• Huge clusters of proteins form molecular machines that do your cells' heavy work, such as copying genes during cell division and making new proteins.
• Antibodies are proteins that help defend your body against foreign invaders, such as bacteria and viruses.
• Receptor proteins stud the outside of cells and transmit signals to partner proteins on the inside of the cells.
• Muscle proteins called actin and myosin enable all muscular movement - from blinking to breathing to rollerblading.
• A protein called alpha-keratin forms your hair and fingernails, and also is the major component of feathers, wool, claws, scales, horns, and hooves.
• The haemoglobin protein carries oxygen in the blood to every part of the body.
• Biuret Test: -

  1. Add sample to distilled water and biuret solution.
  2. Shake solution and then leave for 5 minutes.
  3. A colour change from blue to violet indicates the presence of protein.

  Note: The Biuret test tests for the presence of peptide bonds, so will not detect a protein if the solution contains only free amino acids.
• Two proteins have the same number and type of amino acids but different tertiary structures. Explain why.

  Different sequence of amino acids in the primary structure causes a different tertiary structure as the hydrogen bonds, ionic bonds, and disulphide bridges form in different places.