proteins and enzymes

• proteins are made up of monomers called amino acids.
• fibrous proteins form long parallel chains and have structural functions, such as collagen.
• Globular proteins


  Spherical and have many metabolic functions (e.g., enzymes)
• Amino acid structure


  NH₂-C-COOH, with a side chain (R group)
• The twenty amino acids that are common to all organisms differ from each other only in their side chain (R group)
• Peptide and polypeptide formation

  1. Amino acids join together via a condensation reaction, forming a peptide bond, with water as a by-product

  2. A dipeptide molecule is formed by the condensation of two amino acids

  3. A polypeptide is formed by the condensation of many amino acids

  4. A functional protein can contain one or more polypeptides
• Enzymes


  • Biological catalysts that speed up rates of reaction but remain unchanged and can be used repeatedly
  • Specific to a particular substrate, with only the active site being functional
  • The active site has a specific 3D shape, which is complementary to the substrate
• Enzyme-substrate complex formation and catalysis

  1. When an enzyme binds with its substrate(s), an enzyme-substrate complex is formed

  2. After the reaction, the products leave the active site and the enzyme is free to collide with more substrate

  3. Each enzyme lowers the activation energy of its reaction
• Primary structure

  The number and sequence of amino acids in the polypeptide chain
• Secondary structure

  Hydrogen bonds form between some amino acids to either pleat or twist a polypeptide
• Secondary structure


  • A single hydrogen bond is weak but many hydrogen bonds give these structures stability
  • R-group atoms are not involved
• Tertiary structure

  The final specific 3D shape of the polypeptide is held in place by ionic bonds, disulfide bonds, and hydrogen bonds between R-groups. It is also determined by the hydrophobic and hydrophilic interactions.
• Quaternary structure

  Separate polypeptides linked together. Non-protein, or prosthetic, groups may be associated with proteins with a Quaternary structure.
• Quaternary structure

  • Haemoglobin is made of four polypeptide chains with the prosthetic group haem.
• primary structure functions

  • Amino acid arrangement determines the 3D shape of an enzyme active site
• Biuret test


  Uses biuret reagent that changes colour, from pale blue to lilac, in the presence of protein
• Chromatography


  Can be carried out to identify the components of a mixture of unknown amino acids, when compared to a known standard solution of amino acids
• secondary structure examples

  beta-pleated sheets and alpha helices
• Enzyme concentration

  • This increases the number of active sites
  • More enzyme-substrate complexes form
  • The reaction rate increases until substrate concentration becomes the limiting factor
• Substrate concentration

  • This increases the number of enzyme-substrate complexes formed, so the rate of reaction increases
  • When all of the enzyme active sites are working, substrate concentration will become the rate-limiting factor
• Temperature

  • Raising the temperature increases the kinetic energy
  • Above the optimum temperature, the rate of reaction decreases
  • If the temperature gets too high, the enzyme becomes denatured, and the reaction stops
• pH

  • pH affects hydrogen bonds and ionic bonds holding the active site in its 3D shape
  • Above and below the optimum pH, the rate of reaction decreases
• Optimum temperature


  Each enzyme has its unique optimum
• Competitive inhibitor

  • Competes for the active site of the enzyme
  • An active site blocked by the competitive inhibitor is not able to catalyse a reaction, so the rate of the reaction slows
• Non-competitive inhibitor

  • Binds to a non-functional part of the enzyme and changes the specific shape of the active site
  • The substrate cannot fit into the active site, enzyme-substrate complexes do not form, and the rate of the reaction decreases