1.4 2

• Catalysts


  Reduce the activation energy of reactions
• Measuring the rate of enzyme activity helps us understand protein structure and function
• Active site


  Has a very specific shape that the substrate will fit
• Enzyme specificity


  The active site and substrate slot together to form a complex
• Activation energy


  The energy needed for a reaction to get started
• Formation of enzyme/substrate complex

  1. Lowers the activation energy of the reaction

  2. Affects the bonds in the substrate, making it easier for them to break

  3. Brings the reacting substances close together, making it easier for bonds to form between them
• Raising the temperature


  Increases the rate of a chemical reaction by giving more molecules the energy to react
• Living cells could not survive the temperatures needed to make cellular reactions fast enough, and the energy demands to produce the heat would be enormous</b>
• Enzymes


  Solve the problem of low reaction rates by lowering the activation energy needed for a reaction to take place
• Lock-and-key hypothesis


  A simple model that helps us understand what happens when enzymes work
• The lock-and-key hypothesis is now thought to be an over-simplification
• Induced-fit hypothesis


  The active site of an enzyme is a flexible shape that is modified when the substrate enters to form the active complex
• Reaction rate


  Frequently measured when investigating enzymes and how they act as catalysts
• Enzymes generally increase reaction rates by factors from 10 to 10^14
• Only tiny amounts of most enzymes are needed due to their efficiency as catalysts
• Evidence for the structure of enzymes and how this relates to their functions comes from practical investigations into the effect of different factors on the rate of enzyme-catalysed reactions
• A large excess of substrate must be provided in enzyme experiments, unless the effect of substrate concentration is under investigation
• The initial rate of reaction is when the reaction proceeds at its fastest rate, giving the maximum reaction rate for an enzyme under particular conditions
• Enzymes


  Globular proteins that contain an active site vital to their functioning
• Anything affecting the shape of the protein molecule affects its ability to function, indicating the 3D nature of the molecule is important
• Enzymes change only the rate of a reaction, they do not change or contribute to the end products that form, or affect the equilibrium of the reaction
• Enzymes are very specific to the reactions they catalyse, some will only catalyse one particular reaction
• Substrate concentration


  Affects the rate of an enzyme-catalysed reaction, up to the point where the enzyme becomes saturated
• Temperature coefficient (Q10)


  Expresses the effect of temperature on the rate of a reaction
• Between 0°C and 40°C, Q10 for any reaction is 2 - the rate of the reaction doubles for every 10°C rise in temperature
• Outside the 0°C to 40°C range, Q10 for enzyme-catalysed reactions in humans decreases markedly, while for other reactions it changes only slowly
• At temperatures over 40°C most proteins, including most enzymes, start to lose their tertiary and quaternary structure and denature
• The enzymes of thermophilic bacteria that live in hot springs at up to 85°C are able to work at very high temperatures due to their temperature-resistant proteins
• The optimum temperature of the enzymes of many organisms, including cold water fish and many plants, is much lower than 40°C
• Changes in pH affect the interactions between R groups that hold the 3D structure of protein enzymes together, so different enzymes work best at different pH levels
• RuBisCo is a key but inefficient enzyme for life, catalysing only about 3 reactions per second compared to most enzymes which catalyse around 1000 reactions per second
• Plant cells overcome RuBisCo's inefficiency by making very large quantities of it, about half of the protein in a photosynthetic plant cell
• The active site of RuBisCo can bind to both carbon dioxide and oxygen molecules, with a greater affinity for carbon dioxide, but oxygen binding leads to photorespiration
• RuBisCo evolved in an atmosphere with much less oxygen and more carbon dioxide than today, so oxygen-binding was not a disadvantage at the time