Proteins (enzymes)

• Enzymes

  Proteins that catalyse reactions. The chemical reactions that enzymes bind to are substrates.
• Biological catalyst

  • A substance that speeds up a reaction without being used up and so can be reused
  • Enzymes= proteins that catalyse biochemical reactions
  • Enzymes can act inside or outside cells
  • They determine structure and functions from cellular to whole organism level
• Activation energy


  Specific amount of energy needed to start a chemical reaction and the free energy of the products must be less than that of the substrates
• Activation energy & enzymes

  Enzymes lower the activation energies of chemical reactions inside the cell to increase the rate of reactions by binding to the reactant molecules (substrate) & allowing chemical bonds- breaking and bond forming processes- to happen more easily
• Active site


  • Enzymes catalyse specific reactions
  • Active site has specific shape for each enzyme and is a depression made out of a relatively small number of amino acids
  • Substrates with complementary shape to an active site can bind to form an enzyme-substrate complex
  • Shape of the active site is determined by the tertiary structure of the polypeptide
  • Substrate = held within the active site by bonds that temporarily form between certain amino acids of the active site & groups of the substrate
• Enzyme conditions

  For an enzyme to work it must contact physically with the substrate and have an active site which fits the substrate
• Active site

  • Every enzyme only catalyses 1 specific reaction
  • Every enzyme has a specific site that is complementary to the specific substrate
• Environmental changes

  Can change the tertiary structure of the active site & stop the enzyme from working properly (denatured enzyme)
• Investigating rate of enzyme controlled reactions

  pH: pH fluctuations inside organisms= small, more likely to reduce enzyme's activity than denature it. Changing the pH changes the number of hydroxide ions & hydrogen ions (OH⁻ and H⁺) surrounding the enzyme. These interact with the charges on the enzyme's amino acids, affecting hydrogen bonding and ionic bonding -> changes in the tertiary structure (& substrate can no longer attach to the active site). pH= measure of a solution's hydrogen ion concentration. Any change in the pH decreases the rate from the optimum.
• Denatured enzymes: Increasing or decreasing temp or pH outside optimal range can affect chemical bonds within the active site -> enzyme wont work as well (at extreme temp and pH values) -> enzymes structure may be changed permanently (= denatured enzyme)
• Increasing temperature

  Increase in kinetic energy of the molecules -> increased chance of collision between enzyme & substrate (more effective collisions resulting in more enzyme-substrate complexes being formed) -> more collisions likely in a set period of time (= rate of reaction is faster). Increasing temp by 10°C approx double rate of reaction in most enzyme controlled reactions
• Increasing substrate concentration

  Increases numbers of substrate molecules that can form ES complexes at any one time -> Increases initial rate of reaction but when all enzyme molecules are engaged in ES complexes then rate of reaction cannot increase any further. The rate will then plateau (enzyme is saturated)
• Increasing enzyme concentration

  More enzyme molecules able to catalyse the substrate in a given amount of time
• Competitive inhibitors

  • Inhibitors= chemicals that slow or stop the rate of reaction. ES complexes cannot be formed/ formed at a lower rate
  • Similar to usual substrate shape & affect active site directly blocking access for formation of ES complexes. Increasing the substrate concentration can compensate for competitive inhibitor effects (no permanent damage to shape of active site)
• Non-competitive inhibitors

  • Some have reversible effects but others are irreversible & denature the enzyme
  • They affect another part of the enzyme molecule causing a change to the active site. The active site is no longer complementary to the substrate molecules.
• Calculating rate of reaction

  By measuring the increase in products or decrease in reactants over time. Calculating the gradient of graph of enzyme activity against time also gives rate of reaction. Units depend on quantity measured eg cm³ s⁻¹ (cubic centimetres per second) & gdm⁻¹s⁻¹ (grams per decimeter per second)
• Enzymes have optimal conditions where the enzyme works at its max rate (same conditions= not optimum for every enzyme). Denaturation occur if external conditions cause changes to enzyme tertiary structure, altering shape of the active site so substrate no longer fits
• MODEL OF ENZYME ACTION:

  
• Competitive inhibitors have a similar but not the same shape as the substrate