Enzymes

• Chemical reactions and Metabolism


  A chemical reaction leads to a chemical change in matter. Cells carry out thousands of chemical reactions. The sum of all these reactions is called cellular metabolism.
• Types of metabolic reactions

  • Catabolic reactions - breakdown of substances and release of energy
  • Anabolic reactions - taking in energy to build up complex molecules from simple molecules
• Catabolic or anabolic reactions


  • A glycogen molecule formed from glucose molecules (anabolic)
  • A protein formed from amino acids (anabolic)
  • The digestion of starch to maltose (catabolic)
  • Urea formed from ammonia and carbon dioxide (catabolic)
• Enzymes


  • Biological catalysts made of protein
  • Speed up chemical reactions
  • Specific - each enzyme will catalyse only one reaction
  • Not used up in the reactions they catalyse, can be used again and again
  • Combine with substrates to form enzyme/substrate complexes
  • Only a small amount of enzyme is needed to catalyse a lot of substrate
  • Affected by changes in temperature and pH
  • Many require a cofactor to work
  • Can be slowed down or stopped by inhibitors

  
• Specificity


  Each enzyme can catalyse only one particular reaction, because an enzyme can only react with a specific substrate molecule
• Enzyme-substrate complex formation


  1. Lock-and-key theory - active site has a specific 3D shape that fits the substrate

  2. Induced-fit theory - active site has a flexible shape that moulds around the substrate

  
• Activation Energy


  Energy needed to start a chemical reaction, like pushing a boulder over a hill
• Enzymes lower activation energy


  Enzymes provide a different pathway for the reaction to follow, allowing it to occur at lower temperatures

  
• As temperature increases


  Rate of enzyme-controlled reaction increases up to optimum temperature

  
• Temperature coefficient (Q10)


  Change in rate of reaction for each 10°C rise in temperature
• Above optimum temperature


  Rate of enzyme-controlled reaction falls dramatically due to enzyme denaturation

  
• Optimum temperature


  Temperature at which an enzyme-catalysed reaction has the highest rate, related to the enzyme's usual thermal environment

  
• As temperature increases


  Kinetic energy of substrate and enzyme molecules increases, leading to more collisions and faster reaction rate
• At high temperatures above 40°C


  Bonds holding enzyme in 3D shape are broken, causing denaturation and loss of catalytic activity

  
• Optimum pH


  pH at which an enzyme is most effective, deviations cause bonds to break and change the 3D shape of the enzyme, leading to denaturation

  
• Acidity and alkalinity can affect the active site of an enzyme
• Turnover number


  Number of substrate molecules one enzyme molecule can turn into products in one minute
• If conditions are suitable and there is excess substrate

  Rate of reaction is directly proportional to enzyme concentration

  
• If substrate is restricted


  It may limit the rate of reaction even if more enzyme is added

  
• As substrate concentration increases

  Rate of reaction increases for a given amount of enzyme

  
• Enzyme


  Enzyme that is active over the narrowest range of pH

  
• Enzyme concentration


  • Active site of an enzyme can be used again and again
  • Only a small amount of enzyme is needed to catalyse a lot of substrate
  • After the reaction, the active site is free to accept more substrate

  
• Turnover number


  The number of substrate molecules that one molecule of enzyme can turn into products in one minute
• Enzyme concentration and rate of reaction


  • If conditions are suitable and there is an excess of substrate, the rate of reaction will be directly proportional to the enzyme concentration
  • If substrate is restricted, it may limit the rate of reaction even if you add further enzyme

  
• Substrate concentration and rate of reaction


  • The rate of reaction increases with increase in substrate concentration up to a point
  • At this point, all the active sites are filled and the rate of reaction will not take place any faster by adding more substrates

  
• Cofactors


  Non-protein molecules that modify the chemical structure of the enzyme in some way so that it can function more effectively

  
• Types of cofactors


  • Prosthetic groups
  • Coenzymes
  • Activators
• Prosthetic groups


  Organic molecules that form a permanent attachment to the enzyme
• Coenzymes


  Small, non-protein organic molecules that help enzymes and substrates to bond with each other
• Activators


  Inorganic metal ions that form a temporary attachment to the enzyme and change its active site so that the reaction is more likely to take place
• Allosteric enzymes


  Enzymes that have a second site where non-substrate molecules can attach and regulate enzyme activity through negative feedback

  
• Elastase is a type of protease enzyme produced by white blood cells that can contribute to the progression of diseases
• Enzyme biomarkers can be used to diagnose and monitor the progression of diseases
• Enzyme inhibitors can be used as therapeutic drugs to treat diseases by disrupting the active site of the enzyme biomarkers
• Penicillin and antiviral drugs inhibit enzymes involved in bacterial cell wall formation and viral replication respectively
• Non-reversible inhibitors


  Alter the enzyme permanently, causing the disulphide bonds holding the enzyme together to break and the tertiary structure to be lost

  
• Immobilised enzymes


  • Enzymes are physically confined within an inert support material
  • Allows continuous flow of substrate across the material holding the enzyme
  • Prevents disruption of the tertiary structure of the enzyme

  
• Methods of enzyme immobilisation


  • Adsorption
  • Entrapment
  • Encapsulation
  • Cross linkage

  
• Properties of material used for enzyme immobilisation


  • Must be inert
  • Must be permeable to allow substance and product to pass through it
  • Must be permeable to enzyme if entrapment is being used
  • Insoluble in nature so that the enzyme can be recovered

  
• Examples of immobilised enzymes


  • Clinistix (glucose oxidase)
  • Glucose meters