Enzymes

• What are enzymes?

  Enzymes are biological catalysts made of globular proteins
• What do globular proteins consist of?

  Globular proteins consist of hydrophilic amino acids on its surface and hydrophobic amino acids buried away in its centre.
• Why is the active site specific to only one type of substrate?

  The active site is specific and unique due to specific folding and bonding in the tertiary structure of the protein. This means that enzymes can only attach to substrates that are complementary in shape
• What kind of reactions do enzymes catalyse?

  Enzymes catalyse intracellular and extracellular proteins
• Give an example of an intracellular enzyme and explain its role

  Catalase - Enzyme that works within cells
• Give an example of an extracellular enzyme and explain its role

  Amylase - Enzyme that works outside the cell
• What happens when the active site binds with the substrate?

  An enzyme-substrate complex forms
• Mechanism of Enzyme Action

  Enzyme binds/attaches to the substrate to form an enzyme-substrate complex. The tertiary structure of the active site is complementary to the substrate.

  Once the substrate binds, the amino acids on the surface of the active site form temporary bonds with the substrate. The enzyme catalyses the reaction and forms an enzyme-product complex. The products are then released from the active site
• How do enzymes increase the rate of reaction?

  Enzymes provide an alternative pathway with a lowered activation energy. This means that more substrate molecules have enough energy to cross the barrier and react
• Lock and Key Hypothesis

  This model suggests that the enzyme is like a lock and the substrate is like a key. The active site has a fixed shape and the substrate molecule perfectly fits into the active site, lowering the activation energy and reacting.
• Induced Fit Hypothesis

  As the substrate approaches the active site, the tertiary structure of the enzyme changes shape slightly to mould around the substrate. This ensures the active site fits perfectly with the substrate
• What factors affect enzyme activity?

  - pH

  - Temperature

  - Enzyme concentration

  - Substrate concentration
• Effect of temperature on enzyme activity

  Low temperature: If the temperature is too low, there is insufficient kinetic energy for successful reactions

  High temperature: If the temperature is too high, enzymes will start to denature as the active site changes shape. High temperatures alter the tertiary structure which changes shape of the active site

  At the optimum temperature there is maximum frequency of collisions between substrate and active site

  
• Temperature coefficient

  DOES NOT APPLY TO TEMPERATURE ABOVE OPTIMUM AS ENZYMES DENATURE

  
• Effect of pH on enzyme activity

  Too high/low a pH will interfere with the charges in the amino acids in the active site. This causes ionic/hydrogen bonds to break. This alters the tertiary structure, changes the shape of the active site and the enzyme denatures

  Enzymes have different optimal pH's that they work at

  
• Effect of substrate concentration on enzyme activity

  Low substrate concentration: The reaction will be lower as there would be fewer collisions between substrate and enzyme

  High substrate concentration: The rate of reaction will plateau because all of the enzyme active sites are in use (enzyme is saturated)
• Effect of enzyme concentration on enzyme activity

  Low enzyme concentration: Lower rate of reaction as there will be fewer collisions between enzyme and substrate

  High enzyme concentration: Increases the rate of reaction as enzyme-substrate complexes will be more likely to form. At high concentrations of enzymes (unless a substrate is added), the rate of reaction plateaus as there will be insufficient substrates to bind to the enzymes
• What do non-competitive inhibitors do?

  Non-competitive inhibitors bind to the enzyme away from the active site called the allosteric site. This causes the active site to change shape and therefore the substrate can no longer bind, no matter how much substrate is added. This means fewer enzyme-substrate complexes are formed, and the rate of reaction is lower

  
• What do competitive inhibitors do?

  Competitive inhibitors are the same shape as the substrate and are complementary in shape to the active, therefore they bind to the active site. They compete with the substrate to bind to the active site to form enzyme-inhibitor complexes, reducing the rate of reaction.

  
• What happens if there is a high concentration of substrate?

  If there is a high concentration of substrate, the substrate can knock out the inhibitor and the rate of reaction will increase again

  
• Some competitive inhibitors are reversible and some are...?

  Non-reversible meaning they irreversibly bind to the active site. A high concentration of substrate will have no effect

  
• What are end-product inhibitors and what is the role of end-product inhibition?

  End-product inhibitors are products formed in some reactions that are reversible inhibitors.

  They are involved in controlling reactions, however if lots of product is present, it will inhibit the enzymes and cause the reaction to slow down or stop

  
• Give an advantage of end-product inhibiton

  They prevent resources from being wasted

  
• What is a cofactor?

  A non-protein molecule that helps catalyse the reaction
• What is a coenzyme?

  A cofactor that is an organic molecule

  An organic non-protein molecule that helps catalyse reactions
• What is the role a cofactor/coenzyme?

  They may transfer atoms from one reaction to another in multi-step pathway reactions OR actually bind to the active site to make it complementary to the substrate
• Difference between cofactors and coenzymes?

  Cofactors are inorganic, whereas coenzymes are organic
• Give an example of a cofactor and coenzyme

  • Chloride ion is a cofactor for amylase
  • Vitamins are a source of coenzymes
• What is a prosthetic group?

  A prosthetic group are a type of cofactor, but they are permanently attached to the enzyme to the enzyme.
• Give an example of a prosthetic group

  Zinc ion in carbonic anhydrase
• What is the role of a prosthetic group?

  Prevent enzymes from causing damage within cells