Unit D - Enzymes

• What are enzymes?

  Functional 3-D proteins that act as biological catalysts.

  Enzyme names often end with "-ase" and are named after their substrates.
• What is activation energy?

  Energy that is needed to start a chemical reaction, usually heat.
• What is the problem for activation energy?

  For cells to start reactions, they need to put in energy; but temperature increase can coagulate their protein and the cells die.
• What is the solution for activation energy?

  Enzymes lower the necessary energy of activation.
• What is the Lock and Key model?

  The Lock and Key model is a concept that describes how enzymes and substrates interact. It suggests that enzymes have a specific shape that only allows certain substrates to bind to them, similar to a lock and key fitting together.
• Definition of substrate in Lock and Key model

  The molecule to which the enzyme attaches itself

  Ex. Like a key attaching to a lock.
• Definition of active sight in Lock and Key model

  The area of an enzyme that attaches itself with the substrate molecule.

  Ex. The hole in a lock made specifically for a key.
• What is the first substrate in the lock and key model?

  The substrate, sucrose, made of glucose and fructose, which are bonded together.
• What happens when the substrate binds to the active site?

  Enzyme-substrate formation, sucrase.
• What happens after Enzyme-substrate formation?

  Water is added.
• After water is added, what happens to the Enzyme-substrate formation?

  The bonds between glucose and fructose is broken and the products are released. The enzymes are now free to bind to another sucrose substrate.
• What are the two enzyme helpers?

  Cofactors and co-enzymes.
• What are cofactors?

  Cofactors are inorganic molecules, atoms or ions (Fe2, Zn2, K+) that help enzymes bind to substrate molecules.

  Cofactors are made from minerals.
• What are co-enzymes?

  Co-enzymes are organic molecules that help enzymes bind to substrate molecules.

  Co-enzymes are made from vitamins.
• What are the factors affecting enzyme activity?

  Temperature, pH, substrate concentration, concentration of enzyme, and presence of inhibitors or activators.
• Temperature affecting enzyme activity

  • Increase temperature = faster reaction
  • High temperatures (over 40 degrees) can denature the enzyme, stopping it from working - acting sight can no longer bind substrate molecules.
  • Enzymes work at an optimum temperature.
• pH affecting enzyme activity

  • Each enzymes work at its optimum pH.
  • Change in acidity or basicity can alter enzyme shape.
  • Active site can no longer bind substrate molecules.
• Concentration of substrate affecting enzyme activity

  • Enzymes work slowly with low substrate concentration
  • Usually more substrate increases reaction time
  • When the enzyme is working as fast as it can, more substrate will not make it work faster, the enzyme has reached maximum velocity (Vmax).
• What are the two types of inhibitors?

  Competitive and non-competitive inhibitors.
• Competitive Inhibition key points

  • These inhibitors compete with the substrate for the active site on the enzyme. If they bond to the enzyme, they stop the reaction.
  • The inhibitor is only loosely bound to the active site and can be replaced by the substrate, therefore, the effect is reversible.
  • The degree of inhibition depends on the relative concentrations of the inhibitor and substrate.
• Non-competitive Inhibition key points

  • An inhibitor attaches to the enzyme changing the shape of the active site = the substrate cannot bind.
  • This is an irreversible event. Adding more substrate will not reactivate the enzyme.
• Negative feedback/ Feedback Inhibition key points

  • Enzymes participate in a metabolic pathway where the substrate is modified by a number of enzymes before producing a final product.
  • As the final products accumulates within the cell, it binds to the regulatory site of an enzyme in the pathway, changing its shape, and thus preventing the substrate from binding.
  • The final product is no longer produced until concentrations are reduced.
• Precursor definition

  Accumulation of substrate molecules causes these molecules to attach to the regulatory site of one of the enzymes in a pathway, which improves fit between enzyme and substrate --- increases reaction rate.
• Substrate


  The molecule to which an enzyme attaches itself (reactants)
• Active Site


  The area of an enzyme that attaches with the substrate molecule
• Enzyme Helpers - Cofactors

  Inorganic molecules, atoms or ions (Fe2+, Zn2+, K+) that help enzymes bind to substrate molecules.
• Enzyme Helpers - Co-enzymes


  Organic molecules that help enzymes bind to substrate molecules.

  Made from vitamins.
• Factors Affecting Enzyme Activity: Temperature

  Increase temp = faster reaction

  Too high (over 40) can denature the enzyme, stopping it from working

  Enzymes work at an optimum temperature
• Factors Affecting Enzyme Activity: pH

  Each enzyme work at its optimum pH.

  Change in acidity or basicity can alter enzyme shape

  Active site can no longer bind substrate molecules
• Factors Affecting Enzyme Activity: Concentration of Substrate

  Enzymes work slowly with low substrate concentration.

  Usually more substrate increases reaction rate.

  When the enzyme is working as fast as it can, more substrate will not make it work faster, the enzyme has reached maximum velocity (Vmax)
• Factors Affecting Enzyme Activity: Concentration of Enzyme

  Usually the presence of more enzyme will increase the reaction rate, providing there is an unlimited supply of substrate.
• Factors Affecting Enzyme Activity: Inhibitors

  Two types:

  1. Competitive inhibition
  2. Non-Competitive inhibition
• Factors Affecting Enzyme Activity: Competitive Inhibiton

  Compete with the substrate for the active site on the enzyme. If they bond to the enzyme, they stop the reaction.

  Inhibitor is only loosely bound to the active site and can be replaced by the substrate, therefore the effect is reversible.

  The degree of inhibition depends on the relative concentrations of the inhibitor and substrate.
• Factors Affecting Enzyme Activity: Non-Competitive Inhibitors

  Attaches to the enzyme changing the shape of the active site = the substrate cannot bind.

  This is irreversible.