Enzymes Notes

Enzymes

5.13 Enzymes Speed Up Chemical Reactions by Lowering Energy Barriers

  • Biological molecules possess potential energy that isn't spontaneously released.
  • Activation Energy:
    • The energy barrier that must be overcome for a chemical reaction to begin.
    • Activates the reactants.
  • Enzymes:
    • Function as biological catalysts.
    • Increase the rate of reaction without being consumed.
    • Usually proteins, but some RNA molecules can function as enzymes.
    • Speed up reactions by lowering the activation energy.

Activation Energy and Enzymes

  • Figure 5.13 illustrates the effect of an enzyme in lowering activation energy.
  • The reaction progresses from reactants to products.
  • The activation energy barrier is reduced by the enzyme.

Animation: How Enzymes Work

  • Enzymes work to convert reactants into products.

5.14 A Specific Enzyme Catalyzes Each Cellular Reaction

  • Enzymes are very selective in the reactions they catalyze.
    • Protein-protein interactions contribute to this specificity.
  • The shape of an enzyme determines its specificity.
  • Substrate:
    • The specific reactant an enzyme acts on.
  • Active Site:
    • The location on the enzyme where the substrate fits.
  • Enzymes are specific because only specific substrate molecules fit into their active sites.

Catalytic Cycle of an Enzyme

  • Step 1: The enzyme is available with an empty active site (e.g., sucrase).
  • Step 2:
    • The substrate (e.g., sucrose) enters the active site.
    • The active site enfolds the substrate through induced fit.
  • Step 3: The substrate is converted to products.
  • Step 4: The products (e.g., glucose and fructose) are released.
    • The enzyme is then available for another reaction.

Optimal Conditions for Enzyme Activity

  • Every enzyme has optimal conditions.
  • Temperature affects molecular motion.
  • Optimal temperature produces the highest rate of contact between reactants and the enzyme's active site.
    • Most human enzymes work best at 35-40°C.
  • The optimal pH for most enzymes is near neutrality (like blood).

Cofactors and Coenzymes

  • Cofactors:
    • Non-protein helpers required by many enzymes.
    • Bind to the active site.
    • Function in catalysis.
    • Some cofactors are inorganic ions (e.g., zinc, iron, copper).
  • Coenzymes:
    • Organic cofactors.
    • Most vitamins are coenzymes.

5.15 Enzyme Inhibition Can Regulate Enzyme Activity in a Cell

  • Inhibitor:
    • A chemical that interferes with an enzyme's activity.
  • Competitive Inhibitors:
    • Block substrates from entering the active site.
    • Reduce enzyme productivity.
  • Noncompetitive Inhibitors:
    • Bind to the enzyme somewhere other than the active site.
    • Change the shape of the active site.
    • Prevent the substrate from binding.

Enzyme Inhibition Mechanisms

  • Figure 5.15a shows how inhibitors interfere with substrate binding.
  • Competitive inhibitors compete with the substrate for the active site.
  • Noncompetitive inhibitors bind elsewhere and alter the enzyme's shape.

Regulation of Cell Metabolism

  • Enzyme inhibitors are important in regulating cell metabolism.
  • Feedback Inhibition:
    • The product of a reaction may act as an inhibitor of one of the enzymes in the pathway that produced it.

Feedback Inhibition

  • Figure 5.15b illustrates feedback inhibition in a metabolic pathway, where product D acts as an inhibitor of enzyme 1.

5.16 Connection: Enzyme Inhibitors as Drugs, Pesticides, and Poisons

  • Many beneficial drugs act as enzyme inhibitors, including:
    • Ibuprofen: Inhibits an enzyme involved in the production of prostaglandins (messenger molecules that increase the sensation of pain and inflammation).
    • Some blood pressure medicines.
    • Some antidepressants.
    • Many antibiotics.
    • Protease inhibitors used to fight HIV.
  • Pesticides
  • Deadly poisons for chemical warfare also act as enzyme inhibitors.