Fix the mistake: Cells and Metabolism

How Enzymes Work and Activation Energy

Chemical reactions in living organisms proceed remarkably quickly under mild conditions, such as room temperature and near-neutral pH. Enzymes make this possible. Enzymes are biological catalysts that greatly increase reaction rates by lowering the activation energy required for a reaction.

Basic Knowledge

Chemical nature of enzymes: Most enzymes are proteins (some are RNA, called ribozymes). Because they function as catalysts, they themselves do not change before and after a reaction.
Activation energy and enzymes: For a chemical reaction to proceed, substrate molecules must overcome the "transition state." The energy required for this is the activation energy. Enzymes bind to substrates and stabilize the transition state, lowering the activation energy. They do not change the reaction's change in free energy (ΔG).
Active site and substrate specificity: Only specific substrates can bind to an enzyme's active site. This is called substrate specificity. The lock-and-key model (Fischer) and induced-fit model (Koshland) have been proposed, and the induced-fit model is currently dominant.
Reaction rate and inhibition: At low substrate concentrations, the reaction rate is proportional to substrate concentration, but as the concentration increases, the enzyme becomes saturated and approaches the maximum reaction velocity (Vmax). Competitive inhibition occurs when an inhibitor occupies the active site and can be overcome by increasing substrate concentration. Noncompetitive inhibition occurs when an inhibitor binds somewhere other than the active site and lowers Vmax.
Factors affecting enzyme activity: Temperature (above the optimum temperature, enzymes denature and lose activity), pH (maximum activity at the optimum pH: pepsin about pH2, amylase about pH7, trypsin about pH8), and coenzymes (nonprotein components derived from vitamins, such as NAD⁺ and FAD).

Example Problem
An enzyme promotes a reaction by lowering its activation energy, but it does not change the overall change in free energy (ΔG). Explain why.
Answer: Activation energy is the "barrier" that must be overcome for a reaction to proceed, whereas ΔG is the difference in energy between the starting point and endpoint of the reaction. By binding to the substrate, an enzyme stabilizes the transition state and lowers the barrier, but it does not change the energy difference between the reactants and products themselves. Therefore, ΔG does not change.
Key Points
  • Enzymes = biological catalysts made of proteins (some are RNA)
  • Lower activation energy → reaction rate increases (ΔG remains unchanged)
  • Active site = three-dimensional structure to which only the substrate binds
  • Substrate specificity = lock-and-key and induced-fit models
  • Competitive inhibition = recovery when substrate increases; noncompetitive inhibition = lower Vmax
  • Above the optimum temperature, enzymes denature and lose activity (irreversibly)
  • Coenzymes (NAD⁺, FAD, etc.) = nonprotein components derived from vitamins

Points of Caution

① Enzymes do not change before and after a reaction, but when denatured by high temperatures, strong acids, or strong bases, they permanently lose activity. ② The difference between competitive and noncompetitive inhibition is determined by whether the reaction recovers when the substrate concentration is increased. ③ A ribozyme is an RNA catalyst, not a protein, but it similarly lowers activation energy.

Practice

  1. Explain why enzymes show "substrate specificity" from the structural perspective of the active site.
  2. Compare competitive and noncompetitive inhibition in terms of how the reaction rate changes when the substrate concentration is increased.
  3. What is the approximate optimum pH of pepsin? Also state why optimum pH differs depending on the location in the digestive tract.

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