Enzymes

• Enzymes


  Catalysts that serve the function of catalysis in organisms
• Enzymes


  • They are the most efficient catalysts known, they can increase the rate of a reaction by a factor of up to 10^20 over uncatalyzed reactions
  • They are highly specific, even to the point of being able to distinguish stereoisomers of a given compound
  • Their actions are fine-tuned by regulatory processes
• Enzyme kinetics


  The rate of a reaction
• Enzyme thermodynamics


  The thermodynamic favorability of a reaction
• The rate of a reaction and its thermodynamic favorability are two different topics, although they are closely related
• Standard free energy change (ΔG°)

  The difference between the energies of the reactants (the initial state) and the energies of the products (the final state) of a reaction
• Enzymes, like all catalysts, speed up reactions, but they cannot alter the equilibrium constant or the free energy change
• Activation energy (ΔG°‡)


  The energy input required to initiate the reaction
• Uncatalyzed reaction


  Requires more energy to get started, therefore its rate is slower than that of a catalyzed reaction
• Catalysis


  The process of increasing the rate of chemical reactions
• Enzymes


  Biological catalysts, usually globular proteins, with self-splicing RNA as the only exception
• Standard free energy change

  The difference between the energies of reactants and products under standard conditions
• Activation energy


  The energy required to start a reaction
• Transition state


  The intermediate stage in a reaction in which old bonds break and new bonds are formed
• Enzyme-Substrate Binding


  1. Enzyme binds to substrate to form complex

  2. Formation of transition-state species

  3. Formation of product
• Lock-and-key model


  Description of substrate binding to enzyme where active site and substrate exactly match in shape
• Induced-fit model


  Description of substrate binding to enzyme where enzyme conformation changes to accommodate substrate shape
• Michaelis-Menten Approach to Enzyme Kinetics

  1. Enzyme, E, and substrate, S, form complex, ES

  2. ES complex breaks down to reform E and S or form product, P, and release E
• Steady state


  Condition where concentration of enzyme-substrate complex remains constant despite continuous turnover
• Michaelis constant, KM


  Inverse measure of enzyme's affinity for substrate
• Michaelis-Menten equation


  Defines reaction velocity in terms of substrate concentration, KM, and maximum velocity, Vmax
• Enzyme Mechanisms


  • Ordered mechanism
  • Random mechanism
  • Ping-pong mechanism
• Enzymes can catalyze reactions with multiple substrates, but only one substrate can be studied at a time using Michaelis-Menten approach
• Catalase has a turnover number of 4 x 10^7 and KM of 25
• Carbonic Anhydrase has a turnover number of 1 x 10^6 and KM of 12
• Acetylcholinesterase has a turnover number of 1.4 x 10^4 and KM of 9.5 x 10^-2
• Chymotrypsin has a turnover number of 1.9 x 10^2 and KM of 6.6 x 10^-1
• Lysozyme has a turnover number of 0.5 and KM of 6 x 10^-3
• Enzymes


  Catalysts that serve the function of catalysis in organisms, with the exception of some RNAs (ribozymes)
• Enzymes are the most efficient catalysts known, they can increase the rate of a reaction by a factor of up to 10^20 over uncatalyzed reactions
• Nonenzymatic catalysts typically enhance the rate of reaction by factors of 10^2 to 10^4
• Enzymes


  • They are highly specific, even to the point of being able to distinguish stereoisomers of a given compound
  • They greatly increase the speed of a reaction
  • Their actions are fine-tuned by regulatory processes
• Thermodynamic favorability


  The difference between the energies of the reactants (the initial state) and the energies of the products (the final state) of a reaction, expressed as the standard free energy change (ΔG°)
• Enzymes, like all catalysts, speed up reactions, but they cannot alter the equilibrium constant or the free energy change
• Activation energy (ΔG°‡)


  The energy input required to initiate the reaction
• Uncatalyzed reaction


  Requires more energy to get started, so its rate is slower than that of a catalyzed reaction
• The presence of an enzyme lowers the activation energy needed for substrate molecules to reach the transition state, which increases the concentration of the transition state and results in a much greater rate of the catalyzed reaction compared to the uncatalyzed reaction
• Reaction of glucose and oxygen gas


  • Produces carbon dioxide and water
• The reaction of glucose and oxygen gas is thermodynamically favorable (spontaneous) because its free energy change is negative (ΔG° = -2880 kJ/mol = -689 kcal/mol)
• Spontaneous


  Does not mean "instantaneous", the activation energy must still be supplied to start the reaction