Enzyme Kinetics and Steady State

Enzyme Kinetics

• An intermediate in a series of reactions is said to be in steady state when its rate of synthesis is equal to its rate of degradation.
• The Michaelis constant (Km) is the substrate concentration required to achieve 50% of the maximum velocity of the enzyme.
• Km is a measure of the substrate concentration at which an enzyme achieves half of its maximum reaction velocity (Vmax) when operating at saturating substrate concentrations.

Significance of Km

• Km reflects the affinity of an enzyme for its substrate.
• A low Km value indicates high affinity, meaning the enzyme can achieve half-maximal velocity at relatively low substrate concentrations.
• A high Km value indicates low affinity, requiring higher substrate concentrations to reach half-maximal velocity.
• Km values can provide insights into the substrate specificity of enzymes.

Rate of Reaction

• For molecules to react, they must contain sufficient energy to overcome the energy barrier of the transition state.
• The lower the free energy of activation, the more molecules have sufficient energy to pass through the transition state, and therefore, the faster the rate of the reaction.
• The rate of reaction is affected by substrate concentration, with velocity increasing until it reaches a maximum value (Vmax) and then plateauing.

Enzyme-Saturation

• When substrate concentration increases, velocity (vi) increases until it reaches Vmax, at which point the enzyme is said to be "saturated" with the substrate.
• At saturating conditions, vi depends solely on the rapidity with which product dissociates from the enzyme, allowing it to combine with more substrate.

Steady-State Assumption

• The steady-state assumption is that the rate of formation of ES is equal to that of the breakdown of ES (to E + S and to E + P).
• [ES] does not change with time, meaning the rate of ES synthesis is equal to its rate of degradation.

Enzyme Kinetics and Regulation

• Enzymes are protein catalysts that increase the rate of reactions without being changed in the overall process.
• Virtually all reactions in the body are mediated by enzymes.
• Kinetics is the study of the factors that influence reaction rates.

Energy Changes during the Reaction

• Virtually all chemical reactions have an energy barrier separating the reactants and the products.
• The energy barrier, called the free energy of activation, is the energy difference between the reactants and a high-energy intermediate that occurs during the formation of product.
• The enzyme does not change the free energies of the reactants or products, but accelerates the rate by which equilibrium is reached.

Catalytic Efficiency of Enzymes

• Factors responsible for the catalytic efficiency of enzymes include:
  • Transition-state stabilization: By stabilizing the transition state, the enzyme greatly increases the concentration of the reactive intermediate that can be converted to product.
  • Other mechanisms: The active site can provide catalytic groups that enhance the probability that the transition state is formed.
  • Visualization of the transition state: The enzyme-catalyzed conversion of substrate to product can be visualized as a process similar to removing a sweater from an uncooperative infant.