Notes on Enzyme Kinetics and Regulation

Enzyme Kinetics

• Definition: Study of the rate of reactions catalyzed by enzymes and how it is affected by various factors.
• Factors Affecting Enzyme Kinetics:
  • Enzyme concentration
  • Substrate concentration
  • Effectors (activators/inhibitors)
  • Temperature
• Importance of Studying Kinetics:
  • Provides quantitative descriptions of biocatalysis
  • Helps determine the order of substrate binding
  • Elucidates acid-base catalysis
  • Aids in understanding catalytic mechanisms
  • Useful for finding effective inhibitors
  • Informs on regulation of activity

Kinetic Measurements

• Steps for Kinetic Measurements:
  1. Mix enzyme and substrate.
  2. Record the rate of substrate disappearance or product formation over time (initial velocity, V0).
  3. Plot initial velocity vs. substrate concentration.
  4. Alter substrate concentration and repeat measurements.

Substrate Concentration Effects

• Key Observations:
  • Initial rate (V0) is primarily measured; it can be affected by substrate concentrations.
  • At low [S], V0 increases almost linearly.
  • At high [S], V0 increases minimally, leading to saturation.
• Michaelis-Menten Kinetics:
  • Proposed by Michaelis and Menten in 1913.
  • Enzyme forms an ES complex, which can convert to product (E + P).
  • Steady-state approximation is used for kinetic analysis, where the concentration of ES remains relatively constant.

Michaelis-Menten Equation Interpretation

• Key Relationships:
  1. When Km = [S]:
     $V<em>0 = \frac{V</em>{max}[S]}{[S] + [S]} = \frac{V_{max}}{2}$
  2. When Km >> [S]:
     $V<em>0 = \frac{V</em>{max}[S]}{Km}$
  • First-order reaction with respect to [S].
  1. When [S] >> Km:
     $V<em>0 = V</em>{max}$
  • Zero-order reaction; rate does not change with increasing substrate concentration.

Kinetic Parameters Comparison

• Key Parameters:
  • Km: Michaelis constant; varies widely among enzymes and substrates. Indicates substrate affinity.
  • Vmax: Maximum rate when the enzyme is saturated with substrate.
  • kcat: Turnover number; indicates the number of substrate molecules converted to product per unit time per active site.
  • $k<em>{cat} = k</em>2 ext{ (rate constant for rate limiting step)}$
• Normalization:
  • Compare efficiency using the ratio $\frac{k<em>{cat}}{K</em>m}$.

Two-Substrate Reactions

• Mechanisms:
  • Sequential Mechanism: All substrates bind to the enzyme before any product is released.
  • Ping-Pong Mechanism: One substrate is released before the second substrate binds; forms a transient ES complex.

Enzyme Inhibition

• Types of Inhibitors:
  1. Irreversible Inhibitors: Permanently inactivate the enzyme (e.g., nerve agents, penicillin).
  2. Reversible Inhibitors: Bind non-permanently; can be divided further into:
  • Competitive: Compete with substrate for binding to the active site.
    • Do not change Vmax but increase Km.
  • Uncompetitive: Bind only to the ES complex, reducing effective enzyme concentration.
    • Lower both Vmax and Km.
  • Noncompetitive: Bind enzyme regardless of substrate presence, decreasing Vmax but not affecting Km.

Regulation of Enzyme Activity

• Modulation Mechanisms:
  1. Allosteric Control: Regulatory sites affect enzymatic activity; example: CTP inhibits ATCase.
  2. Isoenzymes: Enzymes that catalyze the same reaction but are different in structure or expression.
  3. Covalent Modification: Enzyme activity modified by adding/removing functional groups (e.g., phosphorylation).
  4. Proteolytic Activation: Activation of enzymes that begins in an inactive form (zymogens).
  5. Control of Enzyme Amount: Regulation via gene expression affecting enzyme production.