Enzyme Kinetics: A Detailed Guide

What is Enzyme Kinetics?

• The study of the rates of enzyme-catalyzed reactions.
• Helps understand how enzymes work and how they are regulated.
• Analogy: Managing a factory (enzyme) that converts raw materials (substrate) into finished goods (product).

The Enzyme-Substrate Handshake

• Basic Reaction: E + S ⇌ ES → E + P
  • E: Enzyme (worker)
  • S: Substrate (raw material)
  • ES: Enzyme-Substrate Complex (worker holding material)
  • P: Product (finished good)
• Rate Constants: k₁ (binding), k₋₁ (dissociation), k₂ (catalysis).

The Michaelis-Menten Equation

• Equation: v₀ = (Vmax * [S]) / (Km + [S])
  • v₀: Initial reaction velocity (speed).
  • [S]: Substrate concentration.
  • Vmax: Maximum reaction velocity (top speed of the factory).
  • Km: Michaelis constant (efficiency meter).

Vmax: The Speed Limit

• Definition: The theoretical maximum rate of an enzyme-catalyzed reaction.
• Achieved when the enzyme is saturated with substrate (all active sites are occupied).
• Vmax is directly proportional to enzyme concentration.

Km: The Efficiency Meter

• Definition: The substrate concentration [S] at which the reaction velocity v₀ is half of Vmax (Vmax / 2).
• Inverse measure of affinity:
  • Low Km: High affinity (enzyme binds substrate tightly, efficient).
  • High Km: Low affinity (enzyme binds substrate weakly, less efficient).

The Michaelis-Menten Graph

      Vmax |-----------------------(Reaction approaches Vmax)
           |                     /
           |                    /
   v₀      |                   /
           |                  /
Vmax/2 ----|................./ 
           |                /
           |_______________/____________
                    |
                    Km      [S] -->

The Lineweaver-Burk Plot

• Purpose: Converts the hyperbolic Michaelis-Menten curve into a straight line for easier determination of Vmax and Km.
• Equation: 1 / v₀ = (Km / Vmax) * (1 / [S]) + 1 / Vmax
  • Y-intercept: 1 / Vmax
  • X-intercept: -1 / Km
  • Slope: Km / Vmax

Enzyme Inhibition

• Molecules that reduce an enzyme's activity.

1. Competitive Inhibition:
   • Inhibitor resembles substrate, binds to active site.
   • Vmax: Unchanged.
   • Km: Increases (apparent).
   • Lineweaver-Burk: Lines intersect at Y-axis.

Enzyme Inhibition (Continued)

2. Uncompetitive Inhibition:

   • Inhibitor binds only to the ES complex.
   • Vmax: Decreases.
   • Km: Decreases (apparent).
   • Lineweaver-Burk: Lines are parallel.

3. Non-competitive Inhibition:

   • Inhibitor binds to a site other than the active site (allosteric site).
   • Vmax: Decreases.
   • Km: Unchanged.
   • Lineweaver-Burk: Lines intersect at X-axis.