Overview
This lecture reviews enzyme kinetics, focusing on the Michaelis-Menten curve and Lineweaver-Burk plot, and shows how different types of enzyme inhibition affect these graphs.
Michaelis-Menten Curve Basics
- The Michaelis-Menten curve is a rectangular hyperbola relating reaction velocity (V₀) to substrate concentration ([S]).
- At low [S], velocity increases linearly with [S] (first-order kinetics).
- At high [S], the curve plateaus; velocity approaches Vmax, showing zero-order kinetics.
- Vmax is the maximum velocity when all enzyme is saturated with substrate.
- Km is the substrate concentration at which reaction velocity is half of Vmax (½ Vmax).
Types of Enzyme Inhibition (Michaelis-Menten Plot)
- Competitive inhibition: Increases Km (curve shifts right), Vmax unchanged; can reach Vmax with sufficient substrate.
- Non-competitive inhibition: Km remains unchanged, decreases Vmax (curve plateaus lower); cannot reach original Vmax.
- Uncompetitive inhibition: Both Km and Vmax decrease (curve shifts left and plateaus lower); cannot reach original Vmax.
Lineweaver-Burk Plot
- This is a double-reciprocal (linear) plot: y-axis is 1/V₀, x-axis is 1/[S].
- Slope: Km/Vmax; y-intercept: 1/Vmax; x-intercept: –1/Km.
- Competitive inhibition: Increases Km (x-intercept closer to zero), slope increases, y-intercept (Vmax) unchanged.
- Non-competitive inhibition: Vmax decreases (y-intercept rises), Km unchanged; slope increases.
- Uncompetitive inhibition: Both Km and Vmax decrease; plot shifts, lines remain nearly parallel to the original.
Key Terms & Definitions
- Vmax — Maximum reaction velocity, when enzyme is saturated with substrate.
- Km — Michaelis constant; substrate concentration at half Vmax.
- First-order kinetics — Rate proportional to substrate concentration.
- Zero-order kinetics — Rate independent of substrate concentration.
- Competitive inhibition — Inhibitor competes with substrate, raising Km.
- Non-competitive inhibition — Inhibitor reduces enzyme function regardless of substrate, lowering Vmax.
- Uncompetitive inhibition — Inhibitor binds only to enzyme-substrate complex, lowering both Km and Vmax.
- Lineweaver-Burk plot — Linear representation of Michaelis-Menten data via reciprocals.
Action Items / Next Steps
- Practice drawing and interpreting Michaelis-Menten and Lineweaver-Burk plots for each type of inhibition.
- Review definitions and effects of each inhibitor type on enzyme kinetics.
- Revisit Michaelis-Menten and Lineweaver-Burk equations and understand their derivations.