Biology
Enzyme Kinetics
Enzymes are biological catalysts that speed up reactions by lowering activation energy. The Michaelis-Menten equation describes their kinetics: rate v = V_max·[S] / (K_m + [S]). V_max is the maximum rate at saturating substrate; K_m is the substrate concentration at half V_max — a measure of enzyme-substrate affinity.
How to Approach Enzyme Kinetics
- 01
Plot rate vs substrate concentration
At low [S], rate is approximately linear in [S]. As [S] increases, rate plateaus toward V_max. The curve is a rectangular hyperbola.
- 02
Extract V_max and K_m
V_max is the asymptote — the rate at infinite substrate. K_m is the [S] at which rate = V_max/2. A Lineweaver-Burk plot (1/v vs 1/[S]) linearizes the curve, making both easy to read.
- 03
Analyze inhibition
Competitive inhibitor: K_m increases, V_max unchanged. Noncompetitive: V_max decreases, K_m unchanged. Uncompetitive: both decrease proportionally. The shape of the Lineweaver-Burk plot tells you which.
Frequently Asked Questions
What does K_m physically represent?
The substrate concentration at which the enzyme works at half-max rate. Lower K_m = higher affinity for substrate. Enzymes optimized for low-abundance substrates evolved low K_m values.
Why do enzymes saturate?
Every enzyme molecule can only process so many substrate molecules per second. At high [S], every active site is occupied — adding more substrate doesn't help because the enzymes are all working as fast as they can.
How do drugs use enzyme kinetics?
Many drugs are enzyme inhibitors. Competitive inhibitors (like statins) compete with substrate for the active site — they can be 'overcome' with more substrate. Noncompetitive inhibitors bind elsewhere and can't be.
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