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
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.
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.
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.
Related Topics
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