Use Vmax and Km to compare enzyme affinity
Vmax describes the maximum rate under stated conditions; Km is derived from it and used to compare enzyme–substrate affinity.
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons teach enzyme concepts and experimental reasoning. Written practice is self-checked; it does not demonstrate supervised practical competence or replace teacher-reviewed assessment.
Before you begin
- Recognise that proteins have specific three-dimensional shapes and that reaction rates can be compared using controlled measurements.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How is Km located on a Michaelis–Menten curve?
What you will learn
- Interpret Vmax as the maximum reaction rate reached under stated conditions
- Use half Vmax to identify Km and compare affinity cautiously
Read a saturation curve
As substrate concentration increases, reaction rate rises and then approaches a maximum, Vmax, when enzyme active sites are occupied much of the time under the stated conditions.
Km is obtained from the substrate concentration at half Vmax on the Michaelis–Menten relationship. Use the rate axis to find Vmax, halve that rate, then read the corresponding substrate concentration from the horizontal axis.
Compare affinity using Km
Km is used to compare the affinity of different enzymes for their substrates under comparable conditions. A lower Km means half-maximal rate is reached at a lower substrate concentration and is commonly interpreted as higher apparent affinity.
Make the comparison for the stated enzyme–substrate systems and conditions. Do not infer that a low Km alone means a faster Vmax; Vmax and Km describe different features of the rate relationship.
Pause and explain
What does Vmax describe?
Worked example
A rate curve approaches Vmax = 80 units s⁻¹. The rate is 40 units s⁻¹ at substrate concentration 2.5 mmol dm⁻³. State Km and what it can help compare.
Show the worked solution
- Half Vmax is 40 units s⁻¹.
- Read the substrate concentration at that rate.
Answer Km = 2.5 mmol dm⁻³ because that substrate concentration gives half Vmax. Km can be used to compare apparent affinity for substrate with another enzyme under comparable conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A graph has Vmax = 120 µmol min⁻¹. At what rate should you read across to find Km?
Give me a hint
Km is associated with half Vmax.
Compare my reasoning
- Half of 120 is 60.
- Read the substrate concentration at rate 60 µmol min⁻¹.
Read across from 60 µmol min⁻¹, which is half Vmax, then down to the substrate-concentration axis to determine Km.
Look for these in your work
- Half Vmax calculated
- Correct graph-reading method
Enzyme A has Km = 1.2 mmol dm⁻³ and enzyme B has Km = 4.8 mmol dm⁻³ for their stated substrates under comparable conditions. Which has the lower Km, and what comparison does that support?
Give me a hint
Compare the concentrations needed for half-maximal rate.
Compare my reasoning
- A has the lower Km.
- Lower Km is used to indicate higher apparent substrate affinity under comparable conditions.
Enzyme A has the lower Km. Under comparable conditions, this supports a higher apparent affinity of enzyme A for its substrate; it does not by itself show that A has a higher Vmax.
Look for these in your work
- Lower value identified
- Affinity inference and limit stated
Enzyme X has a higher Vmax but also a higher Km than enzyme Y. Explain why neither value alone settles which enzyme has greater affinity.
Give me a hint
Separate maximum capacity from the half-maximal substrate concentration.
Compare my reasoning
- Vmax describes maximum rate.
- Km is the value used to compare apparent substrate affinity.
Vmax describes maximum rate under the stated conditions, whereas Km is the parameter used to compare apparent substrate affinity. The higher Vmax for X does not mean it has higher affinity; X’s higher Km instead indicates lower apparent affinity under comparable conditions.
Look for these in your work
- Parameters distinguished
- Affinity conclusion based on Km
Common mistakes
- Calling Km the maximum rate
- Reading Km at Vmax rather than half Vmax
- Concluding that lower Km necessarily means higher Vmax
What can you explain now?
How is Km located on a Michaelis–Menten curve?
Compare with the explanation
Read substrate concentration at half Vmax
Km is identified at half-maximal rate.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What does Vmax describe, how is Km read from a curve, and how is Km used in comparing affinity?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.