Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.2.63 · Enzymes

Vmax and the Michaelis-Menten Constant — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 14 of 17 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

3.2.6 Vmax and the Michaelis-Menten Constant — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define Vmax and Km and use them to compare the affinity of enzymes for their substrates.Purpose — it describes the kinetics of enzyme-catalysed reactions.Method — measure th…

  • Two values
  • Definition
  • At this point
  • Inverse relationship
  • Michaelis-Menten model
  • Enzyme kinetics

Part 1 of 6Michaelis-Menten model

Purpose
  • Purpose it describes the kinetics of enzyme-catalysed reactions.
Method
  • Method measure the reaction rate at different substrate concentrations and plot a graph.
Two values
  • Two values the maximum velocity (Vmax) and the Michaelis-Menten constant (Km).

Part 2 of 6Vmax

Definition
  • Definition the maximum rate of reaction at saturating substrate concentration.
Meaning
  • Meaning all active sites are engaged.

Part 3 of 6Km

Definition
  • Definition the substrate concentration at which the enzyme works at ½Vmax.
At this point
  • At this point half of the active sites are occupied by substrate.
Use
  • Use it compares the affinity of different enzymes for their substrates.

Part 4 of 6Km and affinity

Inverse relationship
  • Inverse relationship a lower Km means higher affinity for the substrate.
Reason
  • Reason a high-affinity enzyme needs less substrate to reach half its maximum rate.
High Km
  • High Km an enzyme with a high Km has low affinity.

Part 5 of 6Finding Vmax and Km from a graph

1
Plot initial rate against substrate concentration.
2
Identify the plateau value as Vmax.
3
Calculate ½Vmax.
4
Read across from ½Vmax to the curve.
5
Read down to the x-axis; this concentration is Km.

Part 6 of 6More from the specification

1Competitive vs. Non-Competitive Inhibitors
  • Competitive inhibitors resemble substrate molecules and bind reversibly to the active site,
  • increasing Km without changing Vmax;
    • non-competitive inhibitors bind to allosteric sites, altering the active site shape and lowering Vmax.
2Effect of Temperature on Enzyme Kinetics
  • Increasing temperature increases the kinetic energy and speed of enzyme and substrate molecules, raising the rate of enzyme-substrate collisions until high temperatures disrupt tertiary bonds and denature the enzyme.
3Michaelis-Menten Constant (Km) and Vmax
  • Vmax represents the maximum rate of reaction when all enzyme active sites are saturated with substrate,
  • while Km reflects the substrate concentration needed to achieve half of Vmax, inversely indicating enzyme-substrate affinity.
4Advantages of Immobilised Enzymes
  • Entrapping enzymes in alginate beads prevents enzyme denaturation at elevated temperatures and extreme pH, facilitating enzyme recovery without contaminating the final product.
5Competitive vs. Non-Competitive Inhibitors
  • Competitive inhibitors resemble substrate molecules and bind reversibly to the active site,
  • increasing Km without changing Vmax;
    • non-competitive inhibitors bind to allosteric sites, altering the active site shape and lowering Vmax.
6Effect of Temperature on Enzyme Kinetics
  • Increasing temperature increases the kinetic energy and speed of enzyme and substrate molecules, raising the rate of enzyme-substrate collisions until high temperatures disrupt tertiary bonds and denature the enzyme.
7Michaelis-Menten Constant (Km) and Vmax
  • Vmax represents the maximum rate of reaction when all enzyme active sites are saturated with substrate,
  • while Km reflects the substrate concentration needed to achieve half of Vmax, inversely indicating enzyme-substrate affinity.
8Advantages of Immobilised Enzymes
  • Entrapping enzymes in alginate beads prevents enzyme denaturation at elevated temperatures and extreme pH, facilitating enzyme recovery without contaminating the final product.
9Buffer solution
  • A solution that holds the pH at a fixed value.
10Enzyme-substrate complex
  • The temporary structure formed when a substrate binds to an enzyme's active site.
11Inhibitor
  • A molecule that reduces enzyme activity.
12Lactose
  • A disaccharide of glucose and galactose.
13Optimum
  • The temperature or pH at which activity is highest.
14Buffer
  • A substance that resists pH change by absorbing or releasing H+.
15Successful collision
  • A collision with enough energy and correct orientation.
16Non-competitive inhibitor
  • An inhibitor that binds elsewhere and changes the active site.
17Immobilised enzyme
  • An enzyme fixed to a surface or trapped in a matrix so it can be reused.
18Enzyme inhibitor
  • A molecule that reduces the activity of an enzyme.

Key terms

TermMeaning
VmaxThe maximum rate of reaction at saturating substrate concentration.
KmThe substrate concentration at half Vmax.
AffinityHow strongly an enzyme binds its substrate.
Michaelis-Menten modelA model describing enzyme kinetics.
Enzyme kineticsThe study of how variables affect reaction rates.
Saturating concentrationA substrate concentration at which all active sites are occupied.

Avoid these mistakes

1
Reading Km as the substrate concentration at Vmax — it is at half Vmax.
2
Saying a high Km means high affinity — a low Km means high affinity.
3
Reading Vmax from the steepest part of the curve — read it from the plateau.
4
Plotting final rates rather than initial rates — use initial rates.

Last updated: