What is the Difference Between Kd and Km?
🆚 Go to Comparative Table 🆚The main difference between Kd and Km lies in their definitions and the properties they describe in enzymatic reactions:
- Kd (Dissociation Constant):
- Kd is a thermodynamic constant.
- It represents the affinity of a substrate towards an enzyme.
- A lower Kd indicates a stronger binding affinity between the enzyme and the substrate.
- Kd is calculated as the ratio of the concentrations of free enzyme (E) and free substrate (S) divided by the concentration of the enzyme-substrate complex (ES).
- Km (Michaelis Constant):
- Km is a kinetic constant, not a thermodynamic constant.
- It represents the relationship between substrate concentration and reaction speed.
- A lower Km means that the enzyme has a stronger affinity for the substrate and requires a lower substrate concentration to reach its maximum reaction speed (Vmax).
- Km is related to the concentration of substrate at which the enzyme is half-saturated (Vmax/2).
In summary, Kd is a thermodynamic constant that describes the binding affinity between an enzyme and its substrate, while Km is a kinetic constant that represents the relationship between substrate concentration and reaction speed.
Comparative Table: Kd vs Km
The main difference between Kd and Km lies in their nature and the information they provide about enzymatic reactions. Here is a table summarizing the differences between Kd and Km:
Property | Kd (Dissociation Constant) | Km (Michaelis Constant) |
---|---|---|
Nature | Thermodynamic constant | Kinetic constant |
Role | Represents the affinity of a substrate towards an enzyme | Represents the relationship between substrate concentration and reaction speed |
Affinity | Lower Kd means higher affinity, and higher Kd means lower affinity | Lower Km means higher affinity, and higher Km means lower affinity |
Formation | Kd = [A][B] / [AB] | Km = [E] at 1/2 Vmax |
Relationship | The ratio Km/Kd can approximate binding affinity, but assuming they are equivalent is a common and problematic fallacy | Km is related to Kd, and higher Km generally indicates a higher Kd |
In summary, Kd represents the affinity of a substrate towards an enzyme and is a thermodynamic constant, while Km represents the relationship between substrate concentration and reaction speed, and it is a kinetic constant.
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