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Glossary of Pharmacology 

Michaelis constant (Km)

Definition: 

The Michaelis constant (Km) is a measure of the concentration at which a substrate/ligand causes 50% of its maximum activation potential (Vmax) of its binding partner at saturation. A binding partner with a high Km has low affinity for its substrate and requires a greater concentration of substrate to achieve Vmax.   

Relevance:

The Km of a binding partner relative to the concentration of a substrate/ligand allows us to predict if that binding partner will be activated or not given substrate/ligand availability. If the binding partner is an enzyme, it allows us to predict the rate of product formation depending on the concentration of substrate. Similarly, if the binding partner is a receptor, it allows us to predict the levels of response based on the amount of substrate.  Likewise, if the binding partner is a transporter, it allows us to predict the rate of substrate flux. 


Low Km at physiological levels of substrate suggests that the binding partner will act at a relatively constant rate as long as the substrate/ligand is present in the physiological range.   

High Km at physiological levels of substrate suggests that the binding partners activity will vary depending on the concentration of the substrate/ligand.   

Linked terms: Vmax, saturation, substrate, ligand, enzyme, receptor          

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