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Comparison of enzyme kinetics with radioligand binding
The Michaelis-Menten equation for enzyme activity has a form similar to the equation describing equilibrium binding.
Note these differences between binding experiments and enzyme kinetics.
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It usually takes many minutes or hours for a receptor incubation to equilibrate. It is common (and informative) to measure the kinetics prior to equilibrium. Enzyme assays reach steady state (defined as constant rate of product accumulation) typically in a few seconds. It is uncommon to measure the kinetics of the transient phase before that, although you can learn a lot by studying those transient kinetics (see an advanced text of enzyme kinetics for details). |
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The equation used to analyze binding data is valid at equilibrium - when the rate of receptor-ligand complex formation equals the rate of dissociation. The equation used to analyze enzyme kinetic data is valid when the rate of product formation is constant, so product accumulates at a constant rate. But the overall system is not at equilibrium in enzyme reactions, as the concentration of product is continually increasing. |
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KD is a dissociation constant that measures the strength of binding between receptor and ligand. KM is not a binding constant. Its value includes the affinity of substrate for enzyme, but also the kinetics by which the substrate bound to the enzyme is converted to product |
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Bmax is measured as the number of binding sites normalized to amount of tissue, often fmol per milligram, or sites/cell. Vmax is measured as moles of product produced per minute. |
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