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This curve is known as a rectangular hyperbola, binding isotherm, or saturation binding curve. Y is zero initially, and increases to a maximum plateau value Bmax.

This equation describes the equilibrium binding of a ligand to a receptor as a function of increasing ligand concentration.

X is the concentration of the ligand.

Y is the specific binding.

Bmax is the maximum number of binding sites, expressed in the same units as the Y-axis (usually radioactive counts per minute, sites per cell, or fmol of receptor per mg of tissue).

Kd is the equilibrium dissociation constant, expressed in the same units as the X-axis (concentration). When the drug concentration equals Kd, half the binding sites are occupied at equilibrium.

This equation also describes the activity of an enzyme as a function of substrate concentration. In this case, the variable labeled Bmax is really Vmax, the maximum enzyme activity, and the variable labeled Kd is really Km, the Michaelis-Menten constant.

 

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