The interactive binding of two ligands by an allosteric protein.

The equilibrium constants describing the simultaneous binding of the ligands AMP and 1-anilinonapthalene-8-sulfonate (ansyl) by phosphorylase b have been evaluated. The binding of a single molecule of AMP per dimer unit causes only a moderate reduction in the binding constant for a single molecule of ansyl, but reduces that for a second molecule of ansyl by 10-fold. The binding of two molecules of AMP per dimer unit almost abolishes the binding of a single molecule of ansyl.