Dual-mode cooperative binding of adenosine 5'-triphosphate to poly(L-lysine)

Equilibrium dialysis of adenosine triphosphate interacting with polylysine at low polymer concentration yield Scatchard exhibiting an apparent non-cooperative binding mode superimposed on a positively cooperative one. The same behavior has been reported in the literature for various mononucleotides and basic poly(amino acids). It is pointed out here that both modes must not be considered as independent but to be mutually exclusive. Applying a pertinent theoretical approach yields higher degrees of cooperativity than obtained for mutual independence. Another necessary modification in the analysis of data takes into account that one ligand interacts with more than one equivalent binding contact (i.e., an elementary charge here). This leads to a further enhancement of the actual cooperativity. Increasing the polymer concentration in the binding experiments reveals a cooperative effect also in the first binding mode. This is physically interpreted and theoretically analyzed in terms of bound dimers stabilized by base stacking.