Consequences of the Non-specific Binding of a Protein to a Linear Polymer: Reconciliation of Stoichiometric and Equilibrium Titration Data for the Thrombin–Heparin Interaction

Theoretical aspects of the thermodynamic characterization of cooperative protein interactions with non-specific segments of a linear polymer lattice have been re-examined. This reconsideration has not only provided an alternative derivation of recursive expressions for the stoichiometry of random ligand binding prior to elimination of the parking problem but also extended that treatment to include binding with overlap of additional lattice units. The major obstacle to thermodynamic characterization of non-specific protein–polymer interactions is determination of the lattice capacity for ligand, which in turn defines the length of the polymer segment to which the protein binds. Although these parameters are most readily obtained from studies under conditions that ensure essentially stoichiometric interaction, the endpoint of such a titration is likely to reflect the irreversible rather than the equilibrium binding capacity of the lattice for ligand. Consideration of published results for spectrofluorometric titrations of the thrombin–heparin system under stoichiometric conditions in such terms has permitted their reconciliation with results of a later publication on the interaction under equilibrium conditions.