Distinction of structural reorganisation and ligand binding in the T<==>R transition of insulin on the basis of allosteric models.

Two allosteric models are presented for the T<==>R transition of insulin hexamers in the presence of phenolic ligands which are based on existing experimental information. The transition mainly involves residues 1-8 of the B-chain, i.e. 15% of the molecule, which are extended in the T- and helical in the R-state. The main facts to be accounted for are: 1) the transition is undergone trimer-wise; 2) the transition of the second trimer is disadvantaged compared to the first one; 3) the subunits of a trimer undergo transition in a cooperative process; 4) binding sites for phenolic ligands only exist in R3 trimers; 5) ligands shift the equilibrium by arresting the R-state; 6) the ligand is accommodated in a pocket made up between two adjacent subunits; 7) binding one ligand molecule extends the lifetime of the two other binding sites of a trimer; 8) only ligand-free trimers can undergo transitions. The two models allowed for CD spectroscopic titrations of zinc and cobalt insulin with phenol and m-cresol to be assessed in terms of structural reorganisation and ligand binding, and for the respective standard free energy differences to be calculated. delta G degrees for the reorganisation of the first timer in zinc-insulin is about 8 kJ/mol, and for that of the second trimer, 21kJ/mol. The corresponding values for cobalt-insulin are 12 and 24 kJ/mol, respectively. For both zinc- and cobalt-insulin, the delta G degrees for phenol and m-cresol binding is about -18 kJ/mol. Both models are equally compatible with the titration data.