Allostery of the two-state model of hemoglobin studied by ECEPP energy minimization

Recently, a powerful parallel-vector processor became available for molecular science. A new FORTRUN program was coded to treat the whole hemoglobin molecule with twofold symmetry. Using the X-ray coordinates of deoxyhemoglobin and oxyhemoglobin, minimum energy conformations were obtained for both the T-state and the R-state on the two-state model of allostery. From them, further energy minimization was performed with simple perturbation to bring the proximal region close to the heme group instead of oxygen binding, and the structural changes and energy changes were investigated. The difference of calculated energy changes between T and R was semiquantitatively in agreement with the experimental value 2.7 kcal/mol for one oxygen binding. When the perturbation was exerted on the alpha-subunits, the energy change within the perturbed alpha-subunits in the T-state gave a main contribution, and in the R-state, the structural changes of the alpha-subunits were specifically large. When the perturbation was exerted on the beta-subunits, the intersubunit interaction energy between alpha1 and beta2 (alpha2 and beta1) was dominant in the difference of the energy changes between T and R.