The calculation of the Interaction Energy Between Subunits of Hemoglobin molecule

The interaction energies between subunits are calculated for oxy- and deoxyhemoglobin by taking into account the van der Waals interaction, the electrostatic interaction and the hydrogen bonds. In the transition from deoxy to oxy form, the van der Waals interaction becomes attractive only by 8 kcal/mole as a whole for hemoglobin, although the interaction in each pair of subunits except for the pair of subunits α_1 α_2 is much varied by the order of 15 kcal/mole. Moreover, the electrostatic interaction becomes more repulsive by 5 kcal/mole in the pair of α_1 β_2 (or α_2 β_1) without taking into account the salt bridges. Therefore the energy difference between deoxy and oxy quaternary structure is mainly to be attributed to the twelve salt bridges in which the C-terminals of four subunits and DPG participate. The calculated result leads to a conclusion that the entropy factor due to the rotation of C-terminal is essential in the transition from deoxy quaternary structure to oxy quaternary structure. This substantiates Perutz' model energetically, and give some quide to Monod-Wyman-Changeux's model