Beta-chain allostery in the frozen quaternary T-structure of haemoglobin M Iwate.

Abstract

Two hybrid species of haemoglobin M Iwate exist: α2Mmetβmetβdeoxy and α2Mmetβ2deoxy These species differ in their ligand and effector binding properties.The α2Mmetβmetβdeoxy hybrid is characterized by a Bohr effect, while the Hill coefficient is n= 1.00. The energy of the interaction between the Bohr protons and the ligand‐binding site corresponds to the interaction energy found in monomeric haemoglobins and amounts to 0.55 kcal/mol. One molecule of 2, 3‐bisphosphoglycerate per mole is bound at neutral pH, while at alkaline pH two co‐operative molecules are bound. Binding of 2,3‐bisphosphogly‐cerate reduces the ligand affinity and shifts the pK value of the Bohr proton binding site to the alkaline region.The α2Mmetβ2deoxy hybrid shows a pH‐dependent co‐operativity of the ligand‐binding sites (n= 1.8 at pH 9). The co‐operativity is weakened by binding of protons or 2,3‐bisphospho‐glycerate. The latter lowers the ligand affinity, but in the alkaline region no influence is observed. Furthermore the Bohr effect curve is shifted to the alkaline region by this compound but the amplitude remains unchanged.The results of the present paper are interpreted in terms of intra‐chain and β‐β inter‐chain interactions, the latter being responsible for a consecutive change of the ligand affinity in a “frozen” T‐state molecule.