Chloride Ions Stabilize Human Adult Hemoglobin in the T-State, Competing with Allosteric Interaction of Oxygen Molecules

In the context of a molecular-level understanding of the allostery mechanisms, human adult hemoglobin (HbA) has been extensively studied for over half a century. Chloride ions (Cl-) have been known as one of HbA allosteric effectors, which stabilizes the T-state preferable to release oxygen molecules. The functional mechanisms were individually proposed by Ueno and Perutz several decades ago. Ueno considered that the site-specific Cl- binding is essential, while Perutz proposed the non-site-specific interaction between HbA and Cl-. Each speculation explains the mechanism plausibly since each was tightly associated with its reasonable experimental observation. However, both mechanisms themselves still seem to make their speculations controversial. In the present study, we have theoretically reconsidered these apart from their approaches. Our atomistic molecular dynamics simulations then showed that the increase of Cl- concentration suppresses the conformational conversion from the T-state. Interestingly, chloride ions loosely interact with the amino acid residues inside the HbA central cavity, suggesting that both Perutz's and Ueno's speculations are involved in understanding the microscopic roles of Cl-. In conclusion, we theoretically certified that the effect of Cl- competes against that of solvated O2, i.e., the destabilization of T-state through the non-site-specific interaction, implying the concerted regulation of HbA under physiological conditions.