Anion Bohr effect of human hemoglobin

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Abstract
The pH dependence of O2 affinity of Hb (Bohr effect) is due to ligand-linked pK shifts of ionizable groups. Attempt to identify these groups has produced controversial data and interpretations. In a further attempt to clarify the situation, it was noticed that Hb alkylated in its liganded form lost the Bohr effect while Hb alkylated in its unliganded form showed the presence of a practically unmodified Bohr effect. In spite of this difference, analyses of the extent of alkylation of the 2 compounds failed to identify the presence of specific preferential alkylations. The .alpha.1 valines and .beta.146 histidines appeared to be alkylated to the same extent in the 2 proteins. Focusing attention on the effect of the anions on the functional properties of Hb, the Bohr effect of untreated Hb was measured in buffers made with HEPES [N-(2-hydroxyethyl)piperazine-N''-2-ethanesulfonic acid]. MES [2-(N-morpholino)ethanesulfonic acid] and MOPS [3-(N-morpholino)propanesulfonic acid], which being zwitterions do not need addition to chlorides or other anions for reaching the desired pH. The shape acquired by the Bohr effect curves, either as pH dependence of O2 affinity or as pH dependence of protons exchanged with the solution, was irreconcilable with that of the Bohr effect curves in usual buffers. This indicated the relevance of solvent components in determining the functional properties of Hb. A new thermodynamic model is proposed for the Bohr effect that includes the interaction of Hb with solvent components. The classic proton Bohr effect is a special case of the new theory.

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