Anion Bohr effect of human hemoglobin

Area of Science:

• Biochemistry
• Physiological Chemistry

Background:

• The Bohr effect describes how oxygen affinity of hemoglobin changes with pH.
• Identifying the specific ionizable groups responsible for the Bohr effect has been controversial.
• Previous studies on hemoglobin alkylation yielded conflicting interpretations regarding these groups.

Purpose of the Study:

• To investigate the role of ligand-linked pK shifts in the Bohr effect.
• To clarify the contribution of specific ionizable groups to hemoglobin's pH-dependent oxygen affinity.
• To explore the influence of buffer components on the Bohr effect.

Main Methods:

• Hemoglobin alkylation in both liganded and unliganded states.
• Analysis of alkylation extent at specific sites (alpha 1 valines, beta 146 histidines).
• Measurement of the Bohr effect using zwitterionic buffers (HEPES, MES, MOPS) without added anions.

Main Results:

• Alkylation of liganded hemoglobin abolished the Bohr effect, while unliganded hemoglobin retained it.
• No specific preferential alkylation was identified at alpha 1 valines or beta 146 histidines.
• Hemoglobin's Bohr effect curves differed significantly in zwitterionic buffers compared to conventional buffers, highlighting anion influence.

Conclusions:

• Solvent components, particularly anions, play a crucial role in determining hemoglobin's functional properties.
• A new thermodynamic model for the Bohr effect is proposed, incorporating hemoglobin-solvent interactions.
• The classic proton Bohr effect is presented as a specific instance within this broader model.