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Updated: Jan 21, 2026
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Distinguishing the R2 from the R Quaternary Structure of Hemoglobin: Role of the Tertiary Structure Transition
Kehinde O Okonjo1, Abimbola M Olatunde1, J Chijioke Ajaelu1
1Department of Chemistry, University of Ibadan, Ibadan, Nigeria.
Hemoglobin
|January 20, 2026
Summary
This study uses Ellman's reagent (DTNB) to measure hemoglobin's affinity, separating allosteric effects. Inositol hexakisphosphate alters this affinity differently in R and R2 states, revealing insights into hemoglobin's tertiary structure and oxygen binding.
Area of Science:
- Biochemistry
- Structural Biology
- Protein Chemistry
Background:
- Hemoglobin studies face challenges in distinguishing allosteric and affinity factors influencing oxygen binding.
- The reaction of Ellman's reagent (DTNB) with hemoglobin's 93β sulfhydryl groups offers a method devoid of allosteric influence.
Purpose of the Study:
- To investigate the influence of inositol hexakisphosphate (inositol-P6) on the DTNB affinity of various hemoglobins across a pH range.
- To elucidate how inositol-P6 affects hemoglobin's tertiary structure equilibria (r⇌t) and its impact on R and R2 quaternary states.
Main Methods:
- Equilibrium studies of the reaction between DTNB and thirty-eight heme-liganded hemoglobins.
- Analysis of pH dependence profiles to understand tertiary structure shifts.
Main Results:
- Inositol-P6 decreases DTNB affinities in R2-state hemoglobins by favoring the 'r' tertiary structure.
- Inositol-P6 increases DTNB affinities in R-state hemoglobins by favoring the 't' tertiary structure.
- Observed pH-dependent changes in DTNB affinities provide insights into structural transitions.
Conclusions:
- Inositol hexakisphosphate modulates hemoglobin's tertiary structure, impacting its affinity for ligands.
- Differences in tertiary equilibria may partially explain variations in oxygen affinities among hemoglobins.
- The DTNB reaction serves as a valuable tool for dissecting hemoglobin allostery and affinity.
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