Single residue modification of only one dimer within the hemoglobin tetramer reveals autonomous dimer function

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Human hemoglobin cooperativity is traditionally modeled as a two-state T (low affinity) to R (high affinity) transition.
• This transition involves changes in hydrogen bonds and salt bridges at the dimer-dimer interface.
• Symmetric modifications weakening these contacts decrease cooperativity and increase ligand affinity.

Purpose of the Study:

• To investigate the transmission mechanism of cooperativity within the human hemoglobin tetramer.
• To differentiate between quaternary and dimer-based communication pathways in hemoglobin.

Main Methods:

• Preparation of hybrid hemoglobin tetramers containing one modified dimer and one wild-type dimer.
• Analysis of cooperative free energy of ligand binding to both modified and wild-type dimers within the hybrid tetramer.
• Comparison with results from doubly modified tetramers.

Main Results:

• The cooperative free energy of ligation to the modified dimer was similarly perturbed in both hybrid and doubly modified tetramers.
• The cooperative free energy of ligation to the wild-type dimer remained unperturbed in the hybrid tetramer.
• Hemoglobin tetramers exhibit asymmetric responses to single-site modifications, indicating localized effects.

Conclusions:

• Loss of dimer-dimer contacts is not communicated across the interface but is transmitted through the affected dimer.
• These findings support a dimer-based model of cooperativity with an added quaternary component.
• Hemoglobin cooperativity involves communication pathways localized within individual dimers.