Chain inequivalence in bovine methemoglobin

Area of Science:

• Biochemistry
• Protein dynamics
• Hemoglobin research

Background:

• Methemoglobin (MetHb) is a form of hemoglobin where iron is in the ferric (Fe3+) state.
• Understanding the conformational changes and ligand binding of MetHb is crucial for hemoglobin function.
• Bovine MetHb serves as a model system to study these complex interactions.

Purpose of the Study:

• To investigate the kinetics of oxygen binding and re-oxidation of a reduced heme intermediate in bovine methemoglobin.
• To elucidate the pH-dependent conformational transitions between the R (relaxed) and T (tense) states.
• To identify subunit inequivalences in ligand binding within the tetrameric structure.

Main Methods:

• Pulse radiolysis was employed to generate a transient valence intermediate by reducing a single heme.
• Spectrophotometric monitoring was used to study the kinetics of oxygenation and re-oxidation.
• pH-dependent studies were conducted to analyze conformational state transitions.

Main Results:

• Oxygenation kinetics revealed two species with distinct affinities, linked to R and T quaternary structures.
• A sigmoidal pH-dependent transition curve between R and T states was established.
• Subunit inequivalences in reaction rates with ferricyanide were observed in both R and T states.

Conclusions:

• Bovine methemoglobin exhibits distinct R and T quaternary structures that influence oxygen binding affinity.
• Chain inequivalences in ligand binding are present in both R and T states.
• Inositol hexakisphosphate shifts the pH-dependent T-to-R transition, but does not eliminate observed chain inequivalences.