Iron ligand recognition by monomeric hemoglobins

Area of Science:

• Biochemistry
• Biophysics
• Protein Chemistry

Background:

• Hemoglobin and myoglobin are crucial proteins involved in oxygen transport and storage.
• Understanding ligand binding to heme proteins is essential for elucidating their biological functions.
• Heterocyclic amines and anions are common molecules that can interact with heme proteins.

Purpose of the Study:

• To evaluate and compare the binding affinities of monomeric Glycera dibranchiata hemoglobin for various anions and heterocyclic amines.
• To investigate the influence of ligand structure (steric bulk, flexibility) on protein-ligand interactions.
• To explore the role of heme oxidation state and ligand properties (e.g., basicity) in determining coordination modes and binding strengths.

Main Methods:

• Spectroscopic techniques including Electron Spin Resonance (ESR) and optical spectroscopy were employed.
• Binding affinities were measured for a range of heterocyclic amines (imidazoles, pyrazole, triazole, tetrazole) and anions.
• Thermodynamic parameters (enthalpy and entropy changes) were determined for ligand binding.
• Comparisons were made with sperm whale and horse heart myoglobin.

Main Results:

• Protein affinities for substituted heterocyclic amines are significantly affected by the steric bulk and flexibility of the aromatic ring.
• Ligand coordination mode is dependent on the heme iron's oxidation state, with iron(III) amine adducts showing greater stability than iron(II) adducts.
• Stronger Brønsted-Lowry bases exhibit higher affinities due to their sigma-donor character.
• Morpholinoethylisocyanide acts as a redox-state-dependent ambidentate ligand, binding via nitrogen to iron(III) and carbon to iron(II).
• pH-dependent spectral changes indicate iron-linked ionizations and spin-equilibria within the heme pocket.
• Mutually compensatory enthalpy and entropy changes were observed for the binding process.
• At the compensation temperature, Glycera dibranchiata hemoglobin and sperm whale myoglobin display similar binding affinities for heterocyclic and anionic ligands.

Conclusions:

• The study elucidates the intricate factors governing ligand binding to heme proteins, including steric, electronic, and redox-state effects.
• Glycera dibranchiata hemoglobin exhibits distinct yet comparable ligand-binding characteristics to myoglobins, highlighting conserved and divergent evolutionary strategies.
• The findings contribute to a deeper understanding of heme protein function and the design of biomimetic systems.