Relaxation analysis of ligand binding to the myoglobin reconstituted with cobaltic heme

Area of Science:

• Biochemistry
• Bioinorganic Chemistry
• Protein Chemistry

Background:

• Myoglobin is a key protein for oxygen transport.
• Iron(III) myoglobin exhibits distinct ligand-binding properties.
• Early studies proposed different binding affinities for cobalt(III) myoglobin.

Purpose of the Study:

• To investigate the ligand-binding affinities of myoglobin reconstituted with oxidized cobalt(III) deuteroheme.
• To compare the binding kinetics and electronic spectral changes with iron(III) myoglobin.
• To elucidate the underlying reasons for cobalt(III) myoglobin's unique ligation behavior.

Main Methods:

• Reconstitution of myoglobin with oxidized cobalt(III) deuteroheme.
• Spectroscopic analysis (electronic spectra) to observe ligand-induced changes.
• Relaxation kinetic analysis to determine ligand association and dissociation rates.

Main Results:

• Cobalt(III) myoglobin displayed significant affinities for cyanide, azide, thiocyanate, pyridine, and imidazole.
• Ligand binding in cobalt(III) myoglobin showed less pronounced spectral shifts compared to iron(III) myoglobin due to the absence of spin-state transitions.
• Kinetic analysis revealed slow association and even slower dissociation rates for ligands binding to cobalt(III) myoglobin.
• The high ligand affinities in cobalt(III) myoglobin result from a compensation between slow association and very slow dissociation rates, contrasting with the fast association and slow dissociation typical of iron(III) myoglobin.

Conclusions:

• Cobalt(III) myoglobin exhibits distinct ligand-binding characteristics compared to iron(III) myoglobin.
• The unique behavior is attributed to the properties of cobalt(III), including its charge and stronger metal-ligand bond formation.
• The findings challenge earlier proposals regarding cobalt(III) myoglobin's ligand affinities.