Triangular kinetic schemes applied to the stability of a heme-globin complex

Area of Science:

• Biochemistry
• Chemical Kinetics

Background:

• Myoglobin is a protein that stores oxygen in muscles.
• Heme is a prosthetic group essential for myoglobin function.
• Aplysia californica myoglobin and horse heart apomyoglobin are model systems for studying protein-heme interactions.

Purpose of the Study:

• To investigate the mechanism of heme transfer between myoglobins.
• To elucidate the kinetic and thermodynamic factors governing heme stability.
• To understand the role of the active site environment in heme binding.

Main Methods:

• Spectrophotometric monitoring of heme transfer reactions.
• Kinetic analysis using Laplacian solutions for triangular reaction schemes.
• Investigating the effect of active site modifications on heme stability.

Main Results:

• Horse heart apomyoglobin effectively traps heme released from Aplysia californica myoglobin.
• Kinetic data fit a triangular mechanism, suggesting a biphasic reaction.
• Aplysia metmyoglobin exists in equilibrium between slow (pentacoordinate) and fast (hexacoordinate) donor states.
• Heme release was not observed for various Aplysia myoglobin derivatives or with altered active site residues (HisE7 to Leu/Val).

Conclusions:

• The stability of the prosthetic heme is influenced by the hydrophobicity of the active site.
• The axial ligand's trans effect plays a crucial role in determining heme stability within the myoglobin structure.
• A general equation for absorbance-time curves was derived, enabling rate constant estimation for triangular kinetic schemes.