A heme binding site on myelin basic protein: characterization, location, and significance

Area of Science:

• Biochemistry
• Neuroscience
• Protein-ligand interactions

Background:

• Myelin basic protein (MBP) is a key component of the myelin sheath, crucial for nerve insulation.
• The functional roles of MBP beyond its structural contribution to myelin are not fully understood.
• Heme, an essential prosthetic group in various proteins, has not been extensively studied in relation to myelin proteins.

Purpose of the Study:

• To investigate the binding interaction between myelin basic protein (MBP) and dicyanohemin.
• To characterize the stoichiometry, affinity, and spectral properties of the MBP-heme complex.
• To identify the specific region within MBP responsible for heme binding and explore its functional implications.

Main Methods:

• Spectroscopic analysis, including Soret absorption and fluorescence quenching titrations, to study heme binding.
• Proteolytic digestion of MBP using Staphylococcus aureus V8 protease and cathepsin D.
• Analysis of heme-binding properties of resulting MBP peptides.
• Sequence homology comparison with known heme-binding proteins.

Main Results:

• MBP binds dicyanohemin with a 1:1 stoichiometry and a high extinction coefficient (8 x 10^4 M^-1 cm^-1 at 420 nm).
• Fluorescence quenching indicates a low binding efficiency (20%) and an association constant >= 10 nM^-1.
• A specific peptide fragment (38-118) of MBP retains identical heme-binding properties, highlighting the critical Phe-Phe (42-43) bond cleavage site.
• Sequence analysis reveals homology between this region and mitochondrial cytochrome b heme-binding sites.

Conclusions:

• MBP directly binds dicyanohemin, forming a stable complex with defined spectral characteristics.
• The Phe-Phe bond at residues 42-43 is crucial for dicyanohemin binding to MBP.
• The observed homology suggests a potential, previously unrecognized role for heme-binding in myelin function or related processes.