Structural and dynamic perturbations induced by heme binding in cytochrome b5

Area of Science:

• Biochemistry
• Structural Biology
• Protein Dynamics

Background:

• Rat hepatic cytochrome b(5) is a heme-containing protein crucial for metabolic processes.
• Heme removal from cytochrome b(5) induces significant structural alterations in the protein.

Purpose of the Study:

• To elucidate the solution structure of apocytochrome b(5) and understand the role of heme in its conformational stability.
• To investigate the influence of osmolytes on apocytochrome b(5) structure.
• To determine the necessity of the heme coordination bond for establishing the holoprotein conformation.

Main Methods:

• Heteronuclear NMR spectroscopy to determine the solution structure of apocytochrome b(5).
• Backbone amide hydrogen exchange to characterize protein cooperative units.
• Site-directed mutagenesis (His63Ala substitution) to study heme binding and structural properties.
• Ligand binding studies (carbon monoxide) to probe structural changes.

Main Results:

• Apoprotein structure reveals a stable module and a disordered heme-binding loop.
• TMAO (trimethylamine N-oxide) showed minimal effect on the disordered loop but slight stabilization of the folded region.
• His63Ala mutant exhibited lower heme affinity but retained the holoprotein fold, indicating the coordination bond is not essential.
• Weak heme binding in the mutant induced distant conformational shifts, suggesting uneven cooperative elements.

Conclusions:

• Cytochrome b(5) apoprotein is partially structured, with heme binding playing a key role in stabilizing the complete fold.
• The intrinsic protein sequence and heme interaction, not solely the coordination bond, dictate the holoprotein structure.
• Structural integrity of cytochrome b(5) involves an uneven distribution of cooperative elements.