Autoxidation of oxymyoglobin with the distal (E7) glutamine

Area of Science:

• Biochemistry
• Molecular Biology
• Comparative Physiology

Background:

• Myoglobin structure and function are crucial for oxygen transport.
• Autoxidation of myoglobin is a key process affecting its stability and function.
• Previous studies identified a distal glutamine substitution in Galeorhinus japonicus myoglobin.

Purpose of the Study:

• To determine the amino acid sequence of myoglobin from Heterodontus japonicus.
• To compare the autoxidation rates and pH dependence of H. japonicus and G. japonicus myoglobins with sperm-whale myoglobin.
• To investigate the role of the distal(E7) histidine in myoglobin autoxidation.

Main Methods:

• Amino acid sequencing of H. japonicus myoglobin.
• Determination of myoglobin hydropathy profiles.
• Measurement of oxymyoglobin autoxidation rates across a pH range (4.5-11.5) at 25°C.
• Comparative analysis of pH dependence and reaction kinetics.

Main Results:

• The H. japonicus myoglobin sequence was determined (148 residues, N-terminal acetylation, distal(E7) histidine).
• Hydropathy profiles of shark and sperm-whale myoglobins were similar, suggesting conserved globin folding.
• H. japonicus myoglobin autoxidation showed similar pH dependence to sperm-whale myoglobin but at a 10-fold higher rate, accelerated by H+.
• G. japonicus myoglobin (distal glutamine) exhibited different pH dependence, with no acceleration of autoxidation by H+ in the acidic range.

Conclusions:

• The distal(E7) histidine in H. japonicus myoglobin plays a role in autoxidation, potentially facilitating proton transfer.
• The substitution of distal histidine with glutamine in G. japonicus myoglobin significantly alters autoxidation kinetics, particularly its response to pH.
• These findings highlight the structural and functional importance of the distal histidine residue in myoglobin's autoxidation process.