Lamprey hemoglobin. Structural basis of the bohr effect

Area of Science:

• Biochemistry
• Structural Biology
• Evolutionary Biology

Background:

• Lampreys possess primitive hemoglobins (Hbs) with unique self-association and ligand-binding properties compared to alpha(2)beta(2) tetrameric Hbs of higher vertebrates.
• Ligated lamprey Hb monomers associate into dimers and tetramers upon deoxygenation, dissociating upon oxygenation, which explains cooperative O(2) binding and pH dependence.

Purpose of the Study:

• To investigate the structural basis of lamprey hemoglobin (Hb) self-association and oxygen binding.
• To test proposed models for the dimeric interface of lamprey Hb, specifically the mammalian alpha(1)beta(2) interface versus a clam Hb-like interface involving E and F helices.

Main Methods:

• Sedimentation equilibrium and oxygen equilibrium measurements were performed on nine mutants of the major hemoglobin component (PMII) from Petromyzon marinus.
• Analysis focused on how mutations affect Hb self-association, O(2) affinity, and pH dependence.

Main Results:

• Results strongly support a critical role for the E helix and AB corner in forming the dimer interface, ruling out the alpha(1)beta(2) model.
• Mutant E75Q demonstrated that Glutamate (Glu) 75 is involved in the Bohr effect, influencing both sedimentation and O(2) binding.
• Replacing distal histidine 73 with glutamine abolished deoxy-Hb self-association and significantly increased O(2) affinity.

Conclusions:

• The study confirms that the dimeric interface in lamprey Hb involves the E helix and AB corner, consistent with structural data.
• The findings elucidate the molecular mechanisms behind the distinct cooperative oxygen binding and pH sensitivity of primitive vertebrate hemoglobins.