Hagfish hemoglobins: structure, function, and oxygen-linked association

Area of Science:

• Biochemistry
• Molecular Biology
• Evolutionary Biology

Background:

• Cyclostomes, including hagfish and lampreys, possess unique hemoglobins that polymerize upon deoxygenation.
• The hemoglobin structure and function in these primitive vertebrates offer insights into protein evolution and oxygen transport.

Purpose of the Study:

• To characterize the three major hemoglobin components (HbI, HbII, HbIII) from the hagfish Myxine glutinosa.
• To compare these hagfish hemoglobins with the known structure of lamprey Petromyzon marinus HbV.
• To investigate the functional implications of structural differences, including polymerization, pH dependence, and heteropolymer formation.

Main Methods:

• Ultracentrifugation to determine hemoglobin quaternary structure under varying conditions.
• Infrared spectroscopy to probe the distal pocket and CO binding kinetics.
• Comparative sequence analysis and functional assays (oxygen affinity, Bohr effect).

Main Results:

• HbI and HbIII from M. glutinosa are monomeric and pH-independent, with substitutions at the distal histidine and deoxy dimer interface.
• HbII exhibits sequence similarity to P. marinus HbV, forms dimers at acidic pH, and displays a Bohr effect.
• HbII can form functional heterodimers and heterotetramers with HbI and HbIII, exhibiting cooperativity and linked proton/bicarbonate binding.

Conclusions:

• Hagfish hemoglobins display diverse functional properties, reflecting adaptations in oxygen transport.
• Structural variations, particularly in the distal pocket and dimer interface, correlate with observed functional differences.
• The capacity for heteropolymer formation in hagfish hemoglobins suggests complex regulatory mechanisms for oxygen delivery.