Nested allosteric interactions in extracellular hemoglobin of the leech Macrobdella decora

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Evolution

Background:

• Leech hemoglobin Macrobdella decora is a giant extracellular hexagonal bilayer globin.
• These complexes contain 144 heme-bearing subunits with a specific quaternary structure (2 x (6 x (3 x 4))).
• Basic substructures are tetramers exhibiting cooperative oxygen binding, suggesting a hierarchy in allosteric interactions.

Purpose of the Study:

• To analyze the functional properties of isolated tetramers and the complete 144-subunit hemoglobin complex from Macrobdella decora.
• To elucidate the cooperative oxygen binding mechanisms and allosteric interactions within this giant hemoglobin.
• To compare the interaction models of isolated subunits, tetramers, and the entire oligomer.

Main Methods:

• Thermodynamic analysis of the whole molecule.
• Detailed analysis of isolated tetramer function.
• Application of the Monod-Wyman-Changeux (MWC) model.

Main Results:

• Isolated tetramers function as a trimer of cooperatively interacting subunits and a non-cooperative monomer.
• Thermodynamic analysis supports a nested MWC model with six cooperatively interacting 12-mer allosteric units for the whole molecule.
• Unlike isolated tetramers, all subunits within the 144-mer appear cooperatively coupled.

Conclusions:

• Macrobdella decora hemoglobin exhibits a hierarchical quaternary structure enabling nested allosteric interactions.
• Hexagonal bilayer hemoglobins join hemocyanins and GroEL as proteins demonstrating hierarchical structure-function relationships.
• The functional properties are modulated by the degree of oligomerization, with subunits showing cooperative coupling in the full 144-mer.