The interplay between molten globules and heme disassociation defines human hemoglobin disassembly
Authors: Samuel P.P. , White M.A. , Ou W.C. , Case D.A. , Phillips G.N. , Olson J.S. 
Published: August 2019

Hemoglobin functions as an oxygen transport protein, with each subunit containing a heme cofactor. We have developed a global disassembly model for human hemoglobin, linking hemin (ferric heme) disassociation and apo(heme-free)-protein unfolding pathways. The model was based on the evaluation of circular dichroism and visible absorbance measurements of guanidine hydrochloride-induced disassembly of holo (heme-bound)-hemoglobin and previous measurements of apohemoglobin unfolding. The populations of holo-intermediates and equilibrium disassembly parameters were determined quantitatively for adult and fetal hemoglobins. The key stages for disassembly into unfolded monomers are characterized by hemichrome intermediates with molten globule characteristics. Hemichromes, which occur when both hemin iron axial sites coordinate amino acids, are not energetically favored in native human hemoglobins. However, these hexacoordinate iron complexes are important for preventing hemin disassociation from partially unfolded species during early disassembly and late stage assembly events. Both our model evaluation and independent small angle X-ray scattering measurements demonstrate that heme disassociation during early disassembly leads to loss of tetrameric structural integrity. Dimeric and monomeric hemichrome intermediates occur along the disassembly pathway inside red cells where the hemoglobin concentration is very high. This prediction explains why in the red cells of patients with unstable hemoglobinopathies, misassembled hemoglobins often get trapped as hemichromes that accumulate into insoluble Heinz bodies. These Heinz bodies become deposited on the cell membranes and can lead to hemolysis. Alternatively, when acellular hemoglobin is diluted into blood plasma after red cell lysis, the disassembly pathway is dominated by early hemin disassociation events, which leads to the generation of higher fractions of apo-subunits and free hemin known to damage to the integrity of blood vessel walls. Thus, our model illuminates the pathophysiology of hemoglobinopathies and other disease states associated with unstable globins and red cell lysis, and provides insights into the factors governing hemoglobin assembly during erythropoiesis.

Significance

Our deconvolution and global analysis of spectral data led to both the characterization of “hidden” hemichrome intermediates and the development of a quantitative model for human hemoglobin disassembly/assembly. The importance of this mechanism is several-fold. First, the hemoglobin system serves as a general biological model for understanding the role of oligomerization and cofactor binding in facilitating protein folding and assembly. Second, the fitted parameters provide: (a) estimates of hemin affinity for apoprotein states; (b) quantitative interpretations of the pathophysiology of hemoglobinopathies and other diseases associated with unstable globins and red cell lysis; (c) insights into the factors governing hemoglobin assembly during erythropoiesis; and (d) a framework for designing targeted hemoglobinopathy therapeutics.