The Interplay between Molten Globules and Heme Disassociation Defines Human Hemoglobin Disassembly

Atomistic Simulations of Heme Dissociation Pathways in Human Methemoglobins Reveal Hidden Intermediates

“…However, in our recent thermodynamics experiments of met-HbA and HbF disassembly, spectroscopic evidence clearly indicates that hemichromes act as intermediates 14 and that a 2-state description is incomplete for assessing heme disassociation. We have also already shown experimentally that when heme is extracted from native protein, both HbA and HbF unfold with ~ 30 % loss of helical content and undergo complete disruption of the tetramer interface to form the extremely unstable apoHb 14,26 . Our simulations now further demonstrate and characterize the multi-state nature of the heme disassociation pathways in Hb.…”

“…Heme disassociation events can lead to cardiovascular and cerebrovascular complications by triggering Hb disassembly, causing accumulation and precipitation of the unstable apo (heme-free) Hb disassembly intermediates in red cells, and introducing toxic free heme in the blood plasma [13][14][15][16][17][18] . Accelerated heme disassociation may be triggered by genetic mutations occurring in the heme pocket that are either (i) causing unfavorable heme binding environment 19,20 or (ii) facilitating heme autoxidation into ferric heme (hemin) and so resulting in the autoxidized methemoglobin (metHb) accumulation 20,21 .…”

“…In the study, we were able to draw connectivity between reversible heme binding parameters and apoHb equilibrium folding constants. HbA, composed of α and β subunits, forms the dimeric α1β1 and then the tetrameric α1β2 interfaces during assembly 14,26 . Each heme molecule in a Hb is held within a specific subunit cavity formed by the F helix at the proximal side and the E helix at the distal side (Fig.…”

“…In order to simulate these events in HbA within the MD time scales available to us, we artificially first removed the bond between distal water and the heme iron generally at about 150-250 ns into each of 7 replicate simulations done; the bond to the proximal His was removed next generally at about 300-450 ns. The breaking of bond between the ligand water and heme iron was explored first because such interactions are known experimentally to be weaker relative to the coordination between the proximal His and heme iron 14,33 . Some of the intermediates (hemichromes and pentacoordinated Hbs) observed during our MD simulations have been previously detected through either spectroscopic or X-ray crystallographic measurements 14,46 .…”

Hemoglobin: Some (Dis)Assembly Required

“…As such, it complicates experimental work designed to unravel the mechanistic features of functional tetramer dismantling. In this issue of Biophysical Journal, Samuel et al (1) present an equilibrium model of ferric hemoglobin denaturation. The difficulty of their task is presaged by decades of unfolding studies of myoglobin (2,3), the monomeric counterpart of hemoglobin.…”