Role of Heme Pocket Water in Allosteric Regulation of Ligand Reactivity in Human Hemoglobin

Abstract

Water molecules can enter the heme pocket after the escape of ligand from myoglobins and hemoglobins, hydrogen bond with the distal histidine, and introduce steric barriers to ligand rebinding. The photodissociated CO complexes of human hemoglobin and its isolated α and β chains were subjected to spectrokinetic analysis of the effect of heme hydration on ligand rebinding. A strong coupling was observed between heme hydration and quaternary state. This coupling may contribute significantly to the 20–60-fold difference between the R- and T-state bimolecular CO binding rate constants and thus to the modulation of ligand reactivity that is the hallmark of hemoglobin allostery. Heme hydration proceeded over the course of several kinetic phases in the tetramer, including the R to T quaternary transition. An initial 150 ns hydration phase increased the R-state distal pocket water occupancy, nw(R), to a level similar to that of the isolated α (~60%) and β (~10%) chains, resulting in a modest barrier to ligand binding. A subsequent phase, concurrent with the first step of the R → T transition, further increased the level of heme hydration, increasing the barrier. The final phase, concurrent with the final step of the allosteric transition, brought the water occupancy of the T-state tetramer, nw(T), even higher and close to full occupancy in both the α and β subunits (~90%). This hydration level could present an even larger barrier to ligand binding and contribute significantly to the lower iron reactivity of the T state toward CO.

Roles of Fe-Histidine bonds in stability of hemoglobin: Recognition of protein flexibility by Q Sepharose

“…The binding of O 2 to the tetramers is cooperative, and the most accepted explanations for binding and release behavior are the MWC (concerted) (5) and KNF (sequential) models (6). Much effort has been devoted to elucidating the mechanism of cooperativity, and some reviews and original articles are still periodically republished (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19).…”

Dynamics of Quaternary Structure Transitions in R-State Carbonmonoxyhemoglobin Unveiled in Time-Resolved X-ray Scattering Patterns Following a Temperature Jump

“…This structure−function relationship has long served as a model system for understanding allostery in proteins. 1−9…”

“…Ligand dissociation from the high-affinity R state triggers a quaternary structure transition to the low-affinity T state, which significantly enhances the ability of hemoglobin to transport oxygen from the lungs to the tissues. This structure–function relationship has long served as a model system for understanding allostery in proteins. − …”

Heterogeneity between Two α Subunits of α2β2 Human Hemoglobin and O2 Binding Properties: Raman, 1H Nuclear Magnetic Resonance, and Terahertz Spectra

“…To elucidate cooperative O 2 binding of Hb A, there have been many studies since those of Bohr (Bohr effects, 1904), Hill (Hill plot analysis, 1913), and Adair (Adair equation, 1925) . Even recently, some reviews and papers have been published. − …”