Hemoglobin Bohr Effects: Atomic Origin of the Histidine Residue Contributions

Abstract

The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures. Using recent high resolution structures, the residue pKa values corresponding to the two structures are calculated. The method is based on determining the electrostatic interactions between residues in the protein, relative to those of the residue in solution, by use of the linearized finite difference Poisson-Boltzmann equation and Monte Carlo sampling of protonation states. Given that good agreement is obtained with the available experimental values for the contribution of His residues in HbA to the Bohr effect, the calculated results are used to determine the atomic origin of the pKa shift between deoxy and carbonmonoxy HbA. The contributions to the pKa shift calculated by means of the linear response approximation show that the salt bridge involving His146 plays an important role in the alkaline Bohr effect, as suggested by Perutz but that other interactions are significant as well. A corresponding analysis is made for the contribution of His143 to the acid Bohr effect for which there is no proposed explanation. The method used is summarized and the program by which it is implemented is described in the Appendix .

New Look at Hemoglobin Allostery

“…Tables 4 and 5 (taken from Tables 4 and 5 of ref. 110 ) give both their calculated and our experimentally determined values. Structures, 2DN2 (for deoxy) and 2DN3 (for oxy), with the highest resolution, were used in most of their analyses.…”

“…110 Their calculated pK values for the N-terminal α1Val are 8.60 and 7.80 for deoxy- and oxy-Hb A, respectively. They found that the number of hydrogen ions released from the two α-chains at pH 8.4 is 0.32, corresponding to 16% of the Bohr effect at that pH.…”

Comparing pairwise‐additive and many‐body generalized Born models for acid/base calculations and protein design

“…Following Zheng et al, the atomic coordinates were taken from the high resolution (1.25 Å) X‐ray structures of the deoxy (PDB 2DN2) and oxy forms (PDB 2DN3) . The deoxy form was crystallized at T = 298 K, pH = 6.5 with ammonium sulfate and ammonium phosphate, and is thought to provide a good representation of the solution structure. The oxy form was crystallized at T = 100 K, pH = 6.7, with sodium phosphate, potassium phosphate and glycerol.…”

“…Second, we have tested the method on a much larger set of acid/base constants, 149 instead of 78 previously. One of the proteins was hemoglobin, for which we also computed the Bohr effect: the number of protons that bind when the protein switches from its deoxy to its oxy form, which is of physiological interest . The FDB performance was slightly but systematically better than NEA, with an rms deviation between the computed and experimental p K a 's of 1.04 instead of 1.16 with NEA, a higher correlation with experiment, fewer large errors, and an improved Bohr effect.…”

Talasemilerde Solunum Sistemi Bozuklukları

“…9 Hemoglobin molekülü, tetramer yapıda olup; hem ve globin proteinlerinin bir araya gelmesi ile oluşur. Hem oksijene bağlanma özelliğine sahip Fe +2 (ferröz demir) ve protoporfirin IX halkasından oluşur 10 .…”