The Stereochemical Mechanism of the Cooperative Effects in Hemoglobin Revisited

Abstract

In 1970, Perutz tried to put the allosteric mechanism of hemoglobin, proposed by Monod, Wyman and Changeux in 1965, on a stereochemical basis. He interpreted their two-state model in terms of an equilibrium between two alternative structures, a tense one (T) with low oxygen affinity, constrained by salt-bridges between the C-termini of the four subunits, and a relaxed one (R) lacking these bridges. The equilibrium was thought to be governed primarily by the positions of the iron atoms relative to the porphyrin: out-of-plane in five-coordinated, high-spin deoxyhemoglobin, and in-plane in six-coordinated, low-spin oxyhemoglobin. The tension exercised by the salt-bridges in the T-structure was to be transmitted to the heme-linked histidines and to restrain the movement of the iron atoms into the porphyrin plane that is necessary for oxygen binding. At the beta-hemes, the distal valine and histidine block the oxygen-combining site in the T-structure; its tension was thought to strengthen that blockage. Finally, Perutz attributed the linearity of proton release with early oxygen uptake to the sequential rupture of salt-bridges in the T-structure and to the accompanying drop in pKa of the weak bases that form part of them. Almost every feature of this mechanism has been disputed, but evidence that has come to light more than 25 years later now shows it to have been substantially correct. That new evidence is reviewed below.

R‐state hemoglobin bound to heterotropic effectors: models of the DPG, IHP and RSR13 binding sites

“…In this new model, heterotropic effectors are considered as primarily responsible for the regulation of oxygen affinity in that effector-linked tertiary changes -rather than the quaternary transition linked to oxygen binding, regulate cooperativity. The effect of heterotropic effectors on HbA function had long been taken into account in previous models, -most notably by PerutzÕs stereochemical model [14,15], but with a rationale that had effectors bind only T-state HbA. In this view, effectors lower the oxygen affinity by stabilizing the Tstate salt bridges between the b-subunits as they bind the larger T-state central cavity.…”

Unusual proximal heme pocket geometry in the deoxygenated Thermobifida fusca: A combined spectroscopic investigation

“…The results of the fitting procedure applied to h-Mb EXAFS data are shown in the left panel of Figure 3. The spectrum is dominated by the first-shell two-body signals associated with the four nitrogen atoms from the porphyrin (γ (2) Fe-N p ) and the axial nitrogen atom from the histidine (γ (2) Fe-N h ). The latter contribution was treated as a separate single-scattering signal as the Fe-N h bond length is about 0.07 Å longer than the average Fe-N p bond length, according to the crystallographic values (PDB code=1A6N, see Table 1).…”

“…The latter contribution was treated as a separate single-scattering signal as the Fe-N h bond length is about 0.07 Å longer than the average Fe-N p bond length, according to the crystallographic values (PDB code=1A6N, see Table 1). In addition to the first-shell contributions, the simulation requires four carbon atoms at about 3.4 Å derived from the connecting methylene groups (γ (2) Fe-C h ). A good fit of the experimental data required the inclusion of the Fe-N p -C 14 three-body (γ (3) ) and the Fe-N p -C 14 -C 23 four-body (η (4) ) total contributions.…”

“…The experimental oscillations of the two samples show marked differences in the range between 7 and 12 Å -1 . Inspection of the theoretical signals shows that these differences originate from the modification in frequency and phase of the two body γ (2) Fe-N h signals. As a consequence different first-shell distances have been obtained from the fitting procedure, with a factor index value R=1.009×10 -5 .…”

“…Exogeneous ligand binding to the sixth coordination position entails the displacement of the iron atom (about 0.5 Å) towards the heme plane with consequent shortening of the iron heme-pyrrole nitrogen bonds, and decreasing of the stretching frequency of the proximal ironhistidine bond. In hemoglobins, the proximal histidine follows the iron atom in its movement thus pulling the F helix in a rigid body displacement, that is subsequently transmitted to the helices that form the subunit interface [2]. Multiple spectroscopic investigations have focused on the fine structural and dynamic properties of the iron-histidine bond in order to assess the stereochemical details at the basis of the ligand binding process in hemoglobins and myoglobins.…”