Ligand binding kinetics of des arginine haemoglobin

Structure changes in hemoglobin upon deletion of C-terminal residues, monitored by resonance Raman spectroscopy

Loss of C-terminal residues in hemoglobin raises oxygen affinity and reduces both cooperativity and the Bohr effect. These functional changes are expected from the loss of C-terminal salt bridges, which are seen crystallographically to stabilize the T quaternary structure. Ultraviolet resonance Raman (UVRR) difference spectroscopy confirms that the strength of the T state contacts is diminished when the C-terminal and also the penultimate residues are removed chemically. Deoxy minus CO difference signals arising from the Trpbeta37-Aspalpha94 and Tyralpha42-Aspbeta99 H bonds at the alpha1 beta2 subunit interface are diminished, and at pH 9, the difference spectra reveal a shift to the R quaternary structure. These effects are small for desHisbeta146 Hb and large for desArgalpha141 Hb, consistent with the order of functional changes. In addition, the H bond between the A and E helices is strengthened by removal of Argalpha141 and is further strengthened when the effector molecule IHP (inositol hexaphosphate) is added to deoxy-desArgalpha141 Hb or when its pH is lowered to 5.8. This effect is attributed to the loss of the C-terminal anchor of the alpha chain H helix, which supports the F and A helices. The beta chain is not as sensitive because it has extra F-H interhelix H bonds. Removal of both Hisbeta146 and Tauyrbeta145 produce UVRR changes which are intermediate between desHisbeta146 and desArgalpha141 Hb, although the functional consequences are greater than for desArgalpha141 Hb. Removal of Tyralpha140 as well as Argalpha141 abolishes cooperative binding as well as the Bohr effect, and the UVRR difference signals are also lost, suggesting that quaternary constraints are removed in both the T and the R states. When the approximately 220 cm-1 iron-histidine stretching vibration of the deoxy-proteins is examined, using Raman excitation in resonance with the heme Soret band, the frequency is observed to diminish toward that of deoxyHb A (215 cm-1) as the pH is lowered and IHP is added and to increase toward a completely relaxed value (223 cm-1) as the pH is raised to 9. The relaxation is in the same order as the functional perturbations: desHisbeta146 < desArgalpha141 < desHisbeta146-Tyrbeta145 < desArgalpha141-Tyralpha140. However, even desArgalpha141-Tyralpha140 Hb shows significant reduction in the Fe-His frequency as IHP is added at low pH. The Fe-His frequency is sensitive to both tertiary and quaternary structure changes and is a global indicator of forces at the heme. The order of affinity changes can be understood on the basis of the number of stabilizing H bonds between the F and H helices. Titration curves of the Fe-His frequency against pH are not sigmoidal, consistent with a multiplicity of contributions to the Bohr effect.

Contribution of arginine (HC3) 141 alpha to the Bohr effect of the fourth binding step in the reaction of ligand with human hemoglobin

A few years ago we reported that histidine (HC3) 146 beta plays a major role in the pH-dependent properties of the R-state of human hemoglobin, accounting for close to 50% of the R-state Bohr effect. We have extended these studies by examining the role of arginine 141 alpha, another group known to affect the overall Bohr effect. We have compared the pH dependencies of the rate constants for the dissociation and combination of the fourth carbon monoxide molecule, l4 and l'4, respectively, for native hemoglobin A (HbA) and a control reconstituted HbA, and des-(Arg 141 alpha) HbA, the hemoglobin molecule resulting from the enzymatic removal of the C-terminal arginine of the alpha-chain of human Hb. From these kinetic constants the pH dependence of L4, the affinity constant for the fourth carbon monoxide molecule, has been estimated. We find that the removal of arginine 141 alpha reduces the pH dependence of log L4 by about 80% between pH 6 and 8, where the alkaline Bohr effect normally occurs. The sum of the effects of the removal of His 146 beta and of Arg 141 alpha is greater than 100%. This suggests that at least one of these modifications alters the contributions of other residues of this Bohr effect.

Allosteric interpretation of haemoglobin properties

It is our purpose to review recent experiments on haemoglobin in order to discuss them in terms of the two state model of cooperativity. Excellent previous reviews are available of the chemistry of haemoglobin (Antonini & Brunori, 1971; Gibson, 1959b) which are referred to when possible. The plethora of data necessitates that a selection must be made in a review. An intentionally wide range of experiments is selected to exhibit