A comparative approach to protein- and ligand-dependence of the Root effect for fish haemoglobins

Allosteric effect of water in fish and human hemoglobins

Prompted by the reported lack of solvation effects on the oxygen affinity of fish (trout I) hemoglobin that questioned allosteric water binding in human hemoglobin A (Bellelli, A., Brancaccio, A., and Brunori, M. (1993) J. Biol. Chem. 268, 4742-4744), we have investigated solvation effects in fish and human hemoglobins by means of the osmotic stress method and allosteric analysis. In contrast to the earlier report, we demonstrate that water potential does affect oxygen affinity of trout hemoglobin I in the presence of inert solutes like betaine. Moreover, we show that upon oxygenation electrophoretically anodic hemoglobin from trout and eel bind a similar number of water molecules as does human hemoglobin A, whereas the cathodic hemoglobins of trout and eel bind smaller, but mutually similar, numbers of water molecules. Addition of cofactors strongly increases the number of water molecules bound to eel hemoglobin A (as in human hemoglobin) but only weakly affects water binding to eel hemoglobin C.

Anticooperative ligand binding properties of recombinant ferric Vitreoscilla homodimeric hemoglobin: a thermodynamic, kinetic and X-ray crystallographic study

Thermodynamics and kinetics for cyanide, azide, thiocyanate and imidazole binding to recombinant ferric Vitreoscilla sp. homodimeric hemoglobin (Vitreoscilla Hb) have been determined at pH 6.4 and 7.0, and 20.0 degrees C, in solution and in the crystalline state. Moreover, the three-dimensional structures of the diligated thiocyanate and imidazole derivatives of recombinant ferric Vitreoscilla Hb have been determined by X-ray crystallography at 1.8 A (Rfactor=19.9%) and 2.1 A (Rfactor=23.8%) resolution, respectively. Ferric Vitreoscilla Hb displays an anticooperative ligand binding behaviour in solution. This very unusual feature can only be accounted for by assuming ligand-linked conformational changes in the monoligated species, which lead to the observed 300-fold decrease in the affinity of cyanide, azide, thiocyanate and imidazole for the monoligated ferric Vitreoscilla Hb with respect to that of the fully unligated homodimer. In the crystalline state, thermodynamics for azide and imidazole binding to ferric Vitreoscilla Hb may be described as a simple process with an overall ligand affinity for the homodimer corresponding to that for diligation in solution. These data suggest that the ligand-free homodimer, observed in the crystalline state, is constrained in a low affinity conformation whose ligand binding properties closely resemble those of the monoligated species in solution. From the kinetic viewpoint, anticooperativity is reflected by the 300-fold decrease of the second-order rate constant for cyanide and imidazole binding to the monoligated ferric Vitreoscilla Hb with respect to that for ligand association to the ligand-free homodimer in solution. On the other hand, values of the first-order rate constant for cyanide and imidazole dissociation from the diligated and monoligated derivatives of ferric Vitreoscilla Hb in solution are closely similar. As a whole, ligand binding and structural properties of ferric Vitreoscilla Hb appear to be unique among all Hbs investigated to date.

Proton-linked subunit kinetic heterogeneity for carbon monoxide binding to hemoglobin from Chelidonichthys kumu

The pH dependence of CO binding kinetics to Chelidonichthys kumu hemoglobin (Hb) and human adult Hb has been investigated between pH 2.0 and 9.0 at 20 degrees C. For both Hbs, CO binding kinetics is characterized by two proton-linked transitions, with different pKa values for alpha- and beta-chains in C. kumu Hb, leading to a relevant functional kinetic heterogeneity at most pH values. On the other hand, in human adult Hb the CO binding does not display a functional heterogeneity. Lowering the pH from 9 to 6 brings about a decrease of the CO binding rate constants, to a different extent for human adult Hb and the two chains of C. kumu Hb. Further lowering the pH from 6 to 2 induces an enhancement of CO binding rate constants, probably related to the protonation of proximal HisF8 Nepsilon atom and the cleavage (or severe weakening) of the HisF8-Fe bond. The presence of physiological concentrations of ATP (approximately 3 mM) affects the pH dependence of CO binding kinetics to C. kumu. Moreover, the effect of temperature (between 8 degrees C and 38 degrees C) on CO binding kinetics has been investigated in the absence of ATP at different pH values. These results allow to interpret the functional kinetic heterogeneity of C. kumu Hb on the basis of different regulatory aspects in the alpha- and beta-subunits, as suggested by structural considerations.

Analysis of oxygen binding by Xenopus laevis hemoglobin: implications for the Root effect

We have measured oxygen binding properties for red cell suspensions and stripped hemolysate of Xenopus laevis (XL) in order to answer the question whether XL hemoglobin exhibits a Root effect. The present results show that under physiological conditions XL red cells do not exhibit all the criteria for a Root effect. Compared to human Hb A, stripped XL hemoglobin has a low oxygen affinity, a normal alkaline Bohr effect and a lower interaction with 2,3-diphosphoglycerate, its physiological allosteric effector in red cells. The values for the Hill coefficients are lower than those for human hemoglobin but XL Hb remains cooperative (n50 approximately 2), even at pH values below 6. Attempts to mimick some of the criteria for the Root effect were carried out in pure Hb A solution upon addition of potent allosteric effectors. This leads to low cooperativity and less than 100% oxygen saturation under room air at acidic pH. Under these conditions, mammalian or XL Hb have a maximum proton release at pH approximately 8 and an increased reverse Bohr effect. By contrast, a Root effect Hb exhibits a maximum proton release at neutral pH with the absence of reverse Bohr effect. Therefore the Root effect in fish Hb and the extreme stabilization of the T-state with effectors in mammalian Hb are not an identical phenomenon. Without crystallographic analyses of fish Hb exhibiting a Root effect, the molecular interpretation of this functional property still remains unexplained.

Is there a Root effect in Xenopus hemoglobin?

The reaction of Xenopus hemoglobin with oxygen and carbon monoxide has been reinvestigated over the pH range 8.5-6.0, in the absence and presence of organic phosphates (2,3-diphosphoglycerate or inositol hexakisphosphate), to establish if the tetramer can be stabilized in a T-quaternary state by protons and polyphosphate; the equilibrium and kinetic data indicate that Xenopus hemoglobin does exhibit a Root effect. These new results are discussed with reference to those reported by Bridges et al. [(1985) Resp. Physiol. 61, 125-136] on Xenopus blood and, more generally, to the molecular definition and the structural basis of the Root effect as an extreme form of the Bohr effect.

Thermodynamic characterization of the allosteric transition in trout hemoglobin

The pH induced spectral changes in the Soret region occurring in the carbomonoxy derivative of trout HbIV have been measured under carefully controlled conditions of temperature and organic phosphate concentration. Parallel experiments on the kinetics of carbon monoxide dissociation by NO replacement have been performed over the same pH range. Both sets of results agree satisfactorily with a thermodynamic scheme independently drawn on the basis of functional data previously analyzed within the framework of a two state allosteric model. Thus, the whole body of data strongly supports the idea that the spectral changes are themselves an indication of the pH induced structural transition, thereby reflecting the stabilization of the liganded T-state of the molecule at low pH. This allows us to definitely conclude that the functional changes induced in trout HbIV-CO by protons are associated with a red shift of the Soret absorption band.

A two-state thermodynamic and kinetic analysis of the allosteric functioning of the haemoglobin of an extreme poikilotherm

The blood of the extreme poikilotherm Trematomus borchgrevinki contains one major haemoglobin component, which may be separated from the minor species by ion-exchange chromatography. This haemoglobin shows co-operative CO-binding isotherms at pH 8.2. An analysis of the temperature-dependence of the binding curves has allowed the thermodynamic constants associated with the two-state allosteric parameters L, KR and KT to be measured. The binding of CO at lower pH (6.2) is characterized by the maintenance of the T-state to relatively high degrees of saturation. Kinetic investigations with the use of flash photolysis of the haemoglobin-CO complex under various conditions has allowed the determination of the thermodynamic parameters association with the T-state and R-state rate constants. An amalgamation of these data has allowed the mathematical simulation of predicted time courses for CO binding to this haemoglobin, by using the two-state model, which very closely represents those obtained experimentally. The overall findings show that this haemoglobin closely follows the two-state model of co-operative interaction.

The reaction of trout hemoglobins with isocyanides

Binding of alkylisocyanides of different bulkiness to the two major components of trout hemolysate is presented. In the case of trout hemoglobin I isocyanide binding is pH-independent, similar to O2 and CO, and the bulkiness of the ligand is related to the endothermicity of ligand binding to the T quaternary state. On the other hand, in trout hemoglobin IV the size of the ligand seems to affect the pH dependence of affinity and cooperativity. A comparison with other ligands, like O2, allows us to hint at possible stereochemical determinants of ligand binding in these two hemoglobins.