Sequence of oxygen binding by hemoglobin

Oximetry with the NMR signals of hemoglobin Val E11 and Tyr C7

The NMR visibility of the signals from erythrocyte hemoglobin (Hb) presents an opportunity to assess the vascular PO₂ (partial pressure of oxygen) in vivo to gather insight into the regulation of O₂ transport, especially in contracting muscle tissue. Some concerns, however, have arisen about the validity of using the Val E11 signal as an indicator of PO₂, since its intensity depends on tertiary structural changes, in contrast to the quaternary structure changes associated with relaxed (R) and tense (T) transition during O₂ binding. We have examined the Val E11 and Tyr C7 signal intensity as a function of Hb saturation by developing an oximetry system, which permits the comparative analysis of the NMR and spectrophotometric measurements. The spectrophotometric assay defines the Hb saturation level at a given PO₂ and yields standard oxygen-binding curves. Under defined PO₂ and Hb saturation values, the NMR measurements have determined that the Val E11 signal, as well as the Tyr C7 signal, tracks closely Hb saturation and can therefore serve as a vascular oxygen biomarker.

Oxygen binding by alpha(Fe2+)2beta(Ni2+)2 hemoglobin crystals

Oxygen binding by hemoglobin fixed in the T state either by crystallization or by encapsulation in silica gels is apparently noncooperative. However, cooperativity might be masked by different oxygen affinities of alpha and beta subunits. Metal hybrid hemoglobins, where the noniron metal does not bind oxygen, provide the opportunity to determine the oxygen affinities of alpha and beta hemes separately. Previous studies have characterized the oxygen binding by alpha(Ni2+)2beta(Fe2+)2 crystals. Here, we have determined the three-dimensional (3D) structure and oxygen binding of alpha(Fe2+)2beta(Ni2+)2 crystals grown from polyethylene glycol solutions. Polarized absorption spectra were recorded at different oxygen pressures with light polarized parallel either to the b or c crystal axis by single crystal microspectrophotometry. The oxygen pressures at 50% saturation (p50s) are 95 +/- 3 and 87 +/- 4 Torr along the b and c crystal axes, respectively, and the corresponding Hill coefficients are 0.96 +/- 0.06 and 0.90 +/- 0.03. Analysis of the binding curves, taking into account the different projections of the alpha hemes along the optical directions, indicates that the oxygen affinity of alpha1 hemes is 1.3-fold lower than alpha2 hemes. Inspection of the 3D structure suggests that this inequivalence may arise from packing interactions of the Hb tetramer within the monoclinic crystal lattice. A similar inequivalence was found for the beta subunits of alpha(Ni2+)2beta(Fe2+)2 crystals. The average oxygen affinity of the alpha subunits (p50 = 91 Torr) is about 1.2-fold higher than the beta subunits (p50 = 110 Torr). In the absence of cooperativity, this heterogeneity yields an oxygen binding curve of Hb A with a Hill coefficient of 0.999. Since the binding curves of Hb A crystals exhibit a Hill coefficient very close to unity, these findings indicate that oxygen binding by T-state hemoglobin is noncooperative, in keeping with the Monod, Wyman, and Changeux model.

T state hemoglobin binds oxygen noncooperatively with allosteric effects of protons, inositol hexaphosphate, and chloride

Hemoglobin is the paradigm of allosteric proteins. Over the years, cooperative oxygen binding has been explained by different models predicting that the T state of hemoglobin binds oxygen either noncooperatively or with some degree of cooperativity or with strong cooperativity. Therefore, a critical test that discriminates among models is to determine the oxygen binding by the T state of hemoglobin. Fixation of hemoglobin in the T state has been achieved either by crystallization from polyethylene glycol solutions or by encapsulation in wet porous silica gels. Hemoglobin crystals bind oxygen noncooperatively with reduced affinity compared with solution, with no Bohr effect and with no influence of other allosteric effectors. In this study, we have determined accurate oxygen-binding curves to the T state of hemoglobin in silica gels with the same microspectrophotometric apparatus and multiwavelengths analysis used in crystal experiments. The T state of hemoglobin in silica gels binds oxygen noncooperatively with an affinity and a Bohr effect similar to those observed in solution for the binding of the first oxygen molecule. Other allosteric effectors such as inositol hexaphosphate, bezafibrate, and chloride significantly affect oxygen affinity. Therefore, T state hemoglobins that are characterized by strikingly different functional properties share the absence of cooperativity in the binding of oxygen. These findings are fully consistent with the Monod, Wyman, and Changeux model and with most features of Perutz's stereochemical model, but they are not consistent with models of both Koshland and Ackers.

Electron-electron spin-spin interaction in spin-labeled low-spin methemoglobin

Nitroxyl free radical electron spin relaxation times for spin-labeled low-spin methemoglobins were measured between 6 and 120 K by two-pulse electron spin echo spectroscopy and by saturation recovery electron paramagnetic resonance (EPR). Spin-lattice relaxation times for cyano-methemoglobin and imidazole-methemoglobin were measured between 8 and 25 K by saturation recovery and between 4.2 and 20 K by electron spin echo. At low temperature the iron electron spin relaxation rates are slow relative to the iron-nitroxyl electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(III) become comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl electron spin echo decay rate. Throughout the temperature range examined, interaction with the Fe(III) increases the spin lattice relaxation rate (1/T1) for the nitroxyl. The measured relaxation times for the Fe(III) were used to analyze the temperature-dependent changes in the spin echo decays and in the saturation recovery (T1) data for the interacting nitroxyl and to determine the interspin distance, r. The values of r for three spin-labeled methemoglobins were between 15 and 15.5 A, with good agreement between values obtained by electron spin echo and saturation recovery. Analysis of the nitroxyl spin echo and saturation recovery data also provides values of the iron relaxation rates at temperatures where the iron relaxation rates are too fast to measure directly by saturation recovery or electron spin echo spectroscopy. These results demonstrate the power of using time-domain EPR measurements to probe the distance between a slowly relaxing spin and a relatively rapidly relaxing metal in a protein.

Structure-specific model of hemoglobin cooperativity

A generalization of the Szabo-Karplus statistical mechanical model for hemoglobin cooperativity is formulated. The model fits the available thermodynamic and spectroscopic data with assumptions that are consistent with structural results and empirical energy function calculations. It provides a mechanism of hemoglobin cooperativity that is a generalization of the proposals of Monod, Wyman, and Changeux and of Perutz. The role of nonsalt-bridge related sources of constraints on ligand affinity and the mode of salt-bridge coupling to tertiary-quaternary structural changes are examined within the framework of the model. Analysis of proton release data for a range of pH values indicates that a pH-independent part of cooperativity must be present. The pH dependence of the first and last Adair constants point to partial linkage of salt bridges to ligation in the deoxy state and to a destabilized intra-beta-chain salt bridge in the unliganded oxy state.

Proton nuclear magnetic resonance and biochemical studies of oxygenation of human adult hemoglobin in deuterium oxide

The proton nuclear magnetic resonance spectrum of human adult deoxyhemoglobin in D2O in the region from 6 to 20 ppm downfield from the proton resonance of residual water shows a number of hyperfine shifted proton resonances that are due to groups on or near the alpha and beta hemes. The sensitivity of these resonances to the ligation of the heme groups and the assignment of these resonances to the alpha and beta chains provide an opportunity to investigate the cooperative oxygenation of an intact hemoglobin molecule in solution. By use of the nuclear magnetic resonance correlation spectroscopy technique, at least two resonances, one at approximately 18 ppm downfield from HDO due to the beta chain and the other at approximately 12 ppm due to the alpha chain, can be used to study the binding of oxygen to the alpha and beta chains of hemoglobin. The present results using approximately 12% hemoglobin concentration in 0.1 M Bistris buffer at pD 7 and 27 degrees C with and without organic phosphate show that there is no significant line broadening on oxygenation (from 0 to 50% saturation) to affect the determination of the intensities or areas of these resonances. It is found that the ratio of the intensity of the alpha-heme resonance at 12 ppm to that of the beta-heme resonance at 18 ppm is constant on oxygenation in the absence of organic phosphate but decreases in the presence of 2,3-diphosphoglycerate or inositol hexaphosphate, with the effect of the latter being the stronger. On oxygenation, the intensities of the alpha-heme resonance at 12 ppm and of the beta-heme resonance at 18 ppm decreases more than the total number of deoxy chains available as measured by the degree of O2 saturation of hemoglobin. This shows the sensitivity of these resonances to structural changes which are believed to occur in the unligated subunits upon the ligation of their neighbors in an intact tetrameric hemoglobin molecule. A comparison of the nuclear magnetic resonance data with the populations of the partially saturated hemoglobin tetramers (i.e., hemoglobin with one, two, or three oxygen molecules bound) leads to the conclusion that in the presence of organic phosphate the hemoglobin molecule with one oxygen bound maintains the beta-heme resonance at 18 ppm but not the alpha-heme resonance at 12 ppm. These resluts suggest that some cooperativity must exist in the deoxy quaternary structure of the hemoglobin molecule during the oxygenation process. Hence, these results are not consistent with the requirements of two-state concerted models for the oxygenation of hemoglobin. In addition, we have investigated the effect of D2O on the oxygenation of hemoglobin by measuring the oxygen dissociation curves of normal adult hemoglobin as a function of pH in D2O andH2O media. We have found that (1) the pH dependence of the oxygen equilibrium of hemoglobin (the Bohr effect) in higher pH in comparison to that in H2O medium and (2) the Hill coefficients are essentially the same in D2O and H2O media over the pH range from 6.0 to 8.2...