Oxygen bonding in human hemoglobin and its isolated subunits: a XANES study

The X-ray absorption near edge structure (XANES) spectra of the human adult and foetal hemoglobin, of the isolated alpha and beta chains, in the oxygenated forms, and of the oxymyoglobin and carp oxyhemoglobin have been measured at the wiggler beam line of the Frascati Synchrotron radiation facility. The bonding angle of oxygen molecule at the iron site in these hemoproteins in solution, has been measured using the multiple scattering theory for data analysis.     

Ethylisocyanide equilibria of hemoglobins M Iwate, M Boston, M Hyde Park, M Saskatoon, and M Milwaukee-I in half-ferric and fully reduced states.

The ethylisocyanide equilibria of all the five known hemoglobins M, namely Hb M Iwate (alpha287 Tyrbeta2), Hb M Boston (alpha258 Tyrbeta2), Hb M Hyde Park (alpha2beta292 Tyr), Hb M Saskatoon (alpha2beta263 tyr), and Hb M Milwaukee-I (alpha2beta267 Glu), were studied both in the half-ferric and fully reduced heme states. In the half-ferric state, no heme-heme interaction was observed for Hb M Iwate, Hb M Boston, and Hb M Hyde Park, but Hb M Saskatoon and Hb M Milwaukee-I show small but definite heme-heme interaction with Hill's n of 1.3. The beta chain mutants, Hb M Hyde Park and Hb M Saskatoon, have almost normal affinity for ethylisocyanide and a normal Bohr effect, whereas the alpha chain mutants, Hb M Iwate and Hb M Boston, have abnormally low affinity and almost no Bohr effect. Hb M Milwaukee-I showed a large Bohr effect and low affinity. These results are consistent qualitatively with those on oxygen equilibria reported previously. In the fully reduced state, in which all four hemes were in the ferrous state and capable of binding ethylisocyanide distinct differences were found in the extent of heme-heme interaction. Namely, the n values for proximal histidine mutants, Hb M Iwate and Hb M Hyde Park, were 1.1 and 1.0, respectively, whereas the distal histidine mutants, Hb M Boston and Hb M Saskatoon, showed high n values of 2.4 and 1.6, respectively. Hb M Milwaukee-I also exhibited a high n value of 2.0 The ethylisocyanide affinity of the four histidine mutants was high compared with that of Hb A, while that for Hb M Milwaukee-I was almost normal. All five Hbs M had approximately normal magnitudes of Bohr effect. In the half-ferric state, the proximal and distal histidine mutants of the same chain showed similar affinity for ethylisocyanide and Bohr effect, rather different from those of the mutants of the opposite chain. These differences seem to be derived from the difference of abnormal bonding of ferric iron to tyrosine or glutamic acid. On the other hand, the reduction of iron, which abolished the abnormal bonding and made all of the chains capable of binding ligand, extinguished the differences of alpha and beta chains, and the effect of amino acid side chains close to iron on ligand binding properties became clear. Proximal histidine, which is considered to trigger the transition between the T and R states, seems to be essential to the heme-heme interaction.     

Thermodynamics of equilibria of hemoglobins M Milwaukee-I and Saskatoon and isolated chains of hemoglobin a with carbon monoxide

The thermodynamic parameters of the CO-equilibria of isolated chains of hemoglobin A and of two α-chains in hemoglobins M Milwaukee-I and Saskatoon at 25°, pH 7.0 were determined. The parameters for the binding of the first CO molecule to the hemoglobins M were ΔH′=?17 and ?18 kcal/mole heme and ΔS′=?30 and ?29 e.u. for hemoglobins M Milwaukee-I and Saskatoon, respectively. In contrast to this the characteristics of the second step of the binding were ΔH′=+5.9^· and +4.3 kcal/mole and ΔS′=+51 and +49 e.u. These values for the second step were also significantly different from those of the isolated α-chain (ΔH′=?15 kcal/mole and ΔS′=?11 e.u.).     

Oxygen-linked CO2 binding to isolated beta subunits of human hemoglobin.

It is known that most of the oxygen-linked carbamate which is formed in normal adult human hemoglobin (Hb A) is confined to the beta subunits rather than to the alpha subunits. In order to find out if similar differences exist in the isolated protomers of Hb A we have measured the effect of various pressures of carbon dioxide (pCO2) on the oxygen affinity in the following heme pigments: isolated alpha and beta subunits with free --SH groups (alphaSH, betaSH), mercurated beta subunits (betaPMB), myoglobin (Mb), and betaSH/PLP in which the terminal alpha-amino group of betaSH was irreversibly blocked with pyridoxal phosphate (PLP). Similar measurements were done on Hb A and the fraction of oxygen-linked carbamate calculated from the effect of pCO2 (at constant pH) on the oxygen half-saturation pressure (p50). A distinct influence of CO2 on p50 was observed in betaSH which was absent in betaSH/PLP and thus indicates that the terminal alpha-amino group mediates the oxygen-linked binding of CO2 in betaSH as it does in the beta subunits of Hb A. However, the fraction of oxygen-linked carbamate was much less dependent on pH and pCO2 in betaSH than in Hb A. Neither alphaSH, betaPMB, or Mb, all of which are known to exist largely or wholly as monomers but have free terminal alpha-amino groups, showed a shift of p50 upon addition of CO2. As both betaSH and betaSH/PLP were shown to be tetrameric molecules, we conclude from this study that homotetramers composed of isolated beta subunits do exhibit a reciprocal interaction between the binding of O2 and CO2.     

Oxidation-reduction reactions of hemoglobin A, hemoglobin M Iwate, and hemoglobin M Hyde Park

The kinetics and equilibrium of the redox reactions of hemoglobin A, hemoglobin M Iwate, and hemoglobin M Hyde Park using the iron (II) and iron (III) complexes of trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetate (CDTA4-) as the reducing and oxidizing agents have been studied. With respect to the equilibrium it was found that hemoglobin M Iwate (where the beta chains were reduced) was more readily reduced than hemoglobin M Hyde Park (where the alpha chains are reduced). This difference was shown to be a result of a difference in the rate constant for reduction but not oxidation. The observed rate contants for the reduction of all three hemoglobins were shown to decrease with increasing pH. This was attributed to a decrease in the [T]/[R] ratio. The observed rate contants for the oxidation reaction were shown to increase with increasing pH. Accompanying this increase was a change in the kinetic profile for hemoglobin A from pseudo first order to one in which the rate increased as the extent of reaction increased. Inositol hexaphosphate had no effect on the rate of oxidation of deoxyhemoglobin A. This was a result of binding of FeCDTA2- or HCDTA3- to the protein. However, in the presence of inositol hexaphosphate, the reduction of methemoglobin A exhibited biphasic kinetics. This result was interpreted in terms of the production of a small amount of a conformation which was more readily reduced.     

Unusual and abnormal canine estrous cycles

Preovulatory serum progesterone concentrations are used to estimate the day of LH peak (day 0), not only to accurately time insemination and predict parturition, but to identify abnormal or unusual estrous cycles due to ovarian dysfunction. Early identification of these disorders is of therapeutic and economic importance. This review discusses anovulation, slow preovulatory progesterone rise, "split heat", insufficient luteal phase, and persistent estrus in the bitch. Some of these were temporary dysfunctions; with appropriate breeding management, pregnancy can be achieved. However, in other cases, these were signs of severe, permanent ovarian dysfunction associated with infertility, with potentially lethal sequelae.     

Unusual and abnormal canine estrous cycles

Preovulatory serum progesterone concentrations are used to estimate the day of LH peak (day 0), not only to accurately time insemination and predict parturition, but to identify abnormal or unusual estrous cycles due to ovarian dysfunction. Early identification of these disorders is of therapeutic and economic importance. This review discusses anovulation, slow preovulatory progesterone rise, “split heat”, insufficient luteal phase, and persistent estrus in the bitch. Some of these were temporary dysfunctions; with appropriate breeding management, pregnancy can be achieved. However, in other cases, these were signs of severe, permanent ovarian dysfunction associated with infertility, with potentially lethal sequelae.     

Mossbauer studies of anhydrous hemoglobin and its subunits

The similarities in the Mossbauer spectra of Hb and its α and β subunits, both in the oxygenated and deoxygenated states, were observed. The relative intensities of the absorption dips in the Mossbauer spectra of ⁵⁷Fe in anhydrohemoglobin (AHb) and anhydrous separated alpha (Aα) and beta (Aβ) chain samples were found to be very sensitive to the conditions of samples preparation. This indicates the formation of hemochromogen as an impurity in the absorbers during the process of preparation. When contributions due to the impurity in the samples were subtracted, Mossbauer spectra of ⁵⁷Fe in AHb, Aα, and Aβ remained similar. Therefore, the electronic structure of the iron cation is the same for Hb and its subunits in the anhydrous state also.     

Mechanical stability of hemoglobin subunits: an abnormality in betaS-subunits of sickle hemoglobin

In order to study the mechanism of the ease of precipitation of oxyhemoglobin S by mechanical shaking, the rates of precipitation of α- and β-subunits of oxyhemoglobin A and oxyhemoglobin S were compared. At pH 8.0, the α^A-subunits precipitated rapidly, while the β^A-subunits were very stable, although a part of β^A-bunits converted to the hemichrome form. At pH 6.0, the β^A-subunits precipitated rapidly while the α^A-subunits were stable. Similar studies with β^S-subunits showed that β^S-subunits precipitated rapidly both at acidic and alkaline pHs. The abnormal precipitation of tetrameric oxyhemoglobin S during mechanical shaking may be due to this instability of the β^S-subunits.     

Developmentally regulated expression of hemoglobin subunits in avascular tissues

We investigated the spatio-temporal profile of hemoglobin subunit expression in developing avascular tissues. Significant up-regulation of hemoglobin subunits was identified in microarray experiments comparing blastocyst inner cell masses with undifferentiated embryonic stem (ES) cells. Hemoglobin expression changes were confirmed using embryoid bodies (derived from in vitro differentiation of ES cells) to model very early development at pre-vascular stages of embryogenesis; i.e. prior to hematopoiesis. We also demonstrate, using RT-PCR, Western blotting and immunocytochemistry, expression of adult and fetal mouse hemoglobin subunits in the avascular ocular lens at various stages of development and maturation. Hemoglobin proteins were expressed in lens epithelial cells (cytoplasmic) and cortical lens fiber cells (nuclear and cell-surface-associated); however, a sensitive heme assay demonstrated negligible levels of heme in the developing lens postnatally. Hemoglobin expression was also observed in the developing eye in corneal endothelium and retinal ganglion cells. Gut sections showed, in addition to erythrocytes, hemoglobin protein staining in rare, individual villus epithelial cells. These results suggest a paradigm shift: hemoglobin subunits are expressed in the avascular lens and cornea and in pre-hematopoietic embryos. It is likely, therefore, that hemoglobin subunits have novel developmental roles; the absence of the heme group from the lens would indicate that at least some of these functions may be independent of oxygen metabolism. The pattern of expression of hemoglobin subunits in the perinuclear region during lens fiber cell differentiation, when denucleation is taking place, may indicate involvement in the apoptosis-like signaling processes occurring in differentiating lens fiber cells.     

Functional cross-linked hemoglobin bis-tetramers: geometry and cooperativity

Hemoglobin-based oxygen carriers have been sought as stable, sterile alternatives to red cells in transfusions. Problems in clinical trials using cross-linked tetramers have led to proposals that larger assemblies of tetramers may alleviate some of the problems. A study of such assemblies requires materials with defined structures and physical properties. Evaluation of cross-linked bis-tetramers with inflexible linear links between the tetramers revealed that these have very low cooperativity in oxygen binding and would thus be inefficient as oxygen carriers. New, more flexible reagents were designed to cross-link and connect tetramers in two modes: with angular connectors that permit torsional movement (1-3) and with linear connectors that resemble previously studied systems (4-6). The resulting cross-linked bis-tetramers were produced in high yield and were isolated and characterized. Digest mapping showed that modifications were specifically introduced as expected at amino groups in the 2,3-bisphosphoglycerate binding sites within beta subunits. Circular dichroism showed that the secondary structure of the globin chains is maintained while the microenvironment of the hemes is altered. The bis-tetramers derived from 1-3 have oxygen affinity (P(50) = 3.6-4.7) and cooperativity (n(50) = 2.2-2.7) that appear to be suitable for efficient oxygen delivery to hypoxic regions along with increased mass that is expected to minimize extravasation.     

Primary structure of abnormal E-like hemoglobin]

An E-like abnormal hemoglobin was detected in a hematological patient and one of the members of her family. The composition of blood hemolyzates was characterized using acetate cellulose electrophoresis. The abnormal beta-chain was isolated by ion-exchange chromatography of total globin on CM-cellulose. Using peptide mapping of the abnormal beta-chain trypsin hydrolysates, it was shown that the amino acid substitution occurs in peptide beta T3. The amino acid analysis and determination of the abnormal fragment C-terminal amino acid allowed to establish the locus and type of this substitution. The first case of hemoglobin E(alpha 2 beta 2 26Glu leads to Lys) identification on the territory of the USSR is reported.     

Fitting abnormal oxygen equilibrium curves of hemoglobin

A growing number of oxygen equilibrium curves for hemoglobin (Hb) mutants, post-translational modifications, or the binding of potent new effectors of Hb cannot be fitted adequately with the two-state model. Examples are curves showing double maxima in the derivative of the Hill plot, or slopes of less than unity. We present such examples of modified hemoglobins and strong effectors in this study and calculate at which substate level the two-state model differs from the data. Analysis of hemoglobin oxygen equilibrium curves is reconsidered using the two-state model extended to allow variation of the individual substate probabilities. In this way the effect on the equilibrium due to perturbations in energy of each substate can be studied as a diagnostic tool.