Properties of hemoglobin M, Milwaukee-I variant and its unique characteristic

Axial coordination of heme in ferric CcmE chaperone characterized by EPR spectroscopy

In Escherichia coli cytochrome c maturation requires a set of eight proteins including the heme chaperone CcmE, which binds heme transiently, yet covalently. Several variants of CcmE were purified and analyzed by continuous-wave electron paramagnetic resonance, electron nuclear double resonance, and hyperfine sublevel correlation spectroscopy to investigate the heme axial coordination. Results reveal the presence of a number of coordination environments, two high-spin heme centers with different rhombicities, and at least one low-spin heme center. The low-spin species was shown to be an artifact induced by the presence of available histidines in the vicinity of the iron. Both of the high-spin forms are five-coordinated, and comparison of the spectra of the wild-type CcmE with those of the mutant CcmE(Y134H) proves that the higher-rhombicity form is coordinated by Tyr134. The low-rhombicity (axial) form does not have a histidine residue or a water molecule as an axial ligand. However, we identified exchangeable protons coupled to the iron ion. We propose that the axial form can be coordinated by a carboxyl group of an acidic residue in the flexible domain of the protein. The two species would represent two different conformations of the flexible alpha-helix domain surrounding the heme. This conformational flexibility confers CcmE special dynamic properties that are certainly important for its function.

Signal transmission between subunits in the hemoglobin T-state

To study allosteric mechanism in hemoglobin, a hydrogen-exchange method was used to measure ligand-dependent changes in structural free energy at defined allosterically sensitive positions. When the two alpha-subunits are CN-met liganded, effects can be measured locally, within the alpha-subunit, and also remotely, within the beta-subunit, even though the quaternary structure remains in the T conformation. When the two beta-subunits are liganded, effects occur at the same positions. The effects seen are the same, independently of whether ligands occupy the alpha-chain hemes or the beta-chain hemes. Control experiments rule out modes of energy transfer other than programmed cross-subunit interaction within the T-state. Cross-subunit transfer may depend on pulling the heme trigger (moving the heme iron into the heme plane) rather than on liganding alone.

Alteration of human myoglobin proximal histidine to cysteine or tyrosine by site-directed mutagenesis: characterization and their catalytic activities

Two mutant proteins of human myoglobin (Mb) that exhibit altered axial ligations were prepared by site-directed mutagenesis of a cloned gene for human Mb. The normal axial ligand residue, histidine 93(F8), was replaced with cysteine or tyrosine, resulting in H93C or H93Y Mb, respectively. Cysteine or tyrosine coordination to the ferric heme iron is verified by electronic absorption, 1H-NMR, EPR spectra, and redox potentials of Fe2+/Fe3+ couple. Their mono-oxygenation activities of styrene are also discussed.

Hb Evans or alpha 262(E11)Val----Met beta 2; an unstable hemoglobin causing a mild hemolytic anemia

Structural analysis of the alpha chain of the hemoglobin from a Caucasian female with a mild hemolytic anemia showed the presence of a variant with a Val----Met substitution at position alpha 62. The valine at this position forms one of the contacts with heme and its replacement by methionine will likely decrease heme binding and cause a distortion of the heme crevice and a decreased stability of the abnormal protein. Dot-blot analysis of amplified DNA with 32P-labeled synthetic oligonucleotide probes confirmed the suspected G----A mutation in the first position of codon 62, and also located the mutation in the alpha 2-globin gene. The mutation was found in the proposita and one of her daughters but was most probably absent in her parents.

Hemoglobin M Iwate is caused by a C----T transition in codon 87 of the human alpha 1-globin gene

DNA restriction, molecular cloning, and sequencing methods have been used to characterize the mutation leading to the methemoglobinemia HbM Iwate. It could be demonstrated that the HbM Iwate defect is caused by a point mutation involving a transition from C to T in the first position of codon 87 of the alpha 1-globin gene. Furthermore, the HbM Iwate mutation can directly be identified upon RsaI digestion. This direct detection of the mutation on the gene level is of significant advantage for differential diagnostic purposes.

Oxygen equilibrium study and light absorption spectra of Ni(II)-Fe(II) hybrid hemoglobins

Ni(II)-Fe(II) hybrid hemoglobins, in which hemes in either the alpha or beta subunit are substituted with Ni(II) protoporphyrin IX, have been prepared and characterized. Since Ni(II) protoporphyrin IX binds neither oxygen nor carbon monoxide, the oxygen equilibrium properties of the Fe subunit in these hybrid hemoglobins were specifically determined. K1 values, namely the equilibrium constants for the first oxygen molecule to bind to hemoglobin, agreed well for these hybrid hemoglobins with the K1 value of native hemoglobin A in various conditions. Therefore, Ni(II) protoporphyrin IX in these hybrid hemoglobins behaves like a permanently deoxygenated heme. Both Ne-Fe hybrid hemoglobins bound oxygen non-co-operatively at low pH values. When the pH was raised, alpha 2 (Fe) beta 2 (Ni) showed co-operativity, but the complementary hybrid, alpha 2 (Ni) beta 2 (Fe), did not show co-operativity even at pH 8.5. The light absorption spectra of Ni(II)-Fe(II) hybrid hemoglobins indicated that the coordination states of Ni(II) protoporphyrin IX in the alpha subunits responded to the structure of the hybrid, whereas those in the beta subunits were hardly changed. In a deoxy-like structure (the structure that looks like that observed in deoxyhemoglobin), four-co-ordinated Ni(II) protoporphyrin IX was dominant in the alpha (Ni) subunits, while under the conditions that stabilized an oxy-like structure (the structure that looks like that observed in oxyhemoglobin), five-co-ordinated Ni(II) protoporphyrin IX increased. The small change observed in the absorption spectrum of the beta (Ni) subunits is not related to the change of the co-ordination number of Ni(II) protoporphyrin IX. Non-co-operative binding of oxygen to the beta subunits in alpha 2 (Ni) beta 2 (Fe) accompanied the change of absorption spectrum in the alpha (Ni) subunits. We propose a possible interpretation of this unique feature.

Analysis of the Hb M Milwaukee mutation at the DNA level

Restriction endonuclease mapping of cellular DNA with the enzyme Sst I has been used to detect the haemoglobin (Hb) Milwaukee mutation directly. Instead of a normal 15.5 kilobase pairs (kb) fragment which contains the normal beta-globin structural genes, in heterozygous Hb M Milwaukee DNA two additional fragments of 9.0 kb and 6.5 kb were obtained that are diagnostic for this anomaly. The position of Sst I sites within the beta-globin gene region could be established.

Differences in Fe(II)-N epsilon(His-F8) stretching frequencies between deoxyhemoglobins in the two alternative quaternary structures

Resonance Raman spectra have been obtained of the alpha deoxy and beta deoxy subunits within valency hybrid hemoglobins both in the high-affinity (R) and low-affinity (T) structures. Upon conversion from the R to the T structure, the vibrational frequency of the Fe(II)-N epsilon(His-F8) bond changes from 223 to 207 or 203 cm-1 in the alpha deoxy subunit and from 224 to 220 or 217 cm-1 in the beta deoxy subunit. We estimate that the Fe(II)-N epsilon(His-F8) bond is stretched by the R leads to T transition 3 times more in the alpha subunit (0.024 A) than in the beta subunit (0.0085 A) and, accordingly, the strain energy developed in that bond is 8 times larger in the alpha than in the beta subunit. Hence, the oxygen affinity of the alpha and beta subunits may be regulated by different mechanisms.

Further studies on the functional properties of hemoglobin M Hyde Park

The oxygen binding properties of Hb M Hyde Park (92 beta, histidine leads to tyrosine) were reinvestigated directing special care to testing the wave length-dependence of the oxygen equilibrium curve and to stabilizing hemoglobin samples using a methemoglobin reductase system. There was no indication that the Hb M Hyde Park fraction separated on a DEAE Sephadex column contained an unknown hemoglobin derivative which appeared in earlier studies. Contrary to earlier observations, there was no significant wave length-dependence of the equilibrium curve of Hb M Hyde Park, verifying the spectrophotometric determination of oxygen saturation. The reductase system satisfactorily reduced the normal alpha chain met hemes without reducing the abnormal beta chain met hemes. The oxygen binding property of Hb M Hyde Park is characterized by 3 to 4 times higher oxygen affinity than that for normal hemoglobin, complete loss of cooperativity, and substantially preserved Bohr effect. These results are consistent in part but not entirely with those observed by earlier investigators. The oxygen affinity of Hb M Hyde Park is between the affinity of the oxy structure and the deoxy structure of normal hemoglobin. Oxygen equilibrium curve of red cell suspension and whole hemolysate containing Hb M Hyde Park were biphasic, indicating that Hb M Hyde Park also exhibited the high oxygen affinity in those samples.

Application of anaerobic ion-exchange chromatography to the separation of hemoglobins in R from T conformational states

As an allosteric protein, hemoglobin can assume at least two different quatenary structures, a deoxy or T for tense conformation and an oxy or R for relaxed conformation (2,3) depending upon its state of ligation. Because the pK values of oxy and deoxy- hemoglobins are different, molecules in the R conformation should behave differently than molecules in the T conformation on ion exchange chromatography. Kilmartin et al. (4) designed an anaerobic cation exchange chromatographic procedure with which they separated the deoxy hemoglobin from oxy hemoglobin and other R conformation hemoglobin components like CO-hemoglobin and sulfohemoglobin. In this report we describe the further development of Kilmartin's procedure to separate two electrophoretically silent hemoglobin mutants, Hb Potomac [beta 101 Glu leads to Asp] (5) a high oxygen affinity variant, and Hb M-Milwaukee [beta 67 Val leads to Glu] (6,8) a low oxygen affinity mutant from Hb A. The modifications we have made include (a) applying the hemolysate at a partial pressure of oxygen determined from an examination of the oxygen equilibrium curve to produce a favorable ratio of T to R conformations of the two hemoglobins to be separated, (b) controlling the partial pressure of oxygen tension of the column effluent during the development of the chromatogram.

Differential effects of pH and inositol hexaphosphate on the spectroscopic properties of the alpha and beta subunits in methemoglobins M Milwaukee and A

The effect of pH and inositol hexaphosphate on the electron spin resonance spectra of the alpha-hemes (g = 6.0) and the beta-hemes (g = 6.7) has been measured in methemoglobin M Milwaukee and compared with that of methemoglobin A (g = 6.0). The beta-hemes are found to be comparatively insensitive to both effectors while the alpha-hemes behave in a manner similar to the heme groups of methemoglobin A. Binding of inositol hexaphosphate enhances the high spin ESR signal of the alpha-hemes in both methemoglobins. Comparison of the optical properties of methemoglobins A and M Milwaukee over the pH range from 5.0 to 8.1 shows that inositol hexaphosphate has a differential effect on the subunit types in these two methemoglobins. At low pH the spectral changes observed upon inositol hexaphosphate binding arise primarily from the beta-hemes, while at neutral and alkaline pH these changes arise from both subunit types. The beta-heme spectral changes appear to be pH independent while those arising from the alpha-hemes are strongly pH dependent. It is concluded that it is the hydroxymet form of the alpha-hemes which undergoes spectral change upon inositol hexaphosphate binding to the beta-subunits. In methemoglobin A the spin state and paramagnetic susceptibility increase only in the neutral and alkaline pH ranges upon inositol hexaphosphate binding (Gupta, R.K. and Mildvan, R.S. (1975) J. Biol. Chem. 250, 246; Perutz, M.F., Sanders, J.K.M., Chenery, D.H., Noble, R.W., Penelly, R.R., Fung, L.W.-M., Ho, C., Giannini, I., Porschke, D. and Winkler, H. (1978) Biochemistry 17, 3640). Therefore the hydroxymet form of the alpha-hemes which is responsible for the observed spectral changes must also be responsible for these increases in the magnetic properties of methemoglobin A. Inositol hexaphosphate can bind to methemoglobin at alkaline pH if the beta-hemes are in the high spin form.

Proton nuclear magnetic resonance studies of hemoglobin M Milwaukee and their implications concerning the mechanism of cooperative oxygenation of hemoglobin

Hemoglobin M Milwaukee (beta67E11 Val leads to Glu) is a naturally occurring valency hybrid containing two permanently oxidized hemes on the beta chains. In this mutant, the two abnormal beta chains cannot combine with ligands whereas the two alpha chains are normal and can combine with oxygen with a Hill coefficient varying from 1.1 to 1.3 [Udem et al. (1970), J Mol. Biol. 48, 489]. High-resolution proton nuclear magnetic resonance spectroscopy at 250 MHz has been used to investigate the exchangeable, ring-current shifted, ferrous and ferric hyperfine shifted resonances of Hb M Milwaukee in the absence and presence of organic phosphates. The alpha-heme environment, as manifested by the ring-current shifted resonances in the liganded form as well as the ferrous hyperfine shifted resonances in unliganded form, and subunit interactions, as manifested by the exchangeable resonances, are similar in Hb M Milwaukee to those in normal adult human hemoglobin. Organic phosphates can partially or completely inhibit the structural transformation which normally accompanies the binding of oxygen or carbon monoxide to Hb M Milwaukee. Upon stepwise addition of oxygen to deoxy Hb M Milwaukee, the hyperfine shifted resonance spectra of ferric beta chains show features which cannot be attributed to either fully deoxy or oxy species. However, the spectra for partially oxygenated Hb M Milwaukee can be described as an appropriately weighted average of the spectra of sero, singly, and doubly oxygenated species. The ferric hyperfine shifted resonance spectrum of the singly oxygenated intermediate has been calculated by a method employing least-squares analysis of the spectra of partially oxygenated Hb M Milwaukee at several values of oxygen saturation. The spectrum of this intermediate exhibits features which cannot be accounted for by a two-structure model. The present results are consistent with a sequential model for the oxygenation of this mutant hemoglobin. In view of the similarities between normal adult hemoglobin and Hb M Milwaukee, it is suggested that a two-state concerted allosteric model does not provide an adequate description of the structure-function relationship in normal adult hemoglobin.

Hb M Milwaukee in a German family

The second occurrence of Hb M Milwaukee is reported in two members of a German family who had cyanosis since early childhood. Contrary to earlier reports, Hb M Milwaukee exhibits a distinct heat instability. It is suggested, that in this family the variant resulted from a new mutation.

Electron paramagnetic resonance of Hb St Louis beta28 (B10) Leu replaced by Gln

Hemoglobin St Louis beta28 (B10) Leu replaced by Gln is a new mutant which occurs as a natural valency hybrid (alpha2beta+2), or hemoglobin M (Cohen-Solal, M., Seligmann, M., Thillet, J. and Rosa, J. (1973) FEBS Lett. 33, 37-41). The electron paramagnetic resonance (EPR) spectrum of native Hb St Louis at pH 6.2 shows a mixture of three species. Two are high spin, one with tetragonal symmetry, like Hb+ A, the other with rhombic distortion. The third is a low-spin form corresponding to a hemichrome with the distal (E7) histidine as the sixth ligand of the ferric iron. The hemichrome is also found in red blood cells. After oxidation to the alpha+2beta+2 form, three EPR species are seen. Surprisingly, there remains only one high-spin signal, with almost tetragonal symmetry. Besides the low-spin hemichrome, a hydroxy signal is observed even at pH 6.2. These observations imply interactions between the alpha and beta hemes.

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

Structure of hemoglobin M Boston, a variant with a five-coordinated ferric heme

X-ray analysis of the natural valency hybrid alpha(2) (+M Boston)beta(2) (deoxy) shows that the ferric iron atoms in the abnormal alpha subunits are bonded to the phenolate side chains of the tyrosines that have replaced the distal histidines; the iron atoms are displaced to the distal side of the porphyrin ring and are not bonded to the proximal histidines. The resulting changes in tertiary structure of the alpha subunits stabilize the hemoglobin tetramer in the quaternary deoxy structure, which lowers the oxygen affinity of the normal beta subunits and causes cyanosis. The strength of the bond from the ferric iron to the phenolate oxygen appears to be the main factor responsible for the many abnormal properties of hemoglobin M Boston.

Cat hemoglobin: pH-dependent cooperativity of oxygen binding

Cat hemoglobin has a lower cooperativity and oxygen affinity than most mammalian hemoglobins. In contrast to the usual invariance of cooperativity with pH, a rise in cooperativity with pH is predicted by the allosteric model for low-affinity hemoglobins. Such a pH-dependent cooperativity for cat hemoglobin has been found.