Studies on Scapharca hemoglobins. Properties of the dimeric protein reconstituted with Fe- or Co-porphyrin

Hydroxide rather than histidine is coordinated to the heme in five-coordinate ferric Scapharca inaequivalvis hemoglobin

The ferric form of the homodimeric Scapharca hemoglobin undergoes a pH-dependent spin transition of the heme iron. The transition can also be modulated by the presence of salt. From our earlier studies it was shown that three distinct species are populated in the pH range 6-9. At acidic pH, a low-spin six-coordinate structure predominates. At neutral and at alkaline pHs, in addition to a small population of a hexacoordinate high-spin species, a pentacoordinate species is significantly populated. Isotope difference spectra clearly show that the heme group in the latter species has a hydroxide ligand and thereby is not coordinated by the proximal histidine. The stretching frequency of the Fe-OH moiety is 578 cm-1 and shifts to 553 cm-1 in H218O, as would be expected for a Fe-OH unit. On the other hand, the ferrous form of the protein shows substantial stability over a wide pH range. These observations suggest that Scapharca hemoglobin has a unique heme structure that undergoes substantial redox-dependent rearrangements that stabilize the Fe-proximal histidine bond in the functional deoxy form of the protein but not in the ferric form.

The unique heme-heme interactions of the homodimeric Scapharca inaequivalvis hemoglobin as probed in the protein reconstituted with unnatural 2,4 heme derivatives

In the homodimeric hemoglobin from Scapharca, HbI, functional communication between the two heme groups is based on their direct structural linkage across the subunit interface through the heme propionates. The heme-protein interactions have been altered in deutero- and meso-HbI by substituting the vinyl groups at positions 2 and 4 of protoheme with hydrogen and ethyl groups, respectively. In meso-HbI the introduction of the ethyl groups in the heme pocket induces significant alterations in the conformation of the heme peripheral substituents, including the propionates, and in the structure of bound CO, as revealed by the resonance Raman spectra. The functional counterpart of these structural changes is the loss of cooperativity in carbon monoxide binding and in the rate of oxygen dissociation. Oxygen pulse and flash photolysis experiments indicate that meso-HbI is locked in the liganded conformation. It is postulated that the ethyl groups, which occupy a larger volume than vinyl ones, impair the ligand-linked movement of the heme relative to its pocket and in turn the expression of cooperativity. In deutero-HbI structural alterations have not been monitored. Functionally, cooperativity in the CO binding kinetics is increased as if hydrogen atoms at positions 2 and 4 permitted more marked movements of the heme than in the native protein.

Protein-heme interactions in hemoglobin from the mollusc Scapharca inaequivalvis: evidence from resonance Raman scattering

Resonance Raman spectra of the Scapharca inaequivalvis homodimeric hemoglobin (HbI) have been measured for the ligand-bound and ligand-free ferrous forms of the protein. In the deoxy derivative, the iron-histidine (Fe-His) stretching mode, proposed as a marker of the oxygen affinity and a conduit linking the hemes to the subunit interface, gives rise to a Raman peak centered at 203 cm-1, an unusually low frequency compared to that reported for other hemoglobins and myoglobins. In the CO-bound derivative, three isotope-sensitive lines at 517, 583, and 1945 cm-1 have been assigned to the Fe-CO stretching, Fe-C-O bending, and C-O stretching modes, respectively. From the frequencies of these modes and from their relative intensities, the Fe-C-O geometry appears to be tilted from axial coordination and shows a bending angle which has been estimated to be about 171 +/- 5 degrees. For the oxygen derivative, only one isotope-sensitive peak has been detected at 570 cm-1, in line with the values found for myoglobin and other hemoglobins. Resonance Raman spectra of HbI modified with p-(chloromercuri)benzoate (PMB) at Cys92 have been measured in parallel with those of the native protein. Despite the large increase in oxygen affinity produced by the PMB modification, the frequency of the Fe-His stretching mode is unshifted in the deoxy derivative.(ABSTRACT TRUNCATED AT 250 WORDS)

Mini-myoglobin. Electron paramagnetic resonance and reversible oxygenation of the cobalt derivative

Mini-myoglobin, obtained by limited proteolysis of horse heart myoglobin (residues 32 to 139), represents a good model for testing the correlation between an exon and a protein domain. We have shown that ligand binding kinetics, spectral and folding features of mini-myoglobin are very similar to those of native myoglobin. In order to develop further the analysis of the structure-function relationship in this mini-protein, mini-globin was reconstituted with the heme moiety in which iron is replaced by cobalt. The Soret absorption spectra of oxy and deoxy cobaltous mini-myoglobin are very similar to those of cobaltous myoglobin derivatives; in addition. Co-mini-myoglobin binds oxygen reversibly with an n value approximately 1 and a p50 value of 45 to 50 mm Hg (the same as Co-myoglobin). Oxy Co-mini-myoglobin shows a well-resolved electron paramagnetic resonance (e.p.r.) spectrum typical of an oxygenated hemoprotein, while the spectrum of the deoxy derivative, although similar to that of deoxy Co-myoglobin, displays a lower resolution of the complex hyperfine structure. Moreover, photodissociation experiments on oxy Co-mini-myoglobin allow e.p.r. detection of an intermediate state, already observed in most hemoproteins and diagnostic for the interaction of bound oxygen with the distal histidine residue. Thus, reconstitution of mini-globin with cobalt protoprophyrin IX has provided, for the first time, a stable oxygenated complex that reflects a correct folding of the protein surrounding the heme pocket and possesses the functional behaviour typical of a hemoprotein.

A cooperative hemoglobin with directly communicating hemes. The Scapharca inaequivalvis homodimer

The unique functional properties of the homodimeric hemoglobin (HbI) extracted from the Arcid blood clam Scapharca inaequivalvis are discussed in the light of the unusual assembly of this protein. At variance with vertebrate hemoglobins, in S. inaequivalvis HbI, the heme-carrying E and F helices form the subunit interface and bring the heme groups almost into direct contact. This creates a new pathway for transferring information about the ligation state of the heme from one subunit to the other which allows cooperativity in the binding of heme ligands to be displayed by a homodimer. The tight coupling between the two subunits and the two heme groups also manifests itself in other reactions that are cooperative in S. inaequivalvis HbI, but not in human hemoglobin, namely, the cleavage of the proximal histidine-heme iron bond and the modification of specific residues located at the subunit interface.

Nuclear-magnetic-resonance investigation of the cooperative homodimeric hemoglobin from the mollusc Scapharca inaequivalvis. Molecular and electronic structure of the cyano-met derivative

The proton nuclear-magnetic-resonance spectra of the cyano-met complexes of the cooperative dimeric and tetrameric hemoglobins from the mollusk Scapharca inaequivalvis have been investigated and compared to those of other structurally characterized oxygen binding hemoproteins. For these proteins, cooperativity is displayed even in the homodimer and preliminary X-ray structural data reveal an unusual back-to-front assembly with intersubunit contacts involving the EF helices [Royer, W. E., Love, W. E. + Fenderson, F. F. (1985) Nature (Lond.) 316, 277-280]. The pattern of hyperfine shifts is very similar for the dimer and tetramer chains, but distinctly different from those of previously characterized low-spin, ferric heme proteins. Individual heme resonances are identified by reconstituting the protein with specifically deuterated hemes. While the axial interactions involving the proximal and distal histidines are very similar to that in myoglobins and other hemoglobins, both the heme contact shift pattern and the amino acid dipolar shift pattern reflect a significantly reduced asymmetry. The decreased spread of the non-cordinated amino acid signals is interpreted in terms of a rotation of the magnetic axes relative to those in myoglobin or other hemoglobins, rather than a change in the magnetic anisotropy. The decreased spread of the heme methyl contact shifts supports this conclusion and is consistent with an orientation of the proximal histidine with the imidazole ring rotated by about 30-40 degrees relative to that in other structurally characterized proteins. Although resonances associated with a complex pattern of alternate heme orientations can be detected immediately after reconstitution of the protein, the isolated protein was found to exhibit insignificant equilibrium heme rotational disorder.

Proton nuclear magnetic resonance study of the ferrous derivatives of the dimeric and tetrameric hemoglobin from the mollusc Scapharca inaequivalvis

Proton NMR spectra have been measured for the two hemoglobins from the mollusc Scapharca inaequivalvis: HbI, a homodimer, and HbII, a heterotetramer. These hemoglobins are endowed with a unique subunit assembly, since the heme carrying E and F helices are involved in the major intersubunit contact. In the far-downfield region of hyperfine-shifted resonances the spectra of HbI and HbII in the deoxy state show respectively one (66.7 ppm) and two (67.8 and 63.6 ppm) exchangeable signals of the proximal histidine N delta H groups, the resonance position being indicative of a significant strain in the iron-imidazole interaction. In the hydrogen-bonded proton region, inter- and intrasubunit hydrogen-bonded proton signals have been detected for both hemoglobins. Deoxy-HbI shows two unique downfield resonances at 11.83 and 11.51 ppm which disappear in the oxygenated state, suggesting that the corresponding hydrogen bonds are involved in the stabilization of the tertiary and/or quaternary structure of the deoxy form. HbII shows even smaller changes in this region upon changes in ligation state. These results therefore provide further proof that, at variance with the vertebrate hemoglobin tetramer, the unique subunit assembly of these proteins is stabilized mainly by hydrophobic interactions.

Aromatic amino-acids and subunit assembly in the hemoglobins from Scapharca inaequivalvis: a fluorescence and CD study of the apoproteins

The dimeric and tetrameric hemoglobins from the mollusc Scapharca inaequivalvis have a unique assembly that places the heme-carrying E and F helices in the inside of the molecule. These helices form the intersubunit contact in the dimer, which represents the structural unit since the tetramer is a dimer of dimers. The E and F helices are highly conserved and contain about 70% of the phenylalanine and tyrosine residues, while the tryptophan residues are near the tetramer contact. The spectroscopic properties (circular dichroism and intrinsic fluorescence) of the aromatic amino-acid residues in the two globins indicate that heme removal brings about a larger conformational change in the tetrameric than in the dimeric protein and that the tryptophan residues acquire a more rigid conformation in the tetramer.

Quaternary structure of erythrocruorin from the nematode Ascaris suum. Evidence for unsaturated haem-binding sites

The quaternary structure of erythrocruorin from the nematode Ascaris suum was studied. The native protein had a sedimentation coefficient, at a protein concentration of 1 mg/ml, of 11.6 +/- 0.3 S and an Mr, as determined by sedimentation equilibrium, of 332,000 +/- 17,000. SDS/polyacrylamide-gel electrophoresis gave one band with a mobility corresponding to an Mr of 43,000 +/- 2000. The Mr of the polypeptide chain was determined to be 41,600 +/- 1,500 by sedimentation equilibrium in 6 M-guanidinium chloride and 0.1 M-2-mercaptoethanol. Cross-linking with glutaraldehyde followed by SDS/polyacrylamide-gel electrophoresis yielded a maximal number of eight bands. The haem content of Ascaris erythrocruorin was observed to vary from one preparation to another. This finding was shown to be due to non-realization of the full binding capacity for haem. By titration with haemin, the haem content was found to attain a maximal value of 2.86 +/- 0.14%, corresponding to a minimal Mr per haem group of 21,000 +/- 1,000. Our findings indicate that Ascaris suum erythrocruorin is composed of eight identical polypeptide chains, carrying two haem sites each.

Amino acid sequence of the cooperative homodimeric hemoglobin from the mollusc Scapharca inaequivalvis and topology of the intersubunit contacts

The dimeric hemoglobin (HbI) from Scapharca inaequivalvis is highly homologous to the other known dimeric Acid hemoglobins. The sequence has a distinctive hydrophobicity profile in the region corresponding to the E and F helices with respect to both the hemoglobin and myoglobin chains from vertebrates due to the presence of several additional hydrophobic residues. The characteristic topology of the E and F helices is conserved in all the known sequences of Arcid hemoglobins including that of the so-called alpha chain of the tetrameric component from Anadara trapezia. The rationale for this conservation lies in the unusual assembly of Arcid hemoglobins where the E and F helices are involved in the interdimeric contact. It is suggested that the extra hydrophobic residues play a major role in the assembly of the basic dimeric unit in these hemoglobins.

Thermodynamic properties of oxygen equilibria of dimeric and tetrameric hemoglobins from Scapharca inaequivalvis

Oxygen equilibrium curves of the dimeric and tetrameric hemoglobin components of Scapharca inaequivalvis were determined at several temperatures. The oxygen equilibrium curves were analyzed by the two-step and four-step oxygen equilibrium schemes of Adair for the dimeric and tetrameric hemoglobins, respectively. The enthalpy and entropy changes for each oxygenation step were determined by the temperature dependences of the Adair constants using van't Hoff equations. Neither dimeric nor tetrameric hemoglobins release protons or anions upon oxygenation under the experimental conditions of the present study. The enthalpy and entropy changes are non-uniform with respect to the oxygenation step and do not need to be corrected for the oxygenation-linked release of protons and anions. For the tetrameric hemoglobin, enthalpy-entropy compensation was observed between the first, second and third steps of oxygenation. The present results suggest that the origin of the co-operative oxygenation is primarily entropic for both hemoglobin components of S. inaequivalvis. Comparison of these data with those obtained on other hemoglobins shows that no simple generalization can be made as to the thermodynamic nature of co-operativity in oxygen binding.