The Bohr proton-binding site in a monomeric haemoglobin. A nuclear-magnetic-resonance study

Solution NMR study of the structural basis of the Bohr effect in the monomeric hemoglobins from Chironomus thummi thummi

The larva of the midge Chironomus thummi thummi possesses two monomeric hemoglobins. HbIII and HbIV, with extensive sequence homology, which exhibit marked but differential Bohr effects (pH influence on ligand affinity). These Hbs serve as ideal models for allosteric control of ligand affinity via tertiary-only structural changes. The cyanomet derivatives of these two Hbs have been shown to possess essentially indistinguishable heme cavity structures in solution at low pH (Zhang et al., 1996) that are also very similar to that of the low pH form of HbIII in the crystal (Steigemann & Weber, 1979). 2D 1H NMR has been utilized to elucidate the solution heme cavity structure of the alkaline form of the cyanomet derivatives of HbIII and HbIV to identify the Bohr proton binding site and characterize the nature of the structural changes that accompany the allosteric transition. Significant structural changes with pH have been identified in two regions of the heme cavity, near the axial His and at the junction of pyrroles B and C. The Bohr proton site is identified as His94, which at low pH makes a salt bridge to the terminal Met136. The rupture of this salt bridge at high pH leads to the expulsion of the Met136 side chain next to the His F8 ring where it serves as a spacer between the heme and F-helix, and leads to a cascade of side chain reorientations in the densely packed hydrophobic interior involving five Phe (65, 66, 128, 129, 133), Val132, and Ile69, all on the E- and H-helices. The terminal member of the cascade, Phe65, which acts as a spacer between the E- and F-helices at low pH, is rotated toward the heme plane. The conversion of the low pH, low-affinity "tense" to the high pH, high-affinity "relaxed" state is primarily due to the removal of the Met136 and Phe65 spacers. A central residue in transmitting the Bohr effect from His94 to Phe65 is residue 132. In HbIV, Val132 provides a cavity in the hydrophobic core to readily accommodate the initial step in rotating the Phe129 side chain. In HbIII, the Ile132 provides tight packing to all neighboring side chains and hence would inhibit the rotation of the Phe129 side chain. It is proposed that the lone internal residue difference between HbIII (Ile132) and HbIV (Val132) is the primary basis for the different amplitudes of their Bohr effect.

Proton nuclear magnetic resonance study of the solution distal histidine orientation in monomeric Chironomus thummi thummi cyanomet hemoglobins. Dynamic stability of the heme pocket as monitored by labile proton exchange

The 1H nuclear magnetic resonance spectral characteristics of the cyano-Met form of Chironomus thummi thummi monomeric hemoglobins I, III and IV in 1H2O solvent are reported. A set of four exchangeable hyperfine-shifted resonances is found for each of the two heme-insertion isomers in the hyperfine-shifted region downfield of ten parts per million. An analysis of relaxation, exchange rates and nuclear Overhauser effects leads to assignments for all these resonances to histidine F8 and the side-chains of histidine E7 and arginine FG3. It is evident that in aqueous solution, the side-chain from histidine E7 does not occupy two orientations, as found for the solid state, rather the histidine E7 side-chain adopts a conformation similar to that of sperm whale myoglobin or hemoglobin A, oriented into the heme pocket and in contact with the bound ligand. Evidence is presented to show that the allosteric transition in the Chironomus thummi thummi hemoglobins arises from the "trans effect". An analysis of the exchange with bulk solvent of the assigned histidine E7 labile proton confirms that the group is completely buried within the heme pocket in a manner similar to that found for sperm whale cyano-Met myoglobin, and that the transient exposure to solvent is no more likely than in mammalian myoglobins with the "normal" distal histidine orientation. Finally, a comparison of solvent access to the heme pocket of the three monomeric C. thummi thummi hemoglobins, as measured from proton exchange rates of heme pocket protons, is made and correlated to binding studies with the diffusible small molecules such as O2.

Protein-structural heterogeneity in a non-allosteric monomeric insect hemoglobin monitored by proton magnetic resonance spectroscopy

Proton NMR has revealed two modes of structural heterogeneity in the monomeric hemoglobin I of Chironomus thummi thummi, CTT I; rotational disorder caused by a 180 degree rotation of the heme about the alpha, gamma-meso axis (primary heterogeneity), which varies for each preparation or reconstitution of this hemoglobin, and a 'silent' amino acid replacement [Thr/Ala exchange in position 98(FG4)] in the vicinity of the heme group, which is invariant under all experimental conditions. The heme rotational disorder (primary heterogeneity) can be removed by reconstitution of CTT I with the symmetrical protoheme III. The secondary splitting is not affected; the ratio of intensities of the two types of resonance remains constant. The 8-methyl and 3-methyl and one of the alpha-vinyl proton resonances for the major heme rotational component and the 5-methyl and 1-methyl and one of the alpha-vinyl proton resonances for the minor heme rotational component have been identified and assigned by reconstitution with deuterium-labeled heme. Decoupling experiments have been employed to assign vinyl beta protons in cis and trans position to the respective vinyl alpha protons. Hyperfine shifts for the heme protons exhibited no pH influence above pH 6, in accord with the lack of the alkaline Bohr effect. Below pH 6, pH effects are most strongly reflected by the 8-methyl and 5-methyl proton resonances possibly reflecting titration of the propionate groups.

Haem-rotational disorder in monomeric allosteric cyano-Met insect haemoglobins monitored by resonance Raman spectroscopy

The haem-rotational disorder (insertion of haem into globin rotated about the alpha, gamma-meso axis by 180 degrees) has been investigated in the cyano-Met form of the monomeric allosteric insect haemoglobins, CTT III and CTT IV, by resonance Raman spectroscopy. The effect of haem disorder on the resonance Raman spectra has been observed in proto-IX, deutero-IX, and meso-IX CTTs. Most importantly, in the absence of overlapping vinyl vibrations, we have identified two Fe-C-N bending vibrations at 401 cm-1 and 422 cm-1 (pH 9.5) for 57Fe deutero-IX CTT IV ligated with 13C15N-, which are attributed to the two haem-rotational components. One Fe-C-N bending mode at 422 cm-1 shows a pH-induced shift to 424 cm-1 (pH 5.5) indicating the t----r conformational transition, whereas the other bending mode is pH-insensitive, representing a non-allosteric component. By replacing the unsymmetrical porphyrins with the "symmetrical" protoporphyrin-III we eliminate the haem disorder. Then, sharpening of the Fe-N epsilon(His) (at 313 cm-1) and Fe-CN (at 453 cm-1) stretching modes is observed and a single Fe-C-N bending mode (at 412 cm-1) appears. In cyano-Met proto-IX CTT III two vinyl bending vibrations at 412 cm-1 and 591 cm-1 assigned by deuteration of the vinyl groups also reflect the haem disorder. The 412 cm-1 vinyl vibration is intensity-enhanced via through-space coupling with one of the Fe-C-N bending modes (at 412 cm-1). In the cyano-Met form of proto-III CTT III this vinyl vibration is shifted to 430 cm-1 resulting in a dramatic drop in intensity. It is most likely that the specific vinyl-protein interaction at position 4 in one of the haem-rotational components is the origin of the coupling between the Fe-C-N and vinyl bending modes. The Fe-N epsilon(proximal His) and the Fe-CN stretching vibrations as well as the Fe-C-N bending vibration have been identified by 54Fe/57Fe and 13C15N/12C15N/13C14N/12C14N isotope exchange.

Resonance Raman evidence for the mechanism of the allosteric control of O2-binding in a cobalt-substituted monomeric insect hemoglobin

The substitution of iron for cobalt in the monomeric insect hemoglobin CTT (Chironomus thummi thummi) III does not alter the Bohr effect for O2-binding. The cobalt substitution in this hemoglobin allows us to identify not only the O-O and Co-O2 stretching mode but also the Co-O-O bending mode by resonance Raman spectroscopy. The assignments were made via 16O2/18O2 isotope exchange. The modes associated with the Co-O-O moiety are pH-dependent. These pH-induced changes of the resonance Raman spectra are correlated with the t = r conformation transition. At high pH (high-affinity state) two unperturbed O-O stretching modes are observed at 1,068 cm-1 (major component) and 1,093 cm-1 (minor component) for the 18O2 complex. These frequencies correspond to split modes at 1,107 cm-1 and 1,136 cm-1 and an unperturbed mode at approximately 1,153 cm-1 for the 16O2 complex. At low pH (low-affinity state) the minor component becomes the major component and vice versa. The Co-O2 stretching frequency varies for approximately 520 cm-1 (pH 5.5) to 537 cm-1 (pH 9.5) indicating a stronger (hence shorter) Co-O2 bond in the high-affinity state. On the other hand, the O-O bond is weakened upon the conversion of the low- to the high-affinity state. The Co-O-O bending mode changes from 390 cm-1 (pH 9.5) to 374 cm-1 (pH 5.5). In the deoxy form the resonance Raman spectra are essentially pH-insensitive except for a vinyl mode at 414 cm-1 (pH 5.5), which is shifted to 416 cm-1 (pH 5.5).

The cobalt-nitrosyl stretching vibration as a sensitive resonance Raman probe for distal histidine-nitrosyl interaction in monomeric hemoglobins

The Co-NO stretching vibration has been assigned in the resonance Raman spectra of various cobalt-substituted monomeric hemoglobins by employing isotope-labeling of nitrosyl (14N16O, 15N16O, 14N18O). Monomeric hemoglobins with a distal histidine (sperm whale myoglobin and leghemoglobin) exhibit this vibration at 573-575 cm-1, whereas hemoglobins without distal histidine (elephant myoglobin and insect hemoglobin from Chironomus thummi thummi, CTT III) show this vibration in the range of 553-558 cm-1. The Fe-NO stretching vibration which occurs in the range of 554-556 cm-1 does not reflect the distal histidine-ligand interaction. Therefore, the Co-NO moiety which is isoelectronic with the Fe-O2 moiety is a good monitor for distal effects on the exogenous ligand of hemoglobins, especially due to the fact that in hemoglobins with distal histidine the Fe-O2 stretching vibration (567-572 cm-1) is similar to the Co-NO stretching vibration.

Mechanism of the control of dioxygen binding in a dimeric cobalt-substituted insect hemoglobin. Resonance Raman evidence for cobalt-axial-ligand bond changes

Resonance Raman spectroscopy has been used to investigate the allosteric control mechanism for O2 binding in a cobalt-substituted dimeric insect hemoglobin (CTT II), which exhibits a large Bohr effect due to a pH-induced transition between two ligand affinity states. Substitution of cobalt for iron in CTT II does not modify the Bohr effect, but permits the resonance enhancement (hence the detection) of Raman lines corresponding to the vibrations of the axial ligand-cobalt bonds. Using 16O2/18O2 isotope substitution the O-O and Co-O2 stretching and the Co-O-O bending mode have been assigned to the two affinity states of this hemoglobin: v (O-O) changes from 1152 cm-1 (pH 5.5; t conformation) to about 1125 cm-1 (pH 9.5, r conformation), v (Co-O2) from 512 cm-1 (pH 5.5) to 537 cm-1 (pH 9.5) and delta (Co-O-O) from 378 cm-1 (pH 5.5) to 390 cm-1 (pH 9.5). The Co-N epsilon (His) stretching mode has also been detected changing from 313 cm-1 (pH 5.5) to 307 cm-1 (pH 9.5). For the first time, reciprocal behaviour between the Co-N epsilon and Co-O2 bonds and between the Co-O2 and the O-O bonds in an allosteric hemoglobin are demonstrated. Furthermore, the pH sensitivity of a vinyl bending mode in the range of 411-415 cm-1 has been investigated and shown also to reflect the t in equilibrium with r conformation transition.

Bohr effect in monomeric insect haemoglobins controlled by O2 off-rate and modulated by haem-rotational disorder

The monomeric insect (Chironomus thummi thummi) haemoglobins CTT III and CTT IV show an alkaline Bohr effect. The amplitude of the Bohr effect curve of CTT IV is about twice as large as that of CTT III. In particular, at low pH a time-dependent 'slow' decrease in p50 upon cyclic oxygenation/deoxygenation is observed which is larger if dithionite, instead of ascorbate, is the reducing agent. The decrease of p50 (increase in affinity) correlates with the ratio of haem-rotational components exhibiting an increase of the 'myoglobin-like' haem-rotational component with high O2 affinity and high stability of the globin-haem complex. The replacement of protohaem IX by mesohaem IX and deuterohaem IX, respectively, causes an increase in O2 affinity following the order: proto less than meso less than deutero CTT Hbs. The Bohr effect, however, seems not to be affected by these porphyrin side-group substitutions. The O2 affinity is modulated by steric effects due to the substituents in position 2 and 4 via variation of the protein-haem interactions which influence the O2 release. The replacement of iron by cobalt in proto and meso CTT IV leads to an increase of the p50 by two to three orders of magnitude. Neither central metal nor vinyl replacement affect the Bohr effect. The natural CTT Hbs III and IV analyzed for mono-componential kinetic systems exhibit pH-dependent O2 off-rate constants: 300 s-1 (at pH 5.6) and 125 s-1 (at pH 9.7) for CTT III, and 550 s-1 (at pH 5.4) and 100 s-1 (at pH 9.0) for CTT IV. Inflection points and amplitudes of the log koff/pH plots correspond to those obtained from the Bohr effect curves indicating again a larger Bohr effect for CTT IV than for CTT III. In contrast, the O2 on-rate constants are pH-independent (kon = 1.15-1.26 X 10(8) M-1 s-1). Thus, the Bohr effect is completely controlled by the off-rate constants. Analysis for bi-componential kinetic systems employing the eigenfunction expansion method clearly identifies two kinetic components for proto-IX and deutero-IX CTT Hbs which can be attributed to the two haem-rotational components x and y (x and y differ due to an 180 degree rotation of the haem group about the alpha,gamma-meso axis; y is the myoglobin-like haem-rotational component).(ABSTRACT TRUNCATED AT 400 WORDS)

Acid Bohr effect of a monomeric haemoglobin from Dicrocoelium dendriticum. Mechanism of the allosteric conformation transition

The dioxygen affinity of Dicrocoelium dendriticum haemoglobin was determined as a function of pH with a thin-layer diffusion technique. From the oxygen dissociation and association curves Hill coefficients h equal 1 were obtained throughout. Ultracentrifugation studies prove this haemoglobin to be monomeric irrespective of pH and ligation state. Thus, Dicrocoelium haemoglobin is a non-cooperative monomer. It has the highest O2 affinity so far known for any monomeric haemoglobin: its half-saturation pressure, p50 value, ranges at 25 degrees C from 0.016 mm Hg to 0.15 mm Hg (2.13-20.0 Pa) dependent on pH. Dicrocoelium haemoglobin shows an acid Bohr effect only and as such it constitutes a new class of haemoglobins. Its log p50 versus pH plot (Bohr effect curve) is characterized by a large amplitude, delta log p50 = 0.96, and an inflection point (Bohr effect pK) at pH 5.0. A model for the acid Bohr effect of D. dendriticum haemoglobin is proposed. By generalization, both the alkaline and the acid Bohr effect in various monomeric haemoglobins may arise from a single Bohr group complex (salt bridge).

Iron-carbon bond lengths in carbonmonoxy and cyanomet complexes of the monomeric hemoglobin III from Chironomus thummi thummi: a critical comparison between resonance Raman and x-ray diffraction studies

Soret-excited resonance Raman spectroscopy yields direct information regarding the iron-carbon bonding interactions in the cyanomet and carbonmonoxy complexes of hemoglobin III from Chironomus thummi thummi (CTT III) in solution. By isotope exchange in cyanide (13CN-, C15N-, and 13C15N-) and carbon monoxide (13CO, C18O, and 13C18O), we have assigned the Fe(III)-CN- stretching at 453 cm-1, the Fe(III)-C-N- bending at 412 cm-1, the Fe(II)-CO stretching at 500 cm-1, the Fe(II)-C-O bending at 574 cm-1, and the C-O stretching at 1960 cm-1. The resonance Raman data, in conjunction with those obtained from heme model complexes with well-known Fe-C bond distances, strongly suggest that the Fe(III)-CN- bond (approximately 1.91 A) is longer (hence weaker) than the Fe(II)-CO bond (approximately 1.80 A). This result disagrees with those of x-ray crystallographic studies [Steigemann, W. & Weber, E. (1979) J. Mol. Biol. 127, 309-338] in which the Fe-C bond lengths were reported as 2.2 A in cyanomet and 2.4 A in carbonmonoxy CTT III. Based on Badger's rule and normal mode calculations, the x-ray data would lead to the prediction of 279 cm-1 for the Fe(II)-CO stretching frequency in CTT III . CO, which was not observed. On the other hand, we estimate the Fe-CO bond as approximately equal to 1.82 A, which is very similar to the 1.80-A value in human Hb . CO crystals. Furthermore, we have used isotope shift data to estimate the Fe-C-O angle as 169 +/- 5 degrees, somewhat larger than the 161 degrees value found by Steigemann and Weber. We therefore conclude that there must be errors in the x-ray crystallographic refinement for the ligand geometry in carbonmonoxy and cyanomet CTT III.

High-resolution proton NMR as indicator of a silent mutation in the haem cavity of a monomeric allosteric haemoglobin

Comparison of the high-field 1H-NMR spectra of the met-cyano complexes of allosteric haemoglobins III and IV of the insect larva Chironomus thummi thummi (CTT-III and CTT-IV) shows that, in addition to the molecular heterogeneity previously described for both Hbs as due to haem orientational disorder [La Mar et al. (1980) J. Biol. Chem. 255, 66-70], CTT-III but not CTT-IV exhibits a second heterogeneity as evidenced by splitting of numerous signals. Reconstitution of CTT-III with modified haems alters the populations of the two haem rotational isomers but leaves the secondary heterogeneity unaffected. This argues directly for locating this secondary heterogeneity solely within the polypeptide chain rather than the result of protein-haem interaction. We assign this secondary heterogeneity found solely in Hb-III to a point mutation in the haem cavity, 57E6 (Ile/Thr); CTT-IV is chemically homogeneous. The observation of significant hyperfine shift differences for the alternative substitution at 57E6, particularly for non-haem single-proton resonances thought to arise from distal residues, indicates some structural consequences in the haem cavity due to the point mutation. While a difference in the allosteric properties cannot be detected in that the point mutation appears to leave the pK for the allosteric transition unaltered, subtle influences on the function of the protein in both affinity states cannot be ruled out at this time.

Bohr-effect and pH-dependence of electron spin resonance spectra of a cobalt-substituted monomeric insect haemoglobin

The monomeric haemoglobin IV from Chironomus thummi thummi (CTT IV) exhibits an alkaline Bohr-effect and therefore it is an allosteric protein. By substitution of the haem iron for cobalt the O2 half-saturation pressure, measured at 25 degrees C, increases 250-fold. The Bohr-effect is not affected by the replacement of the central atom. The parameters of the Bohr-effect of cobalt CTT IV for 25 degrees C are: inflection point of the Bohr-effect curve at pH 7.1, number of Bohr protons -- deltalog p1/2 (O2)/deltapH = 0.36 mol H+/mol O2 and amplitude of the Bohr-effect curve deltalogp1/2 (O2) = 0.84. The substitution of protoporphyrin for mesoporphyrin causes a 10 nm blue-shift of the visible absorption maxima in both, the native and the cobalt-substituted forms of CTT IV. Furthermore, the replacement of vinyl groups by ethyl groups at position 2 and 4 of the porphyrin system leads to an increase of O2 affinities at 25 degrees C which follows the order: proto less than meso less than deutero for iron and cobalt CTT IV, respectively. Again, the Bohr-effect is not affected by the replacement of protoporphyrin for mesoporphyrin or deuteroporphyrin. The electron spin resonance (ESR) spectra of both, deoxy cobalt proto- and deoxy cobalt meso-CTT IV, are independent of pH. The stronger electron-withdrawing effect by protoporphyrin is reflected by the decrease of the cobalt hyperfine constants coinciding with gparallel = 2.035 and by the low-field shift of gparallel. The ESR spectra of oxy cobalt proto- and oxy cobalt meso-CTT IV are dependent of pH. The cobalt hyperfine constants coinciding with gparallel - 2.078 increase during transition from low to high pH. The pH-induced ESR spectral changes correlate with the alkaline Bohr-effect. Therefore, the two O2 affinity states can be assigned to the low-pH and high-pH ESR spectral species. The low-pH form (low-affinity state) is characterized by a smaller, the high-pH form (high-affinity state) by a larger cobalt hyperfine constant in gparallel. The correlation of the cobalt hyperfine constants of the oxy forms with the O2 affinities is discussed for several monomeric haemoglobins. The Co-O-O bond angle in cobalt oxy CTT IV is characterized by an ozonoid type of binding geometry and varies little during the pH-induced conformation transition. Due to the lack of the distal histidine in CTT IV no additional interaction via hydrogen-bonding with dioxygen is possible; this is reflected by the cobalt hyperfine constants.

Isomeric incorporation of the haem into monomeric haemoglobins of Chironomus thummi thummi. 1. Isolation of chemically homogeneous haemoglobins. Evidence for the isomerism of the haem in the component III

With high-resolution NMR spectroscopy it has been possible to detect various types of isomerisms in the monomeric component III of the haemoglobins of Chironomus thummi. In the component III prepared from commercially available larvae of Ch. thummi a chemical heterogeneity has been found to occur at the position E6 (Ile or Thr). The imidazole C-2 proton resonance of the adjacent His-E7 is split because of this alternative occupancy. From larvae of an inbreeding of the single subspecies Chironomus thummi thummi a component III material has been isolated in which the position E6 is occupied by one single amino acid. An additional isomerism of this haemoglobin III is produced by an isomeric incorporation of the haem group into the haem pocket. This isomerism has been established from the complex signal pattern of the mesoproton resonances of the diamagnetic ligated haemoglobin III. Other evidences of this isomerism have been obtained from an analysis of th high-field proton resonances.

Isomeric incorporation of the haem into monomeric haemoglobins of Chironomus thummi thummi 3. Comparative study of components, I, III and IV

13C-NMR studies on 13CO complexes of the components III and IV of the monomeric haemoglobins from the subspecies Chironomus thummi thummi have been carried out in order to confirm the existence of two conformational isomers differing by the isomeric incorporation of the haem group and the extent of the Bohr effect. In addition, the allosteric linkage between the ligand binding site and the Bohr proton binding site in the component IV is described from investigations of various pH-dependent proton resonances including the C-2 proton resonances of the titratable histidines. In comparison to the data obtained for the component III it is assumed that in both conformational isomers of the component IV the allosteric linkage between the distal site of the haem and the main Bohr proton donating group is present. From corresponding NMR investigations of the component I an isomeric incorporation of the haem group into this monomeric haemoglobin seems unlikely. Also, any correlation between the ligand binding site and a titratable group of the protein had not been found in this haemoglobin, in agreement with previous results of other laboratories that a Bohr effect in the component I cannot be detected.

The amino acid sequence of pea (Pisum sativum) leghemoglobin

The amino acid sequence has been determined for leghemoglobin component I from root nodules of pea, Pisum sativum. Pea leghemoglobin is one polypeptide chain composed of 147 amino acids, it contains one methionine residue at position 144, and three histidines, which are at positions 60, 92 and 101. The sequence has at least seven polymorphic residues, but it was not possible to separate the polymorphic protein forms which had identical electric charge. The approximate molecular weight of pea leghemoglobin component I is 16,350. The other major leghemoglobin component (II) from pea has an amino acid composition very similar to that of leghemoglobin component I, suggesting that the gene has duplicated relatively recently. P. sativum leghemoglobin differs from that of Vicia faba by 22--23%, depending on the polymorphic form. The leghemoglobins from Phaseolus vulgaris and Glycine max differ from pea leghemoglobin by 35--44%, and Lupinus luteus leghemoglobins differ from it by 45--48%. The seven leghemoglobins so far sequenced have 50 residues (33%) which are common to all.

Ligand-dependent Bohr effect of Chrionomus hemoglobins

The O2 and CO Bohr effects of monomeric and dimeric hemoglobins of the insect Chironomus thummi thummi were determined as proton releases upon ligation. For the O2 Bohr effect of the monomeric hemoglobin III a maximum value of 0.20 H+/heme was obtained at pH 7.5. Upon ligation with CO, however, only 0.04 H+/heme were released at the same pH. In agreement with this finding isoelectric focusing experiments revealed different isoelectric points for O2-liganded and CO-liganded states of hemoglobin III. Analogous results were obtained in the cases of the monomeric hemoglobin IV and the dimeric hemoglobins of Chironomus thummi thummi; here O2 Bohr effects of 0.43 and 0.86 H+/heme were observed. For the corresponding CO Bohr effects values of 0.08 and 0.31 H+/heme were obtained respectively. On the basis of the available structural data the reduced CO Bohr effect in hemoglobin III is discussed as arising from a steric hindrance of the CO ligand by the side chain of isoleucine-E11, obstructing the movement of the heme-iron upon reaction with carbon monoxide. It should, however, be noted that ligands, according to their different electron donor and acceptor properties, may generally induce different conformational changes and thus different Bohr effects, in those hemoglobins in which distinct tertiary and/or quaternary constraints have not evolved. The general utilization of CO instead of O2 as allosteric effector is ruled out by the results reported here.

Inequivalent conformational response of Chironomus hemoglobins to ligation with O2 and CO. A circular-dichrosim and infrared-spectroscopic study

For the monomeric hemoglobins III and IV and the dimeric hemoglobins of Chironomus the tryptophan circular dichrosim reports different conformational response to binding of different ligands. The fine structure bands at 292 nm are most strongly developed in the unliganded state and considerably reduced in the oxy form. The spectrum of the CO derivative is intermediate but clearly more deoxy-like. This finding correlates well with the corresponding Bohr effect magnitudes determined by measuring proton release upon ligation. Infrared difference spectroscopy on O2 versus CO derivatives in aqueous solution shows a normal O2 stretching band position at 1107 cm-1 characteristic of asymmetric oxygen binding. The CO stretching band, however, is consistently blue-shifted by 11--13 cm-1 from the 1951 cm-1 position observed with mammalian hemoglobins, indicating reduced CO binding strength. Structural factors relevant to an explanation of the observed phenomena are discussed.

A calorimetric study of the CO Bohr effect of monomeric haemoglobins

A calorimetric study has been made of the heats of CO reaction with the monomeric haemoglobins of Chironomus thummi thummi III and IV as a function of pH. The number of Bohr protons released at pH 7.1 was determined from heats of reaction in different buffers as 0.19 and 0.31 mol H+/mol CO for haemoglobin III and IV respectively. The heat of the Bohr ionization process was found to be 6 and 8 kcal/mol H+ (25 and 34 kJ/mol) for the haemoglobins III and IV. These values are consistent with values found for histidine groups. A pH-independent part of the reaction enthalpy was determined as - 19.7 kcal/mol CO (-82.4 kJ/mol). The same reaction with myoglobin is less exothermic. From the combination of deltaG0 and deltaH0 values TdeltaS0 values have been calculated. It was found for both haemoglobins that the entropy of reaction is greater by 2 cal K-1 mol-1 (8.4 JK-1 mol-1) at pH 9.5 as compared to pH 6.0.

Electron-spin resonance of nitrosyl haemoglobins: normal alpha and beta chains and mutants Hb M Iwate and Hb Zürich

At 77 K the electron spin resonance (ESR) spectra of the NO derivatives of the mutant haemoglobins Hb M Iwate and Hb Zurich as well as of the isolated chains of normal haemoglobin were studied. Two types of ESR spectra differing in the g-value and the hyperfine splitting at gzz were observed. The type II spectrum is characterized by a hyperfine structure at gzz = 2.005 with a splitting constant of deltaH = 23 G (14NO) or 32 G (15NO), respectively. In the type I spectrum the splitting constant of the hyperfine structure at gzz = 2.009 amounts to deltaH = 18 G (14NO) or 23 G (15NO), respectively. In some cases this hyperfine structure is coincident with another one at gxx = 2.064 with nearly identical splitting constant. In addition, the type I spectrum is characterized by an increased ESR absorption at gxx = 2.064. At neutral pH the NO derivatives of the isolated chains as well as of the mutant haemoglobins give rise to a type II spectrum. In correspondence with previous results gained with normal NO haemoglobin, the ESR spectra of the NO-alpha chains and NO-Hb Zurich show a transition to type I in the acid region. This transition is favoured by binding of 2,3-bisphosphoglycerate. On the other hand, the ESR spectra of the NO-beta chains and NO-Hb M Iwate are of the type II also at acid pH. The NO-beta chains show a transition of the ESR spectrum from type II to type I only at alkaline pH. These results indicate that in the tetrameric NO haemoglobin only the alpha chains are responsible for the transition of the ESR spectrum from type II to type I in the acid region. The two types of ESR spectra are interpreted in terms of two kinds of haem-NO complexes differing in the iron-NO and iron-imidazole distances. The type II spectrum is attributed to a complex with a relatively short iron-imidazole distance which is responsible for a weakened sigma-bond in trans position. The type I spectrum arises then from a complex with a larger iron-imidazole bond leading to an approach of the NO molecule to the iron. The influence of the protein conformation upon the iron-imidazole bond length is discussed with regard to the ESR spectra of the mutant NO haemoglobins and considering the influence of agents modifying the protein structure.