A proton nuclear magnetic resonance study of sulfmyoglobin cyanide

The proton nuclear magnetic resonance spectrum of sulfmyoglobin cyanide was studied at 400 MHz. The position of a methyl-group resonance at low field is consistent with a chlorin-like structure for the prosthetic group. The proton NMR spectrum of the cyanide derivative of the purified prosthetic group which decomposes upon extraction from the protein was found to be the same as that of the cyanide derivative of the prosthetic group extracted from myoglobin and a sample prepared from hemin-Cl.

Nitrosyliron(III) hemoglobin: autoreduction and spectroscopy

Nitrosyl complexes of the iron(III) forms of myoglobin, human hemoglobin, Glycera dibranchiata hemoglobins (Hbm and Hbh), and model iron(II) and iron(III) synthetic porphyrins including octaethylporphyrin (OEP) have been prepared. The iron(III) heme proteins are electron spin (paramagnetic) resonance (ESR) silent, while hexacoordinate solution structures are indicated for [Fe(OEP)(NO)2]ClO4 and for Hbm(II)NO, which has an ESR spectrum similar to that of Mb(II)NO and the hexacoordinate iron(II) model complex Fe(OEP)NO(BzIm). The splitting of the alpha- and beta-bands in the optical spectrum of Mb(III)NO and Hbh(III)NO contrasts markedly with the sharp, single bands observed in that of Hbm-(III)NO. The nondegeneracy of the dxz and dyz orbitals in Mb(III)NO and Hbh(III)NO is attributed to the influence of the distal histidine. Circular dichroism spectra were obtained for Hbm(III)NO, Hbm(II)NO, Hbh(III)NO, Hbh(II)NO, Mb(II)NO, and Mb(III)NO. The vicinal chiral center contribution that governs the heme protein CD leads to low Kuhn anisotropies, which have been used to assign certain electronic transitions. The Hb(III)NO spectrum is not stable but transforms into that of Hb(II)NO. This autoredox process follows kinetics that are first order in FeIIINO. The relative rates of autoreduction (25 degrees C, 1 atm NO) are Mb(III)NO less than Hbm(III)NO less than Hb alpha(III)NO less than HbA(III)NO. At high NO partial pressure or after "recycling" of HbA, the rates of reduction decrease. The first step in the reaction of NO with the ferric heme is the reversible formation of the formally iron(III) adduct. This reacts with another molecule of NO, generating the final heme(II)-NO via nitrosylation of NO itself or of an endogenous nucleophile. Kinetic and spectroscopic evidence shows involvement of trans-heme-(NO)2 in the reaction. The activation parameters delta H and delta S were determined. The overall reaction is photoenhanced.

X-ray absorption studies of myoglobin peroxide reveal functional differences between globins and heme enzymes

X-ray absorption studies of myoglobin peroxide show that although it is not identical with compound I or II of horseradish peroxidase [Chance, B., Powers, L., Ching, Y., Poulos, T., Yamazaki, I., & Paul, K. G. (1984) Arch. Biochem. Biophys. 235, 596-611], it has some structural features in common with both. As seen in compound I, the Fe-O distance is short, but the iron-pyrrole nitrogen distance is contracted with a longer iron-histidine distance like compound II. The iron has a higher oxidation state than Fe3+, suggesting an oxyferryl ion type species. Comparison of the structures of various peroxidase and myoglobin compounds points out systematic differences that may explain the catalytic activity of the pi cation radical as well as some of the differences between globins and heme enzymes.

Adiabatic compressibility of myoglobin. Effect of axial ligand and denaturation

An ultrasonic technique has been employed to study the adiabatic compressibility of three metmyoglobin derivatives (aquomet-, fluoromet- and azidometmyoglobin) at neutral pH, and aquometmyoglobin as a function of pH in the frequency range of 1-10 MHz at 20 degrees C. No difference was observed in the adiabatic compressibility of the various derivatives. This indicates that the binding of different axial ligands to myoglobin does not affect significantly the conformational fluctuations of the protein. The finding is consistent with the results of the hydrogen exchange rate experiment, indicating that both types of measurements are useful for the study of protein dynamics. Upon acid-induced denaturation, the adiabatic compressibility of myoglobin drops from 5.3 X 10(-12) cm2/dyn to 0.5 X 10(-12) cm2/dyn. Plausible reasons for such a decrease are discussed.

Multiple forms of sulfmyoglobin as detected by 1H nuclear magnetic resonance spectroscopy

The proton nuclear magnetic resonance spectrum of sulfmyoglobin prepared in standard fashion reveals the presence of three forms, A, B, and C, with different chemical reactivity. Conditions for some interconversions of these forms are given. The 1H NMR spectra of the different forms show similar patterns. It appears that the differences between forms involve chemical modification on the porphyrin periphery. The altered heme can be extracted from FeIII(CN) sulfmyoglobin C to give a stable green substance.

[Mechanism of electron transfer between myoglobin derivatives and ferricytochrome C]

Progress in the studies of the electron transport mechanism in biological systems is greatly hindered by the lack of detailed structural information about the components of these systems. That is why a study of electron transfer between protein molecules with the known spatial organization in model reactions in vitro is of great importance. In this respect the MbO2--Cyt C oxidation-reduction reaction offers unique possibilities. Studies of the effects of pH and ionic strength of the medium on the kinetics of this reaction in combination with chemical modification of single amino acid residues of Mb and Cyt C enabled us to identify those parts of the surface of haemoproteins where the molecules come into "active contact". A variation in the number or/and the arrangement of the charged groups at the "active sites" of the molecules induced by both changing the medium pH and chemical modification of some of these groups lowers markedly the probability of electron transfer in the system (e.g. His GH1 and His A10 in Mb) or blocks it entirely (acylation of Lys 72 (73) or Tyr 74 in Cyt C). Based on the results obtained and on the data of Mb and Cyt C X-ray analysis, the figures of spatial arrangement of the groups at the "active sites" of these molecules are presented.

Proton NMR spectroscopy of sulfmyoglobin

The 1H NMR spectra of ferrous sulfmyoglobin, metsulfmyoglobin, and ferric cyanosulfmyoglobin were obtained at 300 MHz. Hyperfine-shifted resonances are observed in the case of metsulfmyoglobin and ferric cyanosulfmyoglobin that have line widths and cover a chemical shift range that are comparable to the corresponding forms of normal myoglobin. Two methyl resonances are observed in the spectrum of ferric cyanosulfmyoglobin at 44.19 and 25.48 ppm (25 degrees C, pH 8.3) that have been assigned to heme methyls at the 8- and 5-positions on the basis of pH titration effects homologous to the corresponding methyl resonances in ferric cyanomyoglobin. Examination of aromatic region resonances and the pH titration profiles of histidine resonances lead to the conclusion that the overall conformation of sulfmyoglobin was highly homologous to that of normal myoglobin and afforded assignments of histidine residues of the former. The most likely position for the addition of a sulfur atom to the heme of sulfmyoglobin is pyrrole ring A, with ring B a possible, but less likely, alternative.

Sulfmyoglobin. Resonance Raman spectroscopic evidence for an iron-chlorin prosthetic group

The green heme protein sulfmyoglobin (SMb) has been suggested to contain a sulfur-modified iron chlorin prosthetic group. To evaluate this hypothesis, we have obtained high-frequency (greater than 1000 cm-1) resonance Raman spectra of both oxidized and reduced SMb with 457.9-, 488.0-, 514.5-, 568.2-, and 647.1-nm excitation. The SMb spectra are compared to those of native met- and deoxymyoglobin (Mb). Vibrational frequencies for SMb are generally similar to those of Mb, suggesting a high-spin state for both the Fe(III) and Fe(II) SMb species, as is typical of native Mb. However, major differences between SMb and Mb occur both for patterns of relative spectral intensities and for depolarization ratios. In particular, all B1g-depolarized porphyrin modes in the Mb spectra have become polarized, totally symmetric vibrational modes in the SMb spectra. These contrasts reflect a dramatic lowering of the effective symmetry for the SMb prosthetic group. Several new bands are observed in SMb spectra that are not present in spectra of either native Mb or iron protoporphyrin IX complexes. The observation of additional polarized bands flanking the oxidation state marker, V4, is of particular interest. In a parallel study, we compared the resonance Raman spectral properties of iron protoporphyrin IX-derived chlorins and metallo-octaethylchlorins with those of the analogous porphyrins: the chlorin spectra exhibited altered intensity patterns, an increased number of totally symmetric (polarized) vibrational bands, and several new vibrational bands, including one or two in the region of the oxidation state marker, V4. Thus, the resonance Raman spectral characteristics of SMb and metallo-chlorins are complementary and strongly support a chlorin prosthetic group for SMb. Furthermore, they establish testable criteria for investigating the prosthetic group structures of other green heme proteins by resonance Raman spectroscopy.

Haem disorder in modified myoglobins. Effect of reconstitution procedures

Apomyoglobin was reconstituted with deuterohaem derivatives under various conditions. The fraction of disordered component, which is characterized by a 180 degree rotation of the haem group, for the various preparations was determined by n.m.r. spectroscopy. By using the procedures described, it was shown that the fraction of disordered component is minimized if the reconstitution is carried out with high-spin ferric haem derivatives within an experimentally determined optimum pH range of 8-9.5. Use of low-spin derivatives in either the ferrous or ferric forms leads to substantial increases in the fraction of disordered form. Attempted removal of the disordered form by selective oxidation and chromatographic purification was not effective.

Effects of ionizing radiation on nitric oxide myoglobin. Part 2. Effects on the globin moiety

Irradiation of nitric oxide myoglobin (NOMb) induces changes in the haem as well as protein moiety of NOMb, especially at doses of 400-800 krad. The changes in the protein include: Conformational changes, with apparent partial denaturation of globin alpha-helix as evidenced by circular dichroism. Preferential scission of the polypeptide chain and dimerization via covalent bond(s) as evidenced by SDS-polyacrylamide gel electrophoresis. Products with a spectrum of hydrodynamic volumes between those of the monomer and the dimer are also formed. The shift of NOMb pIs toward more acidic pHs (probably due to modification and/or destruction of basic amino acid residues by water radiolytic products) as evidenced by isoelectric focusing.

Effects of ionizing radiation on nitric oxide myoglobin. Part 1. Effects on the NO-haem moiety

Bovine nitric oxide myoglobin (NOMb) was irradiated with 40-4000 krad of gamma-radiation, and the effects on the haem studied using absorption spectroscopy and electron spin resonance (e.s.r.) spectroscopy. The results show the following behaviour. The bright red colour of NOMb changes to brown upon irradiation. This is similar to changes observed in radiation sterilized. nitrite-containing meats. NOMb becomes progressively denitrosylated, with met-myoglobin (metMb) as the immediate product. Upon increasing doses of radiation (up to 800 krad) at 0 degrees C parallel to NOMb denitrosylation, metMb is gradually converted, by water radiolytic products, to other products, believed to be ferromyoglobin and ferrimyoglobin peroxide. A minor quantity of 'choleglobin-type' pigments may also be formed at the highest doses. Freezing of NOMb has a substantial protective effect against radiation. Native bovine NOMb behaves as a pentaco-ordinate (hfs of 3 peaks with equal intensity); the bond between iron and N epsilon is thus dramatically stretched and weakened. Using a thermal energy analyser, no NO could be detected over irradiated NOMb solution, indicating rapid reaction of NO liberated from NOMb by radiation, with radiolytic products of water.

Resonance raman spectroscopic studies of axial ligation in oxyhemoglobin, oxymyoglobin, and nitrosylmyoglobin

Raman intensity measurements for the Fe-O2 stretching band of HbO2 (Hb = hemoglobin) have been used to construct an excitation profile, which shows that resonance enhancement occurs mainly via the B and Q transition; no contribution is detectable from an out-of-plane charge-transfer transition. Direct coupling of VFe-O2 to the porphyrin pi-pi* transitions is explained on the basis of competition between the pi* orbitals of porphyrin and O2 for Fe d pi electrons. The RR spectrum of MbNO (Mb = myoglobin) at pH 8.4 is due solely to six-coordinate heme--NO, but lowering the pH to 5.8 converts the RR spectrum to one characteristic of five-coordinate heme--NO, consistent with Fe-ImH (ImH = imidazole) dissociation via protonation. The Fe-NO stretching frequencies are at 553 and 596 cm-1 for the high- and low-pH forms, as expected, but the low-pH form shows an additional 15NO-sensitive band, at 573 cm-1, which is assigned to Fe-N-O bending in the five-coordinate complex. The RR spectrum of MbO2 shows a shoulder at approximately 270 cm-1, which shifts down by approximately 3 cm-1 upon 18O2 substitution, and is suggested to contain the Fe-ImH stretching mode. The weakness of VFe-ImH, relative to VFe-O2, is attributable to the lack of ImH involvement in the heme pi bonding.

Resonance Raman investigation of nitric oxide bonding in nitrosylhemoglobin A and -myoglobin: detection of bound N-O stretching and Fe-NO stretching vibrations from the hexacoordinated NO-heme complex

With excitation at 406.7 nm, we have observed the resonance Raman enhancement of the bound v(N-O) stretch at approximately 1623 cm-1 in nitrosylhemoglobin A and nitrosylmyoglobin, indicating the existence of a charge-transfer transition underlying the strong Soret band. The v(Fe-NO) stretch at 551 cm-1 has also been detected in the Soret as well as in the Q-band region, a phenomenon similar to the v(Fe-O2) and v(Fe-CO) stretches in oxy and carbon monoxy hemoproteins. It appears that these iron-ligand vibrations ay be resonance enhanced via porphyrin pi leads to pi transitions. Upon addition of inositol hexaphosphate at pH 6.0, the v(Fe-NO) stretch at 551 cm-1 and a low-frequency mode at 301 cm-1 exhibit an intensity decrease by approximately one-half. Contrary to the work of Stong et al. [Stong, J. D., Burke, J. M., Daly, P., Wright. P., & Spiro, T. G. (1980) J. Am. Chem. Soc. 102, 5815], who employed an excitation wavelength at 454.5 nm, we observed no intensity increase at 592 cm-1 attributable to the v(Fe-NO) stretch from the pentacoordinated NO-heme complex in the alpha subunits.

Single crystal EPR of myoglobin nitroxide. Freezing-induced reversible changes in the molecular orientation of the ligand

Single crystals of myoglobin nitroxide (MbNO) are examined by the electron paramagnetic resonance spectroscopy at ambient and cryogenic temperatures for both the 14NO and 15NO derivatives. The principal values and the eigenvectors of the g tensor and the hyperfine coupling tensor are determined: g xx = 2.050, g yy = 2.022, and g zz = 1.993; A xi xi = 15.6, A zeta zeta = 21.4, and A eta eta = 26.7 G for the nitrogen in 15NO at ambient temperature. The Fe--N--O bond angle is calculated to be 153 degrees. This result is in good agreement with the x-ray structural result on the six-liganded model compound with the bent Fe--N--O configuration. The principal values and the eigenvectors of the g tensor and the hyperfine coupling tensor are also determined at 77 K for Mb15NO; gxx = 2.076, gyy = 1.979, and gzz = 2.002; A xi xi = 21, A zeta zeta = 24, and A eta eta = 27 G. The Fe--N--O bond angle is calculated to be 109 degrees. The hyperfine splittings attributed to N epsilon atom of proximal histidine are observed in the direction of the gzz at both temperatures. The drastic shift of the EPR spectrum of MbNO single crystal is observed below the freezing point of this crystal. It clearly demonstrates that the conformation of the bonding NO is drastically altered upon freezing. The temperature dependence of the EPR spectra of MbNO below the freezing point cannot be explained only by appropriate combinations of the higher temperature type and the lower temperature type and suggests the contribution from an unpaired electron with the iron dz2 and dyz (or dxz) orbitals. The present EPR results demonstrated that the changes in the molecular orientations are induced by freezing of the biological molecules without disorder of the crystal lattice.

[Comparison of the structures of different forms of sperm whale myoglobin in solution using wide angle x-ray scattering]

Experimental X-ray diffuse scattering curves in a wide range of angles for oxy-, deoxy-, met- and cyan-forms of sperm whale myoglobin in solution have been obtained. It has been found that scattering indicatrices of these ligand forms of myoglobin are well compatible with each other and differ from the theoretical scattering curve of met-myoglobin, calculated from the structure of this protein in crystal. The conclusions have been drawn: (1) on the similarity of myoglobin structures in solution for all the protein forms studied; (2) on the difference in the structure of the above ligand forms of myoglobin in solution from the structure of the protein in crystal by a slight shift (about 2 A) of the "hairpin" GH.

High magnetic field Mössbauer studies of deoxymyoglobin, deoxyhemoglobin, and synthetic analogues

Mössbauer spectra of deoxymyoglobin, deoxyhemoglobin, and the synthetic analogues, iron (II) 2-methylimidazole meso-tetraphenylporphyrin, and iron (II) 1,2-dimethylimidazole meso-tetraphenylporphyrin have been observed in high magnetic fields and over a wide range of temperature. At temperatures greater than 20 K all materials exhibit remarkably similar spectra, with anisotropic internal magnetic fields decreasing as 1/T. All have negative quadrupole interaction, and both this and the magnetic anisotropy imply that the orbital of the odd electron is prolate in the ground quintet, with little unquenched orbital angular momentum. At 4.2 K the spectra differ, suggesting different detailed structure within the quintet. In contrast to the proteins, the 2-methyl model exhibits spectra at 4.2 K which imply that the lowest spin state has high susceptibility in a single direction.

Intensity of highly anisotropic low-spin heme EPR signals

A semi-empirical formula has been derived to calculate the concentration of low-spin heme compounds that are highly anisotropic, i.e. 3 less than gz less than 4, and where information only on the gz absorption is available.

[Quantitative determination of the activity of acid peptidases of industrial origin]

Reagent ninhydrine-Cd++, reacts with free alpha and epsilon amino groups of proteins. Horse-heart apomyoglobin was subjected to exhaustive succinylation, rendering the product non reactive to ninhydrine. The succinylglobin was submitted to enzyme digestion at pH 2.0, 4.0, 4.7 and 6.0. The commercially available enzymes contain mainly pepsin-like and chymosin-like enzymes. The enzymatic digests of succinyl-globin contain new free alpha-amino groups reacting with ninhydrin. Enzymatic digestion was performed under various condition (ratio E/S, pH). The results were compared to those obtained with synthetic substrate: PRO-HIS-LEU-SER-PHE(NO2)-NLEU-ALA-LEU-OME. The price of the synthetic substrate used, was more than 100 times the cost of succinyl-globin, thus the use of this substrate is a valuable tool for the quantitative estimation of peptidase activity in commercially available (pepsin, chymosin-like) enzymes.

Magnetic circular dichroism studies of myoglobin, hemoglobin and peroxidase at room and low temperatures. Ferrous high spin derivatives

The magnetic circular dichroism spectra (MCD) recorded for the visible and near-UV regions of high-spin ferrous derivatives of myoglobin, hemoglobin, hemoglobin dimers and isolated chains as well as of horseradish peroxidase at pH 6.8 and 11.4 have been compared at the room and liquid nitrogen temperatures. The MCD of the Q00- and QV-bands have been shown to be sensitive to structural differences in the heme environment of these hemoproteins. The room temperature visible MCD of native hemoglobin differs from that of myoglobin, hemoglobin dimers and isolated chains as well as from that of model pentacoordinated complex. The MCD of hemoglobin is characterized by the greater value of the MCD intensity ratio of derivative shape A-term in the Q00-band to the A-term in the QV-band. The evidneces are presented for the existence of two pH-dependent forms of ferroperoxidase, the neutral peroxidase shows the "hemoglobin-like" MCD, while the alkaline ferroperoxidase is characterized by the "myoglobin-like" MCD spectrum in the visible region. The differences in the MCD of deoxyhemoglobin and neutral ferroperoxidase as compared with other high-spin ferrous hemoproteins are considered to result from the constraints on heme group imposed by quaternary and/or tertiary protein structure. The differences between hemoporteins which are seen at the room temperature become more pronounced at liquid nitrogen temperature. Except the peak at approximately 580 nm in the MCD of deoxymyoglobin and reduced peroxidase at pH 11.4 the visible MCD does not show appreciable temperature dependent C-terms. The nature of the temperature dependent effect at approximately 580 nm is not clear. The Soret MCD of all hemoproteins studied are similar and are predominantly composed of the derivative-shaped C-terms as revealed by the increase of the MCD peaks approximately in accordance with Boltzmann distribution. The interpretation of temperature-dependent MCD observed for the Soret band has been made in terms of porphyrin to Fe-iron charge-transfer electronic transition which may be assigned as b( pi) leads to 3d. This charge-transfer band is strongly overlapped with usual B(pi --pi*) band resulting in diffuse Soret band. Adopting that only two normal vibrations are sinphase with charge-transfer transition the extracted C-terms of the Soret MCD have been fitted by theoretical dispersion curves.

Myoglobin as an oxygen indicator for measuring the oxygen binding characteristics of a modified myoglobin derivative containing covalently bound mesoheme

By measuring the visible spectrum of a mixture of myoglobin and a modified derivative containing mesoheme in place of the normal protoheme, it is possible to evaluate the relative amounts of the oxidized, reduced, and oxygenated forms of each type of myoglobin. If the oxygen affinity of one myoglobin derivative is known, the oxygen affinity of the other can be determined from measurements at various oxygen partial pressures. In the absence of excess reducing agent, the rate of autoxidation can also be evaluated during the same experiment. The method described is suitable at very low oxygen partial pressures, where most previous methods are inaccurate, and it is very convenient to use, since no time-consuming calibration procedures are required. Using protoheme myoglobin as an oxygen indicator, the oxygen pressure at half saturation (P 1/2) of mesoheme myoglobin was shown to be 11% higher than the P 1/2 of a modified myoglobin derivative containing covalently bound mesoheme. The autoxidation rate of the covalent derivative is faster than that of the noncovalent derivative, but it is less dependent on oxygen pressure.

Infrared magnetic circular dichroism of myoglobin derivatives

By use of a newly constructed CD instrument, infrared magnetic circular dichroism (MCD) spectra were observed for various myoglobin derivatives. The ferric high spin myoglobin derivatives such as fluoride, water and hydroxide complexes, commonly exhibited the MCD spectra consisting of positive A terms. Therefore, the results reinforced the assignment that the infrared band is the charge transfer transition to the degenerate excited state (eg (dpi)). Since the fraction of A term estimated was approximately 80% for myoglobin fluoride and approximately 35% for myoglobin water, the effective symmetry for myoglobin fluoride is determined to be as close as D4h, while that for myoglobin water seems to have lower symmetry components. The ferric low spin derivatives such as myoglobin cyanide, myoglobin imidazole and myoglobin azide showed positive MCD spectra which are very similar to the electronic absorption spectra. These MCD spectra were assigned to the charge transfer transitions from porphyrin pi to iron d orbitals on the ground that they were observed only for the ferric low spin groups and insensitive to the axial ligands. The lack of temperature dependence in the MCD magnitude indicated that the MCD spectra are attributable to the Faraday B terms. Deoxymyoglobin, the ferrous high spin derivative, had fairly strong positive MCD around 760 nm with an anisotropy factor (delta epsilon/epsilon) of 1.4-10(-4). It shows some small MCD bands from 800 to 1800 nm. Among the ferrous low spin derivatives, carbonmonoxymyoglobin did not give any observable MCD in the infrared region while oxymyoglobin seemed to have significant MCD in the range from 700 to 1000 nm.