Structure and function of the myoglobin containing octaethylhemin as a prosthetic group

Utility of heme analogues to intentionally modify heme-globin interactions in myoglobin

Myoglobin reconstitution with various synthetic heme analogues was reviewed to follow the consequences of modified heme-globin interactions. Utility of dimethyl sulfoxide as the solvent for water-insoluble hemes was emphasized. Proton NMR spectroscopy revealed that loose heme-globin contacts in the heme pocket eventually caused the dynamic heme rotation around the iron-histidine bond. The full rotational rate was estimated to be about 1400 s(-1) at room temperature for 1,4,5,8-tetramethylhemin. The X-ray analysis of the myoglobin containing iron porphine, the smallest heme without any side chains, showed that the original globin fold was well conserved despite the serious disruption of native heme-globin contacts. Comparison between the two myoglobins with static and rotatory prosthetic groups indicated that the oxygen and carbon monoxide binding profiles were almost unaffected by the heme motion. On the other hand, altered tetrapyrrole array of porphyrin dramatically changed the dissociation constant of oxygen from 0.0005 mm Hg of porphycene-myoglobin to ∞ in oxypyriporphyrin-myoglobin. Heme-globin interactions in myoglobin were also monitored with circular dichroism spectroscopy. The observation on several reconstituted protein revealed an unrecognized role of the propionate groups in protoheme. Shortening of heme 6,7-propionates to carboxylates resulted in almost complete disappearance of the positive circular dichroism band in the Soret region. The theoretical analysis suggested that the disappeared circular dichroism band reflected the cancellation effects between different conformers of the carboxyl groups directly attached to heme periphery. The above techniques were proposed to be applicable to other hemoproteins to create new biocatalysts. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Various strategies for obtaining oxidative artificial hemoproteins with a catalytic oxidative activity: from "Hemoabzymes" to "Hemozymes"?

The design of artificial hemoproteins that could lead to new biocatalysts for selective oxidation reactions using clean oxidants such as O 2 or H 2 O 2 under ecocompatible conditions constitutes a really promising challenge for a wide range of industrial applications. In vivo, such reactions are performed by heme-thiolate proteins, cytochromes P450, that catalyze the oxidation of drugs by dioxygen in the presence of electrons delivered from NADPH by cytochrome P450 reductase. Several strategies were used to design new artificial hemoproteins to mimic these enzymes, that associate synthetic metalloporphyrin derivatives to a protein that is supposed to induce a selectivity in the catalyzed reaction. A first generation of artificial hemoproteins or "hemoabzymes" was obtained by the non-covalent association of synthetic hemes such as N-methyl-mesoporphyrin IX, Fe(III) -α3β-tetra-o-carboxyphenylporphyrin or microperoxidase 8 with monoclonal antibodies raised against these cofactors. The obtained antibody-metalloporphyrin complexes displayed a peroxidase activity and some of them catalyzed the regio-selective nitration of phenols by H 2 O 2/ NO 2 and the stereo-selective oxidation of sulphides by H 2 O 2. A second generation of artificial hemoproteins or "hemozymes", was obtained by the non-covalent association of non-relevant proteins with metalloporphyrin derivatives. Several strategies were used, the most successful of which, named "host-guest" strategy involved the non-covalent incorporation of metalloporphyrin derivatives into easily affordable proteins. The artificial hemoproteins obtained were found to be able to perform efficiently the stereoselective oxidation of organic compounds such as sulphides and alkenes by H 2 O 2 and KHSO 5.

From “hemoabzymes” to “hemozymes”: towards new biocatalysts for selective oxidations

Two generations of artificial hemoproteins have been obtained: “hemoabzymes”, by non-covalent association of synthetic hemes with monoclonal antibodies raised against these cofactors and “hemozymes”, by non-covalent association of non-relevant proteins with metalloporphyrin derivatives. A review of the different strategies employed as well as their structural and catalytic properties is presented here.

Dynamic motion and rearranged molecular shape of heme in myoglobin: structural and functional consequences

Myoglobin, a simple oxygen binding protein, was reconstituted with various types of synthetic hemes to manipulate the heme-globin interactions. From the paramagnetic NMR analysis, small heme was found to rotate rapidly about the iron-histidine bond upon. This is a novel and typical example for the fluctuation of protein. The dynamic NMR analysis indicated that the 360° rotational rate of a small heme was 1,400 s⁻¹ at room temperature. The X-ray analyses revealed that the tertiary structure of globin containing the smallest heme was closely similar to that of native protein despite extensive destruction of the specific heme-globin interactions. The functional analyses of O₂ binding showed that the loose heme-globin contacts do not significantly affect the oxygen binding. On the other hand, the rearrangement of tetrapyrrole array and the non-planar deformation in porphyrin ring significantly affect the functional properties of myoglobin. These results, taken together, indicate that the essential factors to regulate the myoglobin function are hidden under the molecular shape of prosthetic group rather than in the nonbonded heme-globin contacts.

Dynamics of heme in hemoproteins: proton NMR study of myoglobin reconstituted with iron 3-ethyl-2-methylporphyrin

The asymmetric 3-ethyl-2-methylporphyrin iron complex was synthetized and inserted into apomyoglobin. UV-visible spectroscopic studies demonstrated the capacity of iron to coordinate different exogenous axial ligands in ferrous and ferric forms. The position of synthetic heme into the hydrophobic pocket of the reconstituted myoglobin was investigated by ((1))H NMR spectroscopy. In absence of exogenous ligand, signals of the synthetic prosthetic group were not detected, suggesting a rotational disorder of the synthetic porphyrin into the heme pocket. This direct interconversion behavior is favored since site-specific interactions between the poorly substituted heme and protein in the chiral hydrophobic cavity were weak. Complexion of cyanide to the iron allowed to quench partially the heme reorientation and two interconvertible forms, around the meso-Cα-Cγ axis, were detected in solution.

The contribution of heme propionate groups to the conformational dynamics associated with CO photodissociation from horse heart myoglobin

Photoacoustic calorimetry and transient absorption spectroscopy were used to study conformational dynamics associated with CO photodissociation from horse heart myoglobin (Mb) reconstituted with either Fe protoporphyrin IX dimethylester (FePPDME), Fe octaethylporphyrin (FeOEP), or with native Fe protoporphyrin IX (FePPIX). The volume and enthalpy changes associated with the Fe-CO bond dissociation and formation of a transient deoxyMb intermediate for the reconstituted Mbs were found to be similar to those determined for native Mb (DeltaV1 = -2.5+/-0.6 ml mol(-1) and DeltaH1 = 8.1+/-3.0 kcal mol(-1)). The replacement of FePPIX by FeOEP significantly alters the conformational dynamics associated with CO release from protein. Ligand escape from FeOEP reconstituted Mb was determined to be roughly a factor of two faster (tau=330 ns) relative to native protein (tau=700 ns) and accompanying reaction volume and enthalpy changes were also found to be smaller (DeltaV2 = 5.4+/-2.5 ml mol(-1) and DeltaH2 = 0.7+/-2.2 kcal mol(-1)) than those for native Mb (DeltaV2 = 14.3+/-0.8 ml mol(-1) and DeltaH2 = 7.8+/-3.5 kcal mol(-1)). On the other hand, volume and enthalpy changes for CO release from FePPIX or FePPDME reconstituted Mb were nearly identical to those of the native protein. These results suggest that the hydrogen bonding network between heme propionate groups and nearby amino acid residues likely play an important role in regulating ligand diffusion through protein matrix. Disruption of this network leads to a partially open conformation of protein with less restricted ligand access to the heme binding pocket.

Design and structure analysis of artificial metalloproteins: selective coordination of His64 to copper complexes with square-planar structure in the apo-myoglobin scaffold

apo-Myoglobin (apo-Mb) was reconstituted with three copper complexes: CuII(Sal-Phe) (1; Sal-Phe = N-salicylidene-L-phenylalanato), CuII(Sal-Leu) (2; Sal-Leu = N-salicylidene-L-leucinato), and CuII(Sal-Ala) (3; Sal-Ala = N-salicylidene-L-alanato). The crystal structures of 1.apo-Mb (1.65 Angstrom resolution) and 2.apo-Mb (1.8 Angstrom resolution) show that the coordination geometry around the CuII atom in apo-Mb is distorted square-planar with tridentate Sal-X and a Nepsilon atom of His64 in the apo-Mb cavity and the plane of these copper complexes is perpendicular to that of heme. These results suggest that the apo-Mb cavity can hold metal complexes with various coordination geometries.

Proton NMR study of myoglobin reconstituted with 3,7-diethyl-2,8-dimethyl iron porphyrin: Remarkable influence of peripheral substitution on heme rotation

The iron complex of 3,7-diethyl-2,8-dimethylporphyrin was incorporated into horse heart apomyoglobin to investigate the influence of peripheral substitution on artificial heme rotation. The hyperfine-shifted 1H NMR spectrum of the reconstituted deoxymyoglobin (rMb) revealed the proximal imidazole N-H resonance at 82.5 ppm to indicate the formation of the Fe--N (His93) bond. The pyrrole-protons of the hemin of myoglobin in the absence of external ligand appeared as four resonances between -10 and -18 ppm, indicating a mainly low-spin ferric hemin, with a ligated distal histidine (His64). This also indicates the lost of the symmetry of the hemin, according to an absence of free rotation of the prosthetic group. The 1H NMR spectrum of reconstituted rMbCO revealed a set of four pyrrole-protons and a set of four meso-protons. Accordingly, the prosthetic group without acid side chains interacts specifically with the surrounding globin showing a unique heme orientation in the 1H NMR time-scale, despite the presence of only four alkyl substituents on the porphine ring. This also suggests that two ethyl groups are large enough to avoid the free rotation movement of the heme.

Iron hemiporphycene as a functional prosthetic group for myoglobin

The iron complex of hemiporphycene, a molecular hybrid of porphyrin with porphycene, was incorporated into the apomyoglobin pocket to examine ligand binding ability of the iron atom in the novel porphyrinoid. Apomyoglobin was successfully coupled with a stoichiometric amount of ferric hemiporphycene to afford the reconstituted myoglobin equipped with the iron coordination structure of native protein. Cyanide, imidazole, and fluoride coordinated to the ferric protein with affinities comparable with those for native myoglobin. The ferrous myoglobin was functionally active to bind O(2) and CO reversibly at pH 7.4 and 20 degrees C. The O(2) affinity is 12-fold higher than that of native myoglobin while the CO affinity is slightly lower, suggesting decreased discrimination between O(2) and CO in the heme pocket. The functional anomaly was interpreted to reflect increased sigma-bonding character in the Fe(II)-O(2) bond. In contrast with 6-coordinate native NO protein, the NO myoglobin containing ferrous hemiporphycene is in a mixed 5- and 6-coordinate state. This observation suggests that the in-plane configuration of the iron atom in hemiporphycene is destabilized by NO. Influence of the core deformation was also detected with both the infrared absorption for the ferrous CO derivative and electron paramagnetic resonance for ferric imidazole complex. Anomalies in the ferric and ferrous derivatives were ascribed to the modified iron-N(pyrrole) interactions in the asymmetric metallo core of hemiporphycene.

Unusual spin state equilibrium of azide metmyoglobin induced by ferric corrphycene

Myoglobin was reconstituted with the ferric complex of corrphycene, a novel porphyrin isomer with a rearranged tetrapyrrole array, to investigate the influence of porphyrin deformation on the equilibrium between high-spin (S = 5/2) and low-spin (S = 1/2) states in the azide derivative. The azide affinity, 2.5 x 10(4) M(-1), was 1 order of magnitude lower than the corresponding values of a reference myoglobin containing an electron-deficient diformylheme similar to the corrphycene. Analysis of the visible absorption spectrum over a range of 0-40 degrees C reveals that the population of high-spin iron is 76-82% at room temperature for azide metmyoglobin complexed with ferric corrphycene. The unusual predominance of the high-spin state was verified from the infrared spectrum of coordinating azide, where the high-spin peak at 2046 cm(-1) is 4-fold larger in intensity than the 2023 cm(-1) low-spin band. Electron paramagnetic resonance at 15 K further indicated that the iron-histidine bond is cleaved to form a five-coordinate derivative in some fraction of the myoglobin. The remarkable high-spin bias of the spin equilibrium at room temperature and cleavage of the iron-histidine bond at 15 K could be explained in terms of the contracted and trapezoidal metallo core that weakens the iron-histidine bond of azide metmyoglobin bearing corrphycene.

Properties and reactivity of myoglobin reconstituted with chemically modified protohemin complexes

The synthetic complexes protohemin-6(7)-L-arginyl-L-alanine (HM-RA) and protohemin-6(7)-L-histidine methyl ester (HM-H) were prepared by condensation of suitably protected Arg-Ala or His residues with protohemin IX. HM-RA and HM-H were used for reconstitution of apomyoglobin from horse heart, yielding the Mb-RA and Mb-H derivatives, respectively, of the protein. The spectral, binding and catalytic properties of Mb-RA and Mb-H are significantly different from those of Mb. As shown by MM and MD calculations, these differences are determined by some local structural changes around the heme which are generated by increased mobility of a key peptide segment (Phe43-Lys47), containing the residue (Lys45) that in native Mb interacts with one of the porphyrin carboxylate groups. In the reconstituted Mbs this carboxylate group is bound to the Arg-Ala or His residue and is no longer available for electrostatic interaction with Lys45. The mobility of the peptide segment near the active site allows the distal histidine to come to a closer contact with the heme, and in fact Mb-RA and Mb-H exist as an equilibrium between a high-spin form and a major low-spin, six-coordinated form containing a bis-imidazole ligated heme. The two forms are clearly distinguishable in the NMR spectra, that also show that each of them consists of a mixture of the two most stable isomers resulting from cofactor reconstitution, as also anticipated by MM and MD calculations. Exogenous ligands such as cyanide, azide, or hydrogen peroxide can displace the bound distal histidine, but their affinity is reduced. On the other hand, mobilization of the peptide chain around the heme in the reconstituted Mbs increases the accessibility of large donor molecules at the heme periphery, with respect to native Mb, where a rigid backbone limits access to the distal pocket. The increased active site accessibility of Mb-RA and Mb-H facilitates the binding and electron transfer of phenolic substrates in peroxidase-type oxidations catalyzed by the reconstituted proteins in the presence of hydrogen peroxide.

Novel ligand binding properties of the myoglobin substituted with monoazahemin

The iron complex of alpha-azamesoporphyrin XIII was combined with apomyoglobin to investigate influence of the meso nitrogen on ligand binding properties in the reconstituted protein. Stoichiometric complex formation between the two components was confirmed, and conservation of the native coordination structures in the resultant myoglobin was established with spectroscopic criteria and apparently normal ligand binding. The visible absorption spectra of various ferric and ferrous derivatives are characteristic with less intense Soret peaks and enhanced visible bands. The electron paramagnetic resonance spectrum with g = 5.2 suggests an anomalous intermediate spin (S = 3/2) character for the aquomet protein. The oxygen affinity of reduced azaheme myoglobin, 0.010 mm Hg, is 50 times larger than that of the native myoglobin. In addition, azaheme myoglobin forms stable complexes with imidazole, pyridine, or cyanide in ferrous state. All of these new properties were consistently explained in terms of stronger equatorial ligand field of the heme iron in a narrower coordination cavity. Similarities of azaheme to verdoheme were also pointed out.

Consequence of rapid heme rotation to the oxygen binding of myoglobin

The myoglobin complexed with octamethylhemin was prepared. The visible absorption profiles are typical of general alkylhemin-substituted myoglobins, suggesting normal insertion of the hemin into the hydrophobic cavity. The NMR spectra of various ferric proteins were anomalous, without clearly resolved signals from the prosthetic group, most probably reflecting rapid heme rotation about the iron-histidine bond. The equilibrium and kinetic oxygen bindings were measured to examine the functional significance of the heme motion. Functional comparison with the myoglobin having immobile hemin indicates that rotation of the octamethylheme negligibly affects the myoglobin function. The results suggest that neither the heme peripheral contacts nor the proximal imidazole orientation about the heme normal is the dominant factor to control myoglobin function.

Structure/activity relationships in porphobilinogen oxygenase and horseradish peroxidase. An analysis using synthetic hemins

The apo-enzymes of porphobilinogen oxygenase and horseradish peroxidase were reconstituted with hemin IX, deuterohemin IX, 2,4-diacetyldeuterohemin IX, 2-vinyl-4-deuterohemin IX and hemin I. The apoproteins did not reconstitute with the dimethyl or diethyl esters of hemin IX. The native enzymes and the synthetic hemoproteins showed similar oxygenase activities toward porphobilinogen in the presence of dithionite and oxygen. They also showed peroxidase activity in the presence of H2O2, which was affected by the side-chain substitution pattern of the hemes. Oxygenase activities, however, were not affected by the heme structure. Iron chelators completely inhibited the oxygenase, but not the peroxidase activities. The EPR spectra of the native and synthetic porphobilinogen oxygenase showed that dithionite reduction produced a rapid disappearance of the high-spin heme-iron signal at g = 6.0. It reappeared 1 min later but the enzyme retained its catalytic activity. The changes in the EPR spectra could be correlated with the biphasic kinetics of the oxygenase reaction which was very fast during the first minute and then decreased to a half-value rate. The oxygenase reaction was inhibited by addition of superoxide dismutase during the fast rate phase, but not during the slower phase. These results could be explained by the formation of a superoxide anion during the first minute of the oxygenase reaction, after which a protein-stabilized radical (g = 2.0) is generated (very likely a tyrosyl radical). The latter then oxidizes the substrate porphobilinogen and facilitates its reaction with O2 to give oxopyrrolenines.

Preparation and spectral characterization of the heme d1.apomyoglobin complex: an unusual protein environment for the substrate-binding heme of Pseudomonas cytochrome oxidase

The heme d1 prosthetic group isolated from Pseudomonas cytochrome oxidase combines with apomyoglobin to form a stable, optically well-defined complex. Addition of ferric heme d1 quenches apomyoglobin tryptophan fluorescence suggesting association in a 1:1 molar ratio. Optical absorption maxima for heme d1.apomyoglobin are at 629 and 429 nm before, and 632 and 458 nm after dithionite reduction; they are distinct from those of heme d1 in aqueous solution but more similar to those unobscured by heme c in Pseudomonas cytochrome oxidase. Cyanide, carbon monoxide and imidazole alter the spectrum of heme d1.apomyoglobin demonstrating axial coordination to heme d1 by exogeneous ligands. The cyanide-induced optical difference spectra exhibit isosbestic points, and a Scatchard-like analysis yields a linear plot with an apparent dissociation constant of 4.2 X 10(-5) M. However, carbon monoxide induces two absorption spectra with Soret maxima at 454 or 467 nm, and this duplicity, along with a shoulder that correlates with the latter before binding, suggests multiple carbon monoxide and possibly heme d1 orientations within the globin. The 50-fold reduction in cyanide affinity over myoglobin is more consistent with altered heme pocket interactions than the intrinsic electronic differences between the two hemes. However, stability of the heme d1.apomyoglobin complex is verified further by the inability to separate heme d1 from globin during dialysis and column chromatography in excess cyanide or imidazole. This stability, together with a comparison between spectra of ligand-free and -bound derivatives of heme d1-apomyoglobin and heme d1 in solution, implies that the prosthetic group is coordinated in the heme pocket through a protein-donated, strong-field ligand. Furthermore, the visible spectrum of heme d1.apomyoglobin varies minimally with ligand exchange, in contrast to the Soret, which suggests that much spectral information concerning heme d1 coordination in the oxidase is lost by interference from heme c absorption bands. A comparison of the absorption spectra of heme d1.apomyoglobin and Pseudomonas cytochrome oxidase, together with a critical examination of the previous axial ligand assignments from magnetic resonance techniques in the latter, implies that it is premature to accept the assignment of bishistidine heme d1 coordination in oxidized, ligand-free oxidase and other iron-isobacteriochlorin-containing enzymes.

Etiohemin as a prosthetic group of myoglobin

Sperm whale myoglobin was reconstituted with etioheme and the stoichiometric complex formation was confirmed. The proton NMR spectrum of the deoxy myoglobin exhibits an NH signal from the proximal histidine at 78.6 ppm, indicating heme incorporation into the heme pocket to form the Fe-N(His-F8) bond. The appearance of a single set of the heme-methyl NMR signals shows that etioheme without acid side-chains specifically interacts with the surrounding globin. The visible spectral data suggest retention of a normal iron coordination structure. The functional and NMR spectral properties of etioheme myoglobin are similar to those of mesoheme myoglobin, reflecting the absence of the electron-withdrawing heme vinyl groups.