Heme modification studies of myoglobin

Alteration of Coaxial Heme Ligands Reveals the Role of Heme in Bacterioferritin from Mycobacterium tuberculosis

The uptake and utilization of iron remains critical for the survival/virulence of the host/pathogens in spite of the limitations (low bioavailability/high toxicity) associated with this nutrient. Both the host and pathogens manage to overcome these problems by utilizing the iron repository protein nanocages, ferritins, which not only sequester and detoxify the free Fe(II) ions but also decrease the iron solubility gap by synthesizing/encapsulating the Fe(III)-oxyhydroxide biomineral in its central hollow nanocavity. Bacterial pathogens including Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, encode a distinct subclass of ferritins called bacterioferritin (BfrA), which binds heme, the versatile redox cofactor, via coaxial, conserved methionine (M52) residues at its subunit-dimer interfaces. However, the exact role of heme in Mtb BfrA remains yet to be established. Therefore, its coaxial ligands were altered via site-directed mutagenesis, which resulted in both heme-bound (M52C; ∼1 heme per cage) and heme-free (M52H and M52L) variants, indicating the importance of M52 residues as preferential heme binding axial ligands in Mtb BfrA. All these variants formed intact nanocages of similar size and iron-loading ability as that of wild-type (WT) Mtb BfrA. However, the as-isolated heme-bound variants (WT and M52C) exhibited enhanced protein stability and reductive iron mobilization as compared to their heme-free analogues (M52H and M52L). Further, increasing the heme content in BfrA variants by reconstitution not only enhanced the cage stability but also facilitated the iron mobilization, suggesting the role of heme. In contrary, heme altered the ferroxidase activity to a lesser extent despite facilitating the accumulation of the reactive intermediates formed during the course of the reaction. The current study suggests that heme in Mtb BfrA enhances the overall stability of the protein and possibly acts as an intrinsic electron relay station to influence the iron mineral dissolution and thus may be associated with Mtb's pathogenicity.

Influence of heme propionates on the nitrite reductase activity of myoglobin

The heme propionates in myoglobin (Mb) form a H-bonding network among several residues within its second-sphere coordination, providing a key structural role towards Mb's functional properties. Our work aims to understand the role of the heme propionates on the nitrite reductase (NiR) activity (e.g. reduction of NO₂^- to NO) of this globin by studying an artificial dimethylester heme-substituted horse heart Mb (DME-Mb). The minor structural change brought about by esterification of the heme propionates causes the NiR rate to increase by more than over two-fold (5.6 ± 0.1 M^-1 s^-1) relative to wildtype (wt) Mb (2.3 ± 0.1 M^-1 s^-1). The lower pK_a observed in DME-Mb may enhance the tendency of His64 towards protonation, therefore increasing the NiR rate. In addition, the nitrite binding constant (K_nitrite) for DME-Mb^III is greater than wt Mb^III (350 M^-1 versus 120 M^-1). The disparity in the NiR activity correlates with the differences in electrostatic behavior, which influences the system's reactivity towards the approaching NO₂^- ion, and thus the formation of the Fe^II-NO₂^- intermediate.

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.

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.

Mesohaem substitution reveals how haem electronic properties can influence the kinetic and catalytic parameters of neuronal NO synthase

NOSs (NO synthases, EC 1.14.13.39) are haem-thiolate enzymes that catalyse a two-step oxidation of L-arginine to generate NO. The structural and electronic features that regulate their NO synthesis activity are incompletely understood. To investigate how haem electronics govern the catalytic properties of NOS, we utilized a bacterial haem transporter protein to overexpress a mesohaem-containing nNOS (neuronal NOS) and characterized the enzyme using a variety of techniques. Mesohaem-nNOS catalysed NO synthesis and retained a coupled NADPH consumption much like the wild-type enzyme. However, mesohaem-nNOS had a decreased rate of Fe(III) haem reduction and had increased rates for haem-dioxy transformation, Fe(III) haem-NO dissociation and Fe(II) haem-NO reaction with O2. These changes are largely related to the 48 mV decrease in haem midpoint potential that we measured for the bound mesohaem cofactor. Mesohaem nNOS displayed a significantly lower Vmax and KmO2 value for its NO synthesis activity compared with wild-type nNOS. Computer simulation showed that these altered catalytic behaviours of mesohaem-nNOS are consistent with the changes in the kinetic parameters. Taken together, the results of the present study reveal that several key kinetic parameters are sensitive to changes in haem electronics in nNOS, and show how these changes combine to alter its catalytic behaviour.

The weight loss elicited by cobalt protoporphyrin is related to decreased activity of nitric oxide synthase in the hypothalamus

Administration of cobaltic protoporphyrin IX (CoPP) into the third ventricle of the brain by intracerebroventricular injection in rodents is known to result in transient hypophagia and remarkably prolonged weight loss. The mechanism of action of CoPP in eliciting these effects is unknown. It is known that nitric oxide plays a role in food intake and that the hyperphagia that results from a wide variety of genetic, physiological, and pharmacological stimuli can be blocked by the administration of inhibitors of the enzyme nitric oxide synthase (NOS). We demonstrate that intracerebroventricular administration of compounds that alter nitrergic tone can also change food ingestion and weight gain patterns in normophagic rats. We also demonstrate that CoPP decreases NOS activity but that it paradoxically increases neuronal NOS transcript expression and increases neuronal NOS protein content on Western blotting.

Temperature-dependent studies of NO recombination to heme and heme proteins

The rebinding kinetics of NO to the heme iron of myoglobin (Mb) is investigated as a function of temperature. Below 200 K, the transition-state enthalpy barrier associated with the fastest (approximately 10 ps) recombination phase is found to be zero and a slower geminate phase (approximately 200 ps) reveals a small enthalpic barrier (approximately 3 +/- 1 kJ/mol). Both of the kinetic rates slow slightly in the myoglobin (Mb) samples above 200 K, suggesting that a small amount of protein relaxation takes place above the solvent glass transition. When the temperature dependence of the NO recombination in Mb is studied under conditions where the distal pocket is mutated (e.g., V68W), the rebinding kinetics lack the slow phase. This is consistent with a mechanism where the slower (approximately 200 ps) kinetic phase involves transitions of the NO ligand into the distal heme pocket from a more distant site (e.g., in or near the Xe4 cavity). Comparison of the temperature-dependent NO rebinding kinetics of native Mb with that of the bare heme (PPIX) in glycerol reveals that the fast (enthalpically barrierless) NO rebinding process observed below 200 K is independent of the presence or absence of the proximal histidine ligand. In contrast, the slowing of the kinetic rates above 200 K in MbNO disappears in the absence of the protein. Generally, the data indicate that, in contrast to CO, the NO ligand binds to the heme iron through a "harpoon" mechanism where the heme iron out-of-plane conformation presents a negligible enthalpic barrier to NO rebinding. These observations strongly support a previous analysis (Srajer et al. J. Am. Chem. Soc. 1988, 110, 6656-6670) that primarily attributes the low-temperature stretched exponential rebinding of MbCO to a quenched distribution of heme geometries. A simple model, consistent with this prior analysis, is presented that explains a variety of MbNO rebinding experiments, including the dependence of the kinetic amplitudes on the pump photon energy.

Assembly and activation of heme-deficient neuronal NO synthase with various porphyrins

The heme prosthetic group of NO synthase is critical for catalytic activity as well as assembly of the enzyme to the native homodimeric form. In the current study, we examined if structurally different metal porphyrins could substitute for the native heme prosthetic group in neuronal NO synthase (nNOS) with regard to assembly and catalysis. We established, with the use of a recently developed in vitro method that functionally reconstitutes heme-deficient apo-nNOS, that Fe-mesoporphyrin IX or Fe-deuteroporphyrin IX can substitute for heme and lead to assembly of a functional nNOS, albeit with lower activity. Fe-protoporphyrin IX dimethyl ester or the metal free protoporphyrin IX, however, lead to minimal assembly of nNOS. Protoporphyrin IX compounds where the native Fe was substituted with Zn, Mn, Co, or Sn lead to assembly of nNOS, but no detectable NO was synthesized in the presence of NADPH and L-arginine. Thus, the presence of the metal and propionic acid groups, but not the vinyl moieties, of heme are important structural features in assembly of nNOS. These studies establish that the mechanism of assembly and catalysis of nNOS can be probed with structurally diverse metal porphyrins.

How does heme axial ligand deletion affect the structure and the function of cytochrome b(562)?

We have recently generated a new mutant of cytochrome b(562) (cytb(562)) in which Met7, one of the axial heme ligands, is replaced by Ala (M7A cytb(562)). The M7A cytb(562) can bind heme and the UV-visible absorption spectrum is of a typical high-spin ferric heme. To investigate the effect of the lack of Met7 ligation on the structural integrity of cytb(562), thermal transition analyses of M7A cytb(562) were conducted. From the thermodynamic parameters obtained, it is concluded that the folding of M7A cytb(562) is comparable to the apoprotein despite the presence of heme. On the other hand, exogenous ligands such as cyanide and azide ions are readily bound to the heme iron, indicating that the axial coordination site is available for substrate binding. The peroxidase activity of this mutant is thus examined to evaluate new enzymatic function at this site and M7A cytb(562) was found to catalyze an oxidation reaction of aromatic substrates with hydrogen peroxide. These observations demonstrate that the Met7/His102 bis-ligation to the heme iron is crucial for the stable folding of cytb(562), whereas the functional conversion of cytb(562) is successfully achieved by the loose folding together with the open coordination site.

Design and Semisynthesis of Photoactive Myoglobin Bearing Ruthenium Tris(2,2'-bipyridine) Using Cofactor-Reconstitution

A new strategy for semisynthesis of a photoactivatable redox protein is described. Three protohemin molecules with ruthenium tris(2,2'-bipyridine) attached by different spacers were synthesized. The Ru(bpy)(3)-protohemins were incorporated into the heme crevice of apomyoglobin (apo-Mb) to yield semisynthetic Mbs carrying Ru(bpy)(3) as a photosensitizer (Ru(bpy)(3)-Mb). The photoactivation properties and the reaction mechanisms of Ru(bpy)(3)-Mbs were investigated by steady-state photoirradiation and laser flash photolysis. The photoactivation of Ru(bpy)(3)-Mbs was spectrophotometrically demonstrated by comparison with an intermolecular control, namely an equimolar mixture of Ru(bpy)(3) and native Mb. The spacer structure considerably influenced net activation efficiency over a wide pH range as measured by steady-state visible light irradiation and quantum yield. Laser flash photolysis yielded the rate of the photoinduced electron transfer (ET) from the lifetime of the excited Ru(bpy)(3) (k(et) = 4.4 x 10(7) s(-)(1) for Mb(1b) and k(et) = 3.7 x 10(7) s(-)(1) for Mb(1c)) and the back ET rate (k(back) = (2.0-3.7) x 10(7) s(-)(1) for Mb(1b) and k(back) = (1.4-2.4) x 10(7) s(-)(1) for Mb(1c)) from the decay of the transient absorption. These data consistently explained the results of the net photoreaction as follows. (i) The intermolecular control system was less photoactivated because little ET occurred from the excited state of Ru(bpy)(3) to Mb. (ii) The short lifetime of the charge-separated state after photoinduced ET greatly decreased the photoactivation efficiency of Ru(bpy)(3)-Mb with the shortest spacer. (iii) The photochemical and photophysical data of the other two Ru(bpy)(3)-Mb derivatives (the net photoreaction, quantum yield, and ET/back ET rates) were essentially identical, indicating that flexible spacers consisting of oxyethylene units do not rigidly fix the distance between Ru(bpy)(3) and the heme center of Mb. In addition, Ru(bpy)(3)-Mbs were highly photoactivated under aerobic conditions in a manner similar to that under anaerobic conditions, although O(2) usually quenches the photoexcited state of Ru(bpy)(3). This was probably due to the accelerated intramolecular ET from Ru(bpy)(3) to heme, not to O(2) in Ru(bpy)(3)-Mbs. We therefore showed that visible light affects the content of O(2)-bound Mb even in air.

Incorporation of an artificial receptor into a native protein: new strategy for the design of semisynthetic enzymes with allosteric properties

The sugar-facilitated structure and enzymatic activity change of engineered myoglobins bearing a phenylboronic acid moiety, which were semisynthesized by a cofactor reconstitution method, were studied by the denaturation experiment, spectrophotometric titration of the pKa shift of the axial H2O, circular dichloism (CD), and the kinetics of the myoglobin-catalyzed-aniline hydroxylation reaction. Both boronophenylalanine-appended myoglobin [Mb(m-Bphe)2] and phenylboronic acid-appended myoglobin [Mb(PhBOH)2] were stabilized by approximately 2 kcal/mol upon complexation with D-fructose. CD spectral changes and the sugar-induced pKa shift suggested that the microenvironment of the active site of these myoglobins was re-formed from a partially disturbed state to that comparable to the native state upon D-fructose binding. The correlation of pKa with kcat (for the aniline hydroxylase activity) and the delta GDH2O-kcat profile showed that these structural changes of Mb-(m-Bphe)2 and Mb(PhBOH)2 were closely related to their sugar-enhanced aniline hydroxylase activity. Thus, the results established that an incorporation of the artificial receptor molecule can be a valid methodology for the design of stimuli-responsive semiartificial enzymes.

Functional comparison of the myoglobins reconstituted with symmetric deuterohemes

Deuterohemins III and XIII were coupled with apomyoglobin to examine the influence of the modified heme-globin contacts on the functions of the reconstituted holoproteins. Owing to the molecular symmetry of the prosthetic groups, the resultant proteins are free from the heme orientational disorder. The coordination structures of the two reconstituted myoglobins were found to be normal and closely similar to each other. The equilibrium ligand bindings also resembled with each other in both ferric and ferrous states. The results demonstrate that the different local heme-globin contacts affect the structure and function of the reconstituted myoglobins only slightly. The results therefore suggest that the two asymmetric deuteroheme IX isomers, which are inverted about the alpha, gamma-meso carbon axis of the heme, also exhibit very similar functions in myoglobin.

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.

Monitoring of a conjugation reaction between fluorescein isothiocyanate and myoglobin by capillary zone electrophoresis

The conjugation reaction between the amine reactive fluorescent probe fluorescein isothiocyanate and the protein horse heart myoglobin was directly monitored as the reaction progressed by capillary zone electrophoresis with UV absorbance detection. Kinetic analysis of the data indicated that the order of the reaction was first with respect to myoglobin and 1.3 with respect to fluorescein isothiocyanate. The separation of peaks attributable to the incorporation of n = 1,2, ... 7 fluorochrome labels into the protein has some significance for the generation of single-label protein probes, such as antibodies.

Formation of sulphmyoglobin during expression of horse heart myoglobin in Escherichia coli

Expression of recombinant horse heart myoglobin in Escherichia coli has been found to result in the production of both native and variable amounts (approximately 16-17% total) of two sulphmyoglobin isomers. The recombinant sulphmyoglobin produced consists primarily of the A and B isomers as identified by 1H NMR spectroscopy with no evidence for production of the C isomer. Conversion of recombinant sulphmyoglobin to the native protein can be achieved by reconstitution with protohaem IX. The possible relationship of this observation to recombinant expression of other heme proteins is discussed.

1H NMR study of the role of heme carboxylate side chains in modulating heme pocket structure and the mechanism of reconstitution of cytochrome b5

1H nuclear magnetic resonance spectroscopy was used to assign the hyperfine-shifted resonances and determine the position of a side chain in the heme cavity of wild-type rat apocytochrome b5 reconstituted with a series of synthetic hemins possessing systematically perturbed carboxylate side chains. The hemins included protohemin derivatives with individually removed or pairwise shortened and lengthened carboxylate side chains, as well as (propionate)n(methyl)8-nporphine-iron(III) isomers with n = 1-3 designed to force occupation of nonnative propionate sites. The resonance assignments were effected on the basis of available empirical heme contact shift correlations and steady-state nuclear Overhauser effect measurements in the low-spin oxidized proteins. The failure to detect holoproteins with certain hemins dictates that the stable holoproteins, unlike the case of myoglobin, demand the axial iron-His bonds and cannot accommodate carboxylate side chains at interior positions in the binding pocket. Hence, the heme pocket interior in cytochrome b5 is judged much less polar and less sterically accommodating than that of myoglobin. The propionate occupational preference was greatest as the native 7-propionate site, but also possible at the nonnative crystallographic 5-methyl or 8-methyl positions. Only for a propionate at the crystallographic 8-methyl position was a significant perturbation of the native molecular/electronic structure observed, and this was attributed to an alternative propionate-protein hydrogen bond at the crystallographic 8-methyl position. The structures of the transient protein complexes detected only shortly after reconstitution reveal that the initial encounter complexes during assembly of holoprotein from apoprotein and hemin involve one of the two alternate propionate-protein links at either the 7-propionate or native 8-methyl position. In a monopropionate hemin, this leads to the characterization of a new type of heme orientational disorder involving rotation about a N-Fe-N axis.

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.

Crystallization and preliminary diffraction data for horse heart metmyoglobin

Reddish-brown crystals of metmyoglobin from horse heart have been obtained by both the hanging drop and batch crystallization methods in the space group P2(1), having a = 64.3 A, b = 28.9 A and c = 35.9 A, with beta = 107.1 degree. Morphologically similar crystal forms have been obtained for three derivatives of horse heart myoglobin having modified heme prosthetic groups.

Identification of the titrating group in the heme cavity of myoglobin. Evidence for the heme-protein pi-pi interaction

The pH dependence of the proton NMR chemical shifts of met-cyano and deoxy forms of native and reconstituted myoglobins reflects a structural transition in the heme pocket modulated by a single proton with pK 5.1-5.6. Comparison of this pH dependence of sperm whale and elephant myoglobin and that of the former protein reconstituted with esterified hemin eliminates both the distal histidine as well as the heme propionates as the titrating residue. Reconstitution of sperm whale met-cyano myoglobin with hemin modified at the 2,4-positions leads to a systematic variation in the pK for the structural transition, thus indicating the presence of a coupling between the titrating group and the heme pi system. The results are consistent with histidine FG3 (His-FG3) being the titrating group, and a donor-acceptor pi-pi interaction between its imidazole and the heme is proposed.

The heme environment of leghemoglobins. Absorption and circular dichroism spectra of artificial leghemoglobins and myoglobins

Artificial leghemoglobins were reconstituted from apoleghemoglobin and meso-, deutero- and diacetyldeuteroheme. Absorption and circular dichroism spectra of their high-spin and low-spin derivatives in the ferrous and ferric forms were recorded in the ultraviolet and visible wavelength regions. The substitution of the 2,4-side-chains of heme induced changes in the optical activity, reflecting alterations in the heme environment. The effect on the conformation of aromatic amino acid residues around heme obviously correlates with the sixth axial ligand and the spin state of iron. Absorption and CD spectra of the aquoferric derivatives of artificial myoglobins were recorded in comparison. Strongly electron-withdrawing acetyl side-chains at the 2,4-positions of diacetyldeuteroheme caused a change in the absorption spectra of aquoferric leghemoglobin and myoglobin towards low spin. On the basis of the spectra it was suggested that the displacement of the ferric iron from the pyrrole plane in leghemoglobin derivatives would be smaller than in the corresponding myoglobin derivatives.

Five-coordinate iron-porphyrin as a model for the active site of hemoproteins. Characterization and coordination properties

Preparation of iron(III)-deuteroporphyrin 6(7)-methyl ester, 7(6)-(histidine methyl ester) by coupling histidine methyl ester to deuterohemin has been performed using the mixed carboxylic/carbonic-acid-anhydride method. This compound, which is very soluble in various organic solvents, can be considered as a prosthetic group model for the active site of five-coordinate hemoproteins. In the oxidized state a basic, a neutral or an acid form can be isolated. The basic and acid forms are monomeric at all concentrations. The neutral form is found dimeric in concentrated solutions while it is monomeric at low concentration. The coordination state of iron in these various species is investigated. The neutral form reacts with ligands, such as nitrogenous organic bases, leading to six-coordinate well-known hemichromes which exhibit low-spin electron spin resonance (ESR) spectra. The reaction of anionic ligands, such as F-, CN-, NO-2 and N-3, with the neutral form model leads to unsymmetrical six-coordinate complexes whose optical and ESR spectra are similar to those of synthetic deuteromyoglobin. In benzene, toluene or dichloromethane solutions iron (II)-deuteroporphyrin 6(7)-methyl ester, 7(6)-histidine methyl ester), obtained from ferric forms by heterogeneous reduction with aqueous dithionite, exhibits an optical spectrum characteristic of a five-coordinate high-spin ferrous complex. At low temperature important spectral modifications are observed indicating a dimeric association. At room temperature it binds one nitrogenous base molecule leading to the well-known hemochrome. It reacts also with carbon monoxide with a very high affinity constant (4.5 X 10(8) M-1), comparable to that of the isolated human hemoglobin alpha and beta chains, but much higher than the values reported for other various models, suggesting that the side-chain length bearing the fifth ligand may have an important influence upon the reactivity of the sixth position of the iron ion. At low temperature in toluene or dichloromethane, this compound reversibly binds oxygen without oxidation of the iron ion while oxidation occurs at room temperature. The significance of these results is discussed in relation with the local environment, the electronic nature of the base and the immobilization of the heme group in hemoproteins.