Effects of formylation of vinyl side chains of heme on optical and ligand binding properties of horse heart ferric myoglobin

Heme isomers substantially affect heme's electronic structure and function

Inspection of heme protein structures in the protein data bank reveals four isomers of heme characterized by different relative orientations of the vinyl side chains; remarkably, all these have been reported in multiple protein structures. Density functional theory computations explain this as due to similar energy of the isomers but with a sizable (25 kJ mol^-1) barrier to interconversion arising from restricted rotation around the conjugated bonds. The four isomers, EE, EZ, ZE, and ZZ, were then investigated as 4-coordinate hemes, as 5-coordinate deoxyhemes, in 6-coordinate O₂-adducts of globins and as compound I intermediates typical of heme peroxidases. Substantial differences were observed in electronic properties relevant to heme function: notably, the spin state energy gap of O₂-heme adducts, important for fast reversible binding of O₂, depends on the isomer state, and O₂-binding enthalpies change by up to 16 kJ mol^-1; redox potentials change by up to 0.2 V depending on the isomer, and the doublet-quartet energy splitting of compound I, central to "two-state" reactivity, is affected by up to ∼15 kJ mol^-1. These effects are consistently seen with three distinct density functionals, i.e. the effects are not method-dependent. Thus, the nature of the isomer state is an important but overlooked feature of heme chemistry and function, and previous and future studies of hemes may be reconsidered in this new context.

Modulating the nitrite reductase activity of globins by varying the heme substituents: Utilizing myoglobin as a model system

Globins, such as hemoglobin (Hb) and myoglobin (Mb), have gained attention for their ability to reduce nitrite (NO2(-)) to nitric oxide (NO). The molecular interactions that regulate this chemistry are not fully elucidated, therefore we address this issue by investigating one part of the active site that may control this reaction. Here, the effects of the 2,4-heme substituents on the nitrite reductase (NiR) reaction, and on the structures and energies of the ferrous nitrite intermediates, are investigated using Mb as a model system. This is accomplished by studying Mbs with hemes that have different 2,4-R groups, namely diacetyldeuteroMb (-acetyl), protoMb (wild-type (wt) Mb, -vinyl), deuteroMb (-H), and mesoMb (-ethyl). While trends on the natural charge on Fe and O-atom of bound nitrite are observed among the series of Mbs, the Fe(II)-NPyr (Pyr=pyrrole) and Fe(II)-NHis93 (His=histidine) bond lengths do not significantly change. Kinetic analysis shows increasing NiR activity as follows: diacetyldeuteroMb<wt Mb<deuteroMb<mesoMb. Nitrite binding energy calculations of the different Mb(II)-nitrite conformations demonstrate the N-bound complexes to be more stable than the O-bound complexes for all the different types of heme structures, with diacetyldeuteroMb having the greatest nitrite binding affinity. Spectral deconvolution on the final product generated from the reaction between Mb(II) and NO2(-) for the reconstituted Mbs indicates the formation of 1:1 Mb(III) and Mb(II)-NO. The electronic changes induced by the -R groups on the 2,4-positions do not alter the stoichiometric ratio of the products, resembling wt Mb.

A sensitive chemiluminescence procedure for the determination of carbon monoxide with myoglobin-luminol chemiluminescence system

A novel chemiluminescence method combined with the flow injection technique for the determination of carbon monoxide is presented in this paper. The chemiluminescence signal based on the reaction between myoglobin and luminol in an alkaline medium was remarkably enhanced by carbon monoxide. The enhanced chemiluminescence intensity was linear with carbon monoxide concentration in the range from 0.01 to 10.0 pmol.L(-1), and the detection limit was 3x10(-3) pmol.L(-1) (3sigma). The whole process, including sampling and washing, could be completed in 0.5 min with a relative standard deviation of less than 4.0%. The proposed method was applied successfully in the assay of carbon monoxide in human serum and artificial water samples without any pretreatment procedure.

Structural characterization of non-native states of sperm whale myoglobin in aqueous ethanol or 2,2,2-trifluoroethanol media

The effects of aqueous ethanol or 2,2,2-trifluoroethanol media on the structure of sperm whale myoglobin have been investigated by absorption, CD, and NMR spectra. The structural properties of myoglobin such as heme environments, helix contents, protein folding, and interactions between heme and the protein moiety have been sharply manifested in these spectra. The characterization demonstrated that alcohol-induced conformational change of myoglobin depends on the nature of alcohol and its concentration. It was shown for the first time that, upon the alcohol-induced denaturation of myoglobin, heme is released from partially denatured protein of which helix contents is altered by only about 20% relative to that of native state. Myoglobin has shown to unfold and refold reversibly by controlling the alcohol concentration. Novel methods for the preparation of apomyoglobin and in situ reconstitution of apomyoglobin with heme, based on the alcohol-induced denaturation of the protein, were presented.

1H NMR study of dynamics and thermodynamics of acid-alkaline transition in ferric hemoglobin of a midge larva (Tokunagayusurika akamusi)

One of the components of hemoglobin from the larval hemolyph of Tokunagayusurika akamusi possesses naturally occurring substitution at the E7 helical position (Leu E7) [M. Fukuda, T. Takagi, K. Shikama, Biochim. Biophys. Acta 1157 (1993) 185-191]. Its oxygen affinity is almost comparable to those of mammalian myoglobins and it exhibits Bohr effect. Both acidic and alkaline forms of the ferric hemoglobin have been investigated using 1H NMR in order to gain insight into molecular mechanisms for relatively high oxygen affinity and Bohr effect of this protein. The NMR data indicated that the acidic form of the protein possesses pentacoordinated heme, and that the alkaline form possessing OH- appears with increasing the pH value. pH titration yielded a pK value of 7.2 for the acid-alkaline transition, and this value is the lowest among the values reported so far for various myoglobins and hemoglobins. The kinetic measurements of the transition revealed that the activation energy for the dissociation of the Fe-bound OH-, as well as the dissociation and association rates, decrease with increasing the pH value. These pH dependence properties are likely to be related to the Bohr effect of this protein.

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.

Dynamics and thermodynamics of acid-alkaline transitions in metmyoglobins lacking the usual distal histidine residue

The kinetics of the acid-alkaline transition in the ferric myoglobins from the gastropodic mollusc Dolabella auricularia and the shark Mustelus japonicus, which possess the distal Val E7 and Gln E7, respectively, has been investigated using the paramagnetic 1H-NMR saturation transfer measurements in order to gain insight into functional properties of these non-His distal residues. Both myoglobins possess the penta-coordinated heme below the pK of the transition (7.8 and 10.0 for Dolabella and Mustelus myoglobins, respectively) and bind OH- above the pK. The pH dependence of the transition rates and the relatively high activation barrier (58 +/- 9 kJ/mol) for the dissociation of the Fe-bound OH- in Dolabella myoglobin indicate a strong interaction between the bound ligand and the guanidino NH proton of the Arg E10 in Dolabella myoglobin. Such a strong interaction between Fe-bound OH- and the Arg E10 side-chain in Dolabella myoglobin is also manifested in the EPR spectra. For Mustelus myoglobin, the pH and temperature dependence studies on the kinetics strongly suggest that the distal Gln E7 in this myoglobin does not contribute significantly to stabilize the Fe-bound ligand.

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.

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.