Proton NMR study of the myoglobin reconstituted with meso-tetra(n-propyl)hemin

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.

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.

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.

The effects of heme modification on reactivity, ligand binding properties and iron-coordination structures of cytochrome P450nor

Artificial cytochrome P450nors (nitric oxide reductase) were prepared by replacing the native protoheme with various 2,4-substituted hemes: meso-, deutero-, and diacetyldeutero-hemes. For these samples, the ratio of low spin/high spin states of the ferric resting enzyme were varied, indicating that the coordination of the water molecule at the iron sixth site was affected by the electron withdrawing capacities of the heme 2,4-substituents. The binding of the water molecule reduces the rate of binding of nitric oxide (NO) to the ferric iron. In addition, the reduction reaction of the ferric-NO complex with NADH, which constitutes the second step in the NO reduction, was facilitated by the electron withdrawing capacity of 2,4-substituents. Consequently, proto- (native-) P450nor exhibited the highest overall enzymatic activity (NO reduction activity), while the enzymes containing diacetyl-, deutero-, and meso-hemes had considerably lower activities, since the NO reduction activity is determined by a balance of the reaction rates of the above two steps. The optical absorption spectra of the ferric-NO and the ferrous-CO complexes of the reconstituted enzymes show that the electron density on the heme in both states was modulated by the substituent groups. However, the resonance Raman spectral measurements showed that the Fe-NO and N-O stretching frequencies in the ferric-NO complex were insensitive to the electron density of the heme while the Fe-CO and C-O stretching frequencies in the ferrous-CO complex were sensitively varied by the electron withdrawing capacity of the 2,4-substituent. The differences are discussed in terms of the difference in the iron-ligand bond characters between the ferric-NO and the ferrous-CO complexes.

Structure and function of 6,7-dicarboxyheme-substituted myoglobin

Myoglobin was reconstituted with 6,7-dicarboxy-1,2,3,4,5, 8-hexamethylheme, a compact synthetic heme with the shortest acid side chains, to pursue the structural and functional consequences after intensive disruption of the heme propionate-apoglobin linkages in the native protein. The electron-withdrawing carboxylate groups directly attached to the porphyrin ring lowered the oxygen affinity by 3-fold as compared with native myoglobin. Autoxidation of the oxy derivative to the ferric protein proceeded with 1.6 x 10(-)2 min-1 at pH 7.0 and 30 degrees C. The crystallographic structure of the cyanomet myoglobin with 1.9 A resolution shows that the heme adopts a unique orientation in the protein pocket to extend the two carboxylates toward solvent sphere. The native globin fold is conserved, and the conformations of globin side chains are almost intact except for those located nearby the heme 6,7-carboxylates. The 7-carboxylate only weakly interacts with Ser92 and His97 through two mediating water molecules. The 6-carboxylate, on the other hand, forms a novel salt bridge with Arg45 owing to conformational flexibility of the guanidinium side chain. The proton NMR shows that the small heme does not fluctuate about the iron-histidine bond even at 55 degreesC, suggesting that the salt bridge between Arg45 and heme 6-carboxylate is of critical importance to recognize and fix the heme in myoglobin.

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.

NMR study of the molecular and electronic structure of the heme cavity in Dolabella met-cyano myoglobin

The molecular and electronic structure of the active site of the cyanide-ligated ferric complex of the myoglobin from the mollusc Dolabella auricularia has been investigated using NMR. Analysis of nuclear Overhauser effects has revealed that the correlation times for the internal motion of the heme propionate alpha-CH2 and beta-CH2 groups at ambient temperature are about 5 and 4 ns, respectively. These correlation times indicate that the terminal carboxylate groups of both the heme propionates are not bound to the protein via salt bridges. Although the absence of the propionate-protein interaction does not influence the equilibrium population of the two heme orientational isomers involving rotation about the alpha,gamma-meso axis, it allows the heme to rotate about the iron-His bond in the active site of the myoglobin. Such rotational motion of the heme resulted in an anomalous temperature-dependence of the heme methyl-proton hyperfine shift. Thus the present myoglobin studies provide the first example demonstrating the rotation of the heme about the iron-His bond in native myoglobin.

Kinetic studies on CO binding to reconstituted myoglobins with four synthetic hemes; structural control in ligand binding to myoglobin

Examination was made of CO binding reactions to four kinds of modified sperm whale myoglobin (Mb), whose heme was reconstituted by iron complexes of synthetic porphyrins such as porphine (Por), meso-tetramethylporphyrin (TMeP), meso-tetraethylporphyrin (TEtP) and meso-tetra(n-propyl)porphyrin (TnPrP), using flash photolysis and stopped-flow methods. The CO association rate was found to be 5- to 20-times and dissociation rate 10- to 36-times accelerated by replacement with synthetic hemes. These features could be explained based on characteristic structures of modified Mbs indicated by X-ray crystallography. The side chain of Arg-45 protruded from the heme vicinity into the solvent region and heme was tilted by interactions of meso-alkyl side chains with surrounding peptides, resulting in the formation of widely opened channels and pockets for ligand passage. These structural features indicate the CO ligand to more easily enter or exit from heme pockets of reconstituted myoglobins, compared to native Mb.

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.