Properties and reactivity of myoglobin reconstituted with chemically modified protohemin complexes

Cu-MOF/hemin: a bionic enzyme with excellent dispersity for the determination of hydrogen peroxide released from living cells

The stability, repeatability and sensitivity of an electrochemical biosensor material are closely connected with the dispersibility of metal organic frameworks (MOFs) in aqueous media. Herein, a nanocomposite based on Cu-MOF/hemin, which is not only highly water-soluble but also simple and efficient in synthesis, was used for the construction of a non-enzymatic sensor to detect hydrogen peroxide (H₂O₂). The Cu-MOF/hemin was characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS)-mapping, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA), which indicate that hemin and the Cu-MOF were successfully combined. As a H₂O₂ electrochemical biomimetic enzyme, the Cu-MOF/hemin exhibited excellent electrocatalytic performance, which was confirmed by the electrochemical experiments and chromogenic reactions, and the possible mechanism of the reactions has been deduced. The electrochemical sensor based on the biomimetic enzyme exhibited an extended linear detection range from 0.01-5.0 mM (R = 0.998), low detection limit of 4.14 μM, and high selectivity and stability under the optimized conditions. More importantly, the practical application ability of the sensor was verified by the test of H₂O₂ in human serum samples and it could be used for the real-time detection of H₂O₂ released from living cells with satisfactory results. Therefore, this novel nanocomposite has certain potential in preparing electrochemical sensing platforms for nonenzymatic biosensing and provides a new method for clinical diagnosis and real-time monitoring.

Hemoproteins Reconstituted with Artificial Metal Complexes as Biohybrid Catalysts

In nature, heme cofactor-containing proteins participate not only in electron transfer and O₂ storage and transport but also in biosynthesis and degradation. The simplest and representative cofactor, heme b, is bound within the heme pocket via noncovalent interaction in many hemoproteins, suggesting that the cofactor is removable from the protein, leaving a unique cavity. Since the cavity functions as a coordination sphere for heme, it is of particular interest to investigate replacement of native heme with an artificial metal complex, because the substituted metal complex will be stabilized in the heme pocket while providing alternative chemical properties. Thus, cofactor substitution has great potential for engineering of hemoproteins with alternative functions. For these studies, myoglobin has been a focus of our investigations, because it is a well-known oxygen storage hemoprotein. However, the heme pocket of myoglobin has been only arranged for stabilizing the heme-bound dioxygen, so the structure is not suitable for activation of small molecules such as H₂O₂ and O₂ as well as for binding an external substrate. Thus, the conversion of myoglobin to an enzyme-like biocatalyst has presented significant challenges. The results of our investigations have provided useful information for chemists and biologists. Our own efforts to develop functionalized myoglobin have focused on the incorporation of a chemically modified cofactor into apomyoglobin in order to (1) construct an artificial substrate-binding site near the heme pocket, (2) increase cofactor reactivity, or (3) promote a new reaction that has never before been catalyzed by a native heme enzyme. In pursuing these objectives, we first found that myoglobin reconstituted with heme having a chemically modified heme-propionate side chain at the exit of the heme pocket has peroxidase activity with respect to oxidation of phenol derivatives. Our recent investigations have succeeded in enhancing oxidation and oxygenation activities of myoglobin as well as promoting new reactions by reconstitution of myoglobin with new porphyrinoid metal complexes. Incorporation of suitable metal porphyrinoids into the heme pocket has produced artificial enzymes capable of efficiently generating reactive high valent metal-oxo and metallocarbene intermediates to achieve the catalytic hydroxylation of C(sp³)-H bonds and cyclopropanation of olefin molecules, respectively. In other efforts, we have focused on nitrobindin, an NO-binding hemoprotein, because aponitrobindin includes a β-barrel cavity, which provides a robust structure highly similar to that of the native holoprotein. It was expected that the aponitrobindin would be suitable for development as a protein scaffold for a metal complex. Recently, it was confirmed that several organometallic complexes can bind to this scaffold and function as catalysts promoting hydrogen evolution or C-C bond formation. The hydrophobic β-barrel structure plays a significant role in substrate binding as well as controlling the stereoselectivity of the reactions. Furthermore, these catalytic activities and stereoselectivities are remarkably improved by mutation-dependent modifications of the cavity structure for the artificial cofactor. This Account demonstrates how apoproteins of hemoproteins can provide useful protein scaffolds for metal complexes. Further development of these concepts will provide a useful strategy for generation of robust and useful artificial metalloenzymes.

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.

Rational heme protein design: all roads lead to Rome

Heme proteins are among the most abundant and important metalloproteins, exerting diverse biological functions including oxygen transport, small molecule sensing, selective C-H bond activation, nitrite reduction, and electron transfer. Rational heme protein designs focus on the modification of the heme-binding active site and the heme group, protein hybridization and domain swapping, and de novo design. These strategies not only provide us with unique advantages for illustrating the structure-property-reactivity-function (SPRF) relationship of heme proteins in nature but also endow us with the ability to create novel biocatalysts and biosensors.

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.

Enhancement of enzymatic activity for myoglobins by modification of heme-propionate side chains

The modification of myoglobin is an attractive process not only for understanding its molecular mechanism but also for engineering the protein function. The strategy of myoglobin functionalization can be divided into at least two approaches: site-directed mutagenesis and reconstitution with a non-natural prosthetic group. The former method enables us to mainly modulate the physiological function, while the latter has the advantage of introducing a new function on the protein. Particularly, replacement of the native hemin with an artificially created hemin having hydrophobic moieties at the terminal of the heme-propionate side chains serves as an appropriate substrate-binding site near the heme pocket, and consequently enhances the peroxidase and peroxygenase activities for the reconstituted myoglobin. In addition, the incorporation of the synthetic hemin bearing modified heme-propionates into an appropriate apomyoglobin mutant drastically enhances the peroxidase activity. In contrast, to convert myoglobin into a cytochrome P450 enzyme, a flavin moiety as an electron transfer mediator was introduced at the terminal of the heme-propionate side chain. The flavomyoglobin catalyzes the deformylation of 2-phenylpropanal in the presence of NADH under aerobic conditions through the peroxoanion formation from the oxygenated species. In addition, modification of the heme-propionate side chains has an significant influence on regulating the reactivity of the horseradish peroxidase. Furthermore, the heme-propionate side chain can form a metal binding site with a carboxylate residue in the heme pocket. These studies indicate that modification of the heme-propionate side chains can be a new and effective way to engineer functions for the hemoproteins.

Electron transfer and oxidase activities in reconstituted hemoproteins with chemically modified cofactors

Protoheme IX is a typical iron porphyrin cofactor, showing a variety of reactivities in many hemoproteins under the reaction environments provided by protein matrices. Chemical modification of the protoheme cofactor is expected to be a versatile strategy to design hemoproteins possessing unique functions. This review focuses on the conversion of a hemoprotein, mainly myoglobin (an oxygen-storage hemoprotein), into a protein having different functions from the original ones by replacement of the protoheme cofactor with synthetic cofactors. The myoglobin having anionic patches pended to the heme propionates effectively binds electron-accepting proteins or small cationic organic molecules on the protein surface, resulting in enhanced efficiency of the photoinduced electron transfers from the myoglobin to these electron acceptors. Furthermore, the peroxidase and peroxygenase activities are also enhanced due to the facile substrate accesses. The attachment of the chemically active moiety such as flavin at the heme terminal is also important to give P450-like function to the native myoglobin. The employment of a structural isomer of porphyrin as an artificial cofactor gives rise to remarkably high dioxygen affinity and peroxidase activity in myoglobin, and allows us to easily detect high-valent species of the porphyrin isomer in HRP. These examples provide a clear insight into hemoprotein modifications based on synthetic chemistry as well as genetic amino acid mutations.

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.

Tuning of peroxidase activity by covalently tethered DNA oligonucleotides

We report on the modulation of the peroxidase activity of hybrid catalysts, comprising short DNA oligonucleotides and heme enzymes by means of sequence variation of tethered oligonucleotides. In particular, binary mixtures of native heme (protophorphyrin IX) and single-stranded DNA oligonucleotides as well as the analogous covalent heme-oligonucleotide conjugates were compared with DNA-enzyme conjugates, prepared by reconstitution of apo-myoglobin or apo-horseradisch peroxidase, using the aforementioned covalent heme-oligonucleotide conjugates. In all systems, it was clearly evident that the implemented oligonucleotides markedly influence the catalytic activity in a sequence-dependent matter. Greater than 100-fold changes in catalytic constants were observed, depending on which oligonucleotide was incorporated in the hybrid catalyst. We also observed that the tethered oligomers affect the inhibition of HRP-mediated peroxidation by means of small molecule inhibitors. On the basis of the quantitative description of this phenomenon and consideration of the current state of knowledge, we hypothesize that distinct interactions, such as hydrogen bonding or electrostatic contacts, occur between the oligonucleotides and the heme-containing catalyst, which account for the effects observed.

Computational design and elaboration of a de novo heterotetrameric alpha-helical protein that selectively binds an emissive abiological (porphinato)zinc chromophore

The first example of a computationally de novo designed protein that binds an emissive abiological chromophore is presented, in which a sophisticated level of cofactor discrimination is pre-engineered. This heterotetrameric, C(2)-symmetric bundle, A(His):B(Thr), uniquely binds (5,15-di[(4-carboxymethyleneoxy)phenyl]porphinato)zinc [(DPP)Zn] via histidine coordination and complementary noncovalent interactions. The A(2)B(2) heterotetrameric protein reflects ligand-directed elements of both positive and negative design, including hydrogen bonds to second-shell ligands. Experimental support for the appropriate formulation of [(DPP)Zn:A(His):B(Thr)](2) is provided by UV/visible and circular dichroism spectroscopies, size exclusion chromatography, and analytical ultracentrifugation. Time-resolved transient absorption and fluorescence spectroscopic data reveal classic excited-state singlet and triplet PZn photophysics for the A(His):B(Thr):(DPP)Zn protein (k(fluorescence) = 4 x 10(8) s(-1); tau(triplet) = 5 ms). The A(2)B(2) apoprotein has immeasurably low binding affinities for related [porphinato]metal chromophores that include a (DPP)Fe(III) cofactor and the zinc metal ion hemin derivative [(PPIX)Zn], underscoring the exquisite active-site binding discrimination realized in this computationally designed protein. Importantly, elements of design in the A(His):B(Thr) protein ensure that interactions within the tetra-alpha-helical bundle are such that only the heterotetramer is stable in solution; corresponding homomeric bundles present unfavorable ligand-binding environments and thus preclude protein structural rearrangements that could lead to binding of (porphinato)iron cofactors.

Apoenzyme reconstitution as a chemical tool for structural enzymology and biotechnology

Many enzymes contain a nondiffusible organic cofactor, often termed a prosthetic group, which is located in the active site and essential for the catalytic activity of the enzyme. These cofactors can often be extracted from the protein to yield the respective apoenzyme, which can subsequently be reconstituted with an artificial analogue of the native cofactor. Nowadays a large variety of synthetic cofactors can be used for the reconstitution of apoenzymes and, thus, generate novel semisynthetic enzymes. This approach has been refined over the past decades to become a versatile tool of structural enzymology to elucidate structure-function relationships of enzymes. Moreover, the reconstitution of apoenzymes can also be used to generate enzymes possessing enhanced or even entirely new functionality. This Review gives an overview on historical developments and the current state-of-the-art on apoenzyme reconstitution.

Metal ion-dependent fluorescent dynamics of photoexcited zinc-porphyrin and zinc-myoglobin modified with ethylenediaminetetraacetic acid

The reconstituted zinc-myoglobin (ZnMb) dyads, ZnMb-[M(II)(edta)], have been prepared by incorporating a zinc-porphyrin (ZnP) cofactor modified with ethylenediaminetetraacetic acid (H(4)edta) into apo-Mb. In case of the monomeric ZnP(edta) cofactor coordinated by one pyridine molecule, ZnP(py)(edta), a spontaneous 1:1 complex with a transient metal ion was formed in an aqueous solvent, and the photoexcited singlet state of ZnP, (1)(ZnP)*, was quenched by the [Cu(II)(edta)] moiety through intramolecular photoinduced electron-transfer (ET) reaction. The rate constant for the intramolecular quenching ET (k(q)) at 25 degrees C was successfully obtained as k(q) = 5.1 x 10(9) s(-1). In the case of Co(2+), Ni(2+), and Mn(2+), intersystem crossing by paramagnetic effect was mainly considered between (1)(ZnP)* and the [M(II)(edta)] complex. For the ZnMb-[M(II)(edta)] systems, the intramolecular ET reaction between the excited singlet state of (1)(ZnMb)* and the [Cu(II)(edta)] moieties provided the slower quenching rate constant, k(q) = 2.1 x 10(8) s(-1), compared with that of the ZnP(py)(edta) one. Kinetic studies also presented the efficient fluorescence quenching of the (1)(ZnMb)*-[Co(II)(edta)] dyad. Our study clearly demonstrates that wrapping of the ZnP cofactor by the apoprotein matrix and synthetic manipulation at the Mb surface ensure metal ion-sensitive fluorescent dynamics of ZnMb and provides valuable information to elucidate the complicated mechanism of the biological photoinduced ET reactions of hemoproteins.

Thermostability, solvent tolerance, catalytic activity and conformation of cofactor modified horseradish peroxidase

Artificial prosthetic groups, HeminD1 and HeminD2, were designed and synthesized, which contain one benzene ring and one carboxylic group or two carboxylic groups at the terminal of each propionate side chain of hemin, respectively. HeminD1 and HeminD2 were reconstituted with apo-HRP successfully to produce the two novel HRPs, rHRP1 and rHRP2, respectively. The thermal and solvent tolerances of native and reconstituted HRPs were compared. The cofactor modification increased the thermostability both in aqueous buffer and some organic solvents, and also enhanced the tolerance of some organic solvents. To determine the conformation stability, the unfolding of native and reconstituted HRPs by heat was investigated. Tm was increased from 70.0 degrees C of nHRP to 75.4 degrees C of rHRP1 and 76.5 degrees C of rHRP2 after cofactor modification. Kinetic studies indicated that the cofactor modification increased the substrate affinity and catalytic efficiency both in aqueous buffer and some organic solvents. The catalytic efficiency for phenol oxidation was increased by approximately 55% for rHRP1 in aqueous buffer, and it was also increased by approximately 70% for rHRP1 in 10% ACN. Spectroscopic studies proved that the cofactor modification changed the microenvironment of both heme and tryptophan, increased alpha-helix content, and increased the tertiary structure around the aromatic residue in HRP. The improvements of catalytic properties are related to these changes of the conformation. The introduction of the hydrophobic domain as well as the retention of the moderate carboxylic group in active site is an efficient method to improve the thermodynamic and catalytic efficiency of HRP.

Crystal structure and peroxidase activity of myoglobin reconstituted with iron porphycene

The incorporation of an artificially created metal complex into an apomyoglobin is one of the attractive methods in a series of hemoprotein modifications. Single crystals of sperm whale myoglobin reconstituted with 13,16-dicarboxyethyl-2,7-diethyl-3,6,12,17-tetramethylporphycenatoiron(III) were obtained in the imidazole buffer, and the 3D structure with a 2.25-A resolution indicates that the iron porphycene, a structural isomer of hemin, is located in the normal position of the heme pocket. Furthermore, it was found that the reconstituted myoglobin catalyzed the H2O2-dependent oxidations of substrates such as guaiacol, thioanisole, and styrene. At pH 7.0 and 20 degrees C, the initial rate of the guaiacol oxidation is 11-fold faster than that observed for the native myoglobin. Moreover, the stopped-flow analysis of the reaction of the reconstituted protein with H2O2 suggested the formation of two reaction intermediates, compounds II- and III-like species, in the absence of a substrate. It is a rare example that compound III is formed via compound II in myoglobin chemistry. The enhancement of the peroxidase activity and the formation of the stable compound III in myoglobin with iron porphycene mainly arise from the strong coordination of the Fe-His93 bond.

Photoinduced Intramolecular Electron-Transfer Reactions of Reconstituted Met- and Zinc-Myoglobins Appending Acridine and Methylacridinium Ion as DNA-Binders

Three types of reconstituted met- and zinc-myoglobin (metMb and ZnMb) dyads, ZnMbAc(4)Me+, ZnMbAc(6)Me+, and metMbAc(6) have been prepared by incorporating chemically modified metalloporphyrin cofactor appending an acridine (Ac) or a methylacridinium ion ([AcMe]+) into apo-Mb. In the bimolecular system between ZnMb and [AcMe]+, the photoexcited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by [AcMe]+ to form the radical pair of ZnMb cation (ZnMb*+) and reduced methylacridine ([AcMe]*), followed by a thermal back ET reaction. The rate constants for the intermolecular quenching ET (kq) and the back ET reaction (kb) at 25 degrees C were successfully obtained as kq = (8.8 +/- 0.4) x 10(7) M(-1) s(-1) and kb = (1.2 +/- 0.1) x 10(8) M(-1) s(-1), respectively. On the other hand, in case of the intramolecular photoinduced ET reactions of ZnMbAc(4)Me+ and ZnMbAc(6)Me+ dyads, the first-order quenching rate constants (kET) of 3(ZnMb)* by [AcMe]+ moiety were determined to be kET = 2.6 x 10(3) and 2.5 x 10(3) s(-1), respectively. When such ET occurs along the alkyl spacer via through-bond mechanism at the surface of Mb, the obtained kET is reasonable to provide decay constant of beta (1.0-1.3 A(-1)). Upon photoirradiation of [AcMe]+ moiety, kinetic studies also presented the intramolecular quenching reactions from the excited singlet state, 1([AcMe]+)*, whose likely process is the photoinduced energy-transfer reaction. For metMbAc(6) dyad, steady-state fluorescence was almost quenched, while the signal around 440 nm gradually appeared in the presence of various concentrations of DNA. Our study implies that synthetic manipulation at the Mb surface, by using an artificial DNA-binder coupled with photoinduced reaction, may provide valuable information to construct new Mb-DNA complex and sensitive fluorescent for DNA.

Kinetic Analysis of Semisynthetic Peroxidase Enzymes Containing a Covalent DNA–Heme Adduct as the Cofactor

The reconstitution of apo enzymes with DNA oligonucleotide-modified heme (protoporphyrin IX) cofactors has been employed as a tool to produce artificial enzymes that can be specifically immobilized at the solid surfaces. To this end, covalent heme-DNA adducts were synthesized and subsequently used in the reconstitution of apo myoglobin (aMb) and apo horseradish peroxidase (aHRP). The reconstitution produced catalytically active enzymes that contained one or two DNA oligomers coupled to the enzyme in the close proximity to the active site. Kinetic studies of these DNA-enzyme conjugates, carried out with two substrates, ABTS and Amplex Red, showed a remarkable increase in peroxidase activity of the DNA-Mb enzymes while a decrease in enzymatic activity was observed for the DNA-HRP enzymes. All DNA-enzyme conjugates were capable of specific binding to a solid support containing complementary DNA oligomers as capture probes. Kinetic analysis of the enzymes immobilized by the DNA-directed immobilization method revealed that the enzymes remained active after hybridization to the capture oligomers. The programmable binding properties enabled by DNA hybridization make such semisynthetic enzyme conjugates useful for a broad range of applications, particularly in biocatalysis, electrochemical sensing, and as building blocks for biomaterials.

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.

Aggregation Behavior of Giant Amphiphiles Prepared by Cofactor Reconstitution

We report on biohybrid surfactants, termed "giant amphiphiles", in which a protein or an enzyme acts as the polar head group and a synthetic polymer as the apolar tail. It is demonstrated that the modification of horseradish peroxidase (HRP) and myoglobin (Mb) with an apolar polymer chain through the cofactor reconstitution method yields giant amphiphiles that form spherical aggregates (vesicles) in aqueous solution. Both HRP and Mb retain their original functionality when modified with a single polystyrene chain, but reconstitution has an effect on their activities. In the case of HRP the enzymatic activity decreases and for Mb the stability of the dioxygen myoglobin (oxy-Mb) complex is reduced, which is probably the result of a disturbed binding of the heme in the apo-protein or a reduced access of the substrate to the active site of the enzyme or protein.

pH-dependent redox and CO binding properties of chelated protoheme-l-histidine and protoheme-glycyl-l-histidine complexes

The pH dependence of redox properties, spectroscopic features and CO binding kinetics for the chelated protohemin-6(7)-L-histidine methyl ester (heme-H) and the chelated protohemin-6(7)-glycyl-L-histidine methyl ester (heme-GH) systems has been investigated between pH 2.0 and 12.0. The two heme systems appear to be modulated by four protonating groups, tentatively identified as coordinated H(2)O, one of heme's propionates, N(epsilon) of the coordinating imidazole, and the carboxylate of the histidine residue upon hydrolysis of the methyl ester group (in acid medium). The pK (a) values are different for the two hemes, thus reflecting structural differences. In particular, the different strain at the Fe-N(epsilon) bond, related to the different length of the coordinating arm, results in a dramatic alteration of the bond strength, which is much smaller in heme-H than in heme-GH. It leads to a variation in the variation of the pKa for the protonation of the N(epsilon) of the axial imidazole as well as in the proton-linked behavior of the other protonating groups, envisaging a cross-talk communication mechanism among different groups of the heme, which can be operative and relevant also in the presence of the protein matrix.

Immobilization of myoglobin on phosphate and phosphonate grafted-zirconia nanoparticles

We investigated the adsorption and catalytic activity of myoglobin (Mb) immobilized on colloidal particles of zirconia covalently grafted with phosphoric (ZrO2-P) and benzenephosphonic acid (ZrO2-BP). The maximum adsorption was reached after 1 h of contact and was greater on a hydrophilic support, ZrO2-P, compared to a hydrophobic support, ZrO2-BP. The equilibrium isotherms fitted the Langmuir equation, suggesting the presence of a monolayer of protein molecules on the surface of the nanoparticles. The nanostructured biocomposites are active in the oxidation of 2-methoxyphenol (guaiacol) by hydrogen peroxide. The oxidation catalyzed by immobilized Mb followed a Michaelis-Menten kinetics, similar to that observed in the oxidation by free Mb. Furthermore, the catalytic efficiency is similar to that of free Mb and higher than that of "large-size" biocatalysts (with sizes larger than 1 mum). In the latter case, the kinetic parameters, k(cat) and K(M), indicate that this is mostly due to an increased affinity of the nano-biocomposite for the substrate. The activity of the nano-biocomposites decreases slightly as the amount of adsorbed protein increases. This is mainly due to the formation of a nonordered monolayer, which reduces the accessibility of the substrate to the active center.

Coordinated Design of Cofactor and Active Site Structures in Development of New Protein Catalysts

New methods for the synthesis of artificial metalloenzymes are important for the construction of novel biocatalysts and biomaterials. Recently, we reported new methodology for the synthesis of artificial metalloenzymes by reconstituting apo-myoglobin with metal complexes (Ohashi, M. et al., Angew Chem., Int. Ed. 2003, 42, 1005-1008). However, it has been difficult to improve their reactivity, since their crystal structures were not available. In this article, we report the crystal structures of M(III)(Schiff base).apo-A71GMbs (M = Cr and Mn). The structures suggest that the position of the metal complex in apo-Mb is regulated by (i) noncovalent interaction between the ligand and surrounding peptides and (ii) the ligation of the metal ion to proximal histidine (His93). In addition, it is proposed that specific interactions of Ile107 with 3- and 3'-substituent groups on the salen ligand control the location of the Schiff base ligand in the active site. On the basis of these results, we have successfully controlled the enantioselectivity in the sulfoxidation of thioanisole by changing the size of substituents at the 3 and 3' positions. This is the first example of an enantioselective enzymatic reaction regulated by the design of metal complex in the protein active site.

Catalytic activity, stability, unfolding, and degradation pathways of engineered and reconstituted myoglobins

The structural and functional consequences of engineering a positively charged Lys residue and replacing the natural heme with a heme-L-His derivative in the active site of sperm whale myoglobin (Mb) have been investigated. The main structural change caused by the distal T67K mutation appears to be mobilization of the propionate-7 group. Reconstitution of wild-type and T67K Mb with heme-L-His relaxes the protein fragment around the heme because it involves the loss of the interaction of one of the propionate groups which stabilize heme binding to the protein. This modification increases the accessibility of exogenous ligands or substrates to the active site. The catalytic activity of the reconstituted proteins in peroxidase-type reactions is thus significantly increased, particularly with T67K Mb. The T67K mutation slightly reduces the thermodynamic stability and the chemical stability of Mb during catalysis, but somewhat more marked effects are observed by cofactor reconstitution. Hydrogen peroxide, in fact, induces pseudo-peroxidase activity but also promotes oxidative damage of the protein. The mechanism of protein degradation involves two pathways, which depend on the evolution of radical species generated on protein residues by the Mb active species and on the reactivity of phenoxy radicals produced during turnover. Both protein oligomers and heme-protein cross-links have been detected upon inactivation.

Catalytic activity of myoglobin immobilized on zirconium phosphonates

The adsorption and catalytic activity of myoglobin (Mb) on zirconium phosphonates (a-zirconium benzenephosphonate (alpha-ZrBP), a-zirconium carboxyethanephosphonate (alpha-ZrCEP), and a novel layered zirconium fluoride aminooctyl-N,N-bis(methylphosphonate) (ZrC8)) were investigated. The maximum adsorption was reached after 16 h of contact and was greater on hydrophobic supports such as alpha-ZrBP and ZrC8 compared to hydrophilic supports such as alpha-ZrCEP. The equilibrium adsorption isotherms fitted the Langmuir equation, suggesting the presence of a monolayer of protein molecules on the support surfaces. The catalytic activities of free Mb and of the obtained biocomposites were studied in terms of the oxidation of two aromatic substrates, o-phenylenediamine and 2-methoxyphenol (guaiacol), by hydrogen peroxide. The oxidation catalyzed by immobilized myoglobin followed the Michaelis-Menten kinetics, similar to oxidation by free Mb. The kinetic parameters, kcat and KM, were significantly affected by the adsorption process. Mb/alpha-ZrCEP was the most efficient biocatalyst obtained, probably because of the hydrophilic nature of the support. The effect of immobilization on the stability of Mb toward inactivation by hydrogen peroxide was also investigated, and an increased resistance was found. The biocomposites obtained can be stored at 4 degrees C for months without a significant loss of catalytic activity.

Mechanistic insight into the peroxidase catalyzed nitration of tyrosine derivatives by nitrite and hydrogen peroxide

Peroxidases perform the nitration of tyrosine and tyrosyl residues in proteins, in the presence of nitrite and hydrogen peroxide. The nitrating species is still unknown but it is usually assumed to be nitrogen dioxide. In the present investigation, the nitration of phenolic compounds derived from tyrosine by lactoperoxidase and horseradish peroxidase was studied, with the aim of elucidating the mechanism of the reaction. The results indicate that nitrogen dioxide cannot be the only nitrating species and suggest the presence of two simultaneously operative pathways, one proceeding through enzyme-generated nitrogen dioxide and another through a more reactive species, assumed to be complexed peroxynitrite, which is generated by reaction of hydrogen peroxide with the enzyme-nitrite complex. The importance of the two pathways depends on peroxide and nitrite concentrations. With lactoperoxidase, nitration through the highly reactive intermediate is preferred except at very low nitrite concentration, while with horseradish peroxidase, the nitrogen dioxide driven mechanism is preferred except at very high nitrite concentration. The preferred mechanism for the two enzymes is that operative in the physiological nitrite concentration range.

Engineering peroxidase activity in myoglobin: the haem cavity structure and peroxide activation in the T67R/S92D mutant and its derivative reconstituted with protohaemin-l-histidine

Atomic co-ordinates and structure factors for the T67R/S92D metMbCN mutant have been deposited with the Protein Data Bank, under accession codes 1h1x and r1h1xsf, respectively. Protein engineering and cofactor replacement have been employed as tools to introduce/modulate peroxidase activity in sperm whale Mb (myoglobin). Based on the rationale that haem peroxidase active sites are characterized by specific charged residues, the Mb haem crevice has been modified to host a haem-distalpropionate Arg residue and a proximal Asp, yielding the T67R/S92D Mb mutant. To code extra conformational mobility around the haem, and to increase the peroxidase catalytic efficiency, the T67R/S92D Mb mutant has been subsequently reconstituted with protohaem-L-histidine methyl ester, yielding a stable derivative, T67R/S92D Mb-H. The crystal structure of T67R/S92D cyano-metMb (1.4 A resolution; R factor, 0.12) highlights a regular haem-cyanide binding mode, and the role for the mutated residues in affecting the haem propionates as well as the neighbouring water structure. The conformational disorder of the haem propionate-7 is evidenced by the NMR spectrum of the mutant. Ligand-binding studies show that the iron(III) centres of T67R/S92D Mb, and especially of T67R/S92D Mb-H, exhibit higher affinity for azide and imidazole than wild-type Mb. In addition, both protein derivatives react faster than wild-type Mb with hydrogen peroxide, showing higher peroxidase-like activity towards phenolic substrates. The catalytic efficiency of T67R/S92D Mb-H in these reactions is the highest so far reported for Mb derivatives. A model for the protein-substrate interaction is deduced based on the crystal structure and on the NMR spectra of protein-phenol complexes.

Hybridization of Modified-Heme Reconstitution and Distal Histidine Mutation to Functionalize Sperm Whale Myoglobin

To modulate the physiological function of a hemoprotein, most approaches have been demonstrated by site-directed mutagenesis. Replacement of the native heme with an artificial prosthetic group is another way to modify a hemoprotein. However, an alternate method, mutation or heme reconstitution, does not always demonstrate sufficient improvement compared with the native heme enzyme. In the present study, to convert a simple oxygen storage hemoprotein, myoglobin, into an active peroxidase, we applied both methods at the same time. The native heme of myoglobin was replaced with a chemically modified heme 2 having two aromatic rings at the heme-propionate termini. The constructed myoglobins were examined for 2-methoxyphenol (guaiacol) oxidation in the presence of H2O2. Compared with native myoglobin, rMb(H64D.2) showed a 430-fold higher kcat/Km value, which is significantly higher than that of cytochrome c peroxidase and only 3-fold less than that of horseradish peroxidase. In addition, myoglobin-catalyzed degradation of bisphenol A was examined by HPLC analysis. The rMb(H64D.2) showed drastic acceleration (>35-fold) of bisphenol A degradation compared with the native myoglobin. In this system, a highly oxidized heme reactive species is smoothly generated and a substrate is effectively bound in the heme pocket, while native myoglobin only reversibly binds dioxygen. The present results indicate that the combination of a modified-heme reconstitution and an amino acid mutation should offer interesting perspectives toward developing a useful biomolecule catalyst from a hemoprotein.

Pseudoperoxidase activity of myoglobin: kinetics and mechanism of the peroxidase cycle of myoglobin with H2O2 and 2,2-azino-bis(3-ethylbenzthiazoline-6-sulfonate) as substrates

Using 2,2-azino-bis(3-ethylbenzthiazoline-6-sulfonate) (ABTS) as substrate, it has been shown that the increased peroxidase activity for decreasing pH of myoglobin activated by hydrogen peroxide is due to a protonization of ferrylmyoglobin, MbFe(IV)=O, facilitating electron transfer from the substrate and corresponding to pK(a) approximately 5.2 at 25.0 degrees C and ionic strength 0.16, rather than due to specific acid catalysis. On the basis of stopped flow absorption spectroscopy with detection of the radical cation ABTS(.+), the second-order rate constant and activation parameters for the reaction between MbFe(IV)=O and ABTS were found to have the values k = 698 +/- 32 M(-1) s(-1), DeltaH# = 66 +/- 4 kJ mol(-1), and DeltaS# = 30 +/- 15 J mol(-1) K(-1) at 25.0 degrees C and physiological pH (7.4) and ionic strength (= 0.16 M NaCl). At a lower pH (5.8) corresponding to the conditions in meat, values were found as follows: k = 3.5 +/- 0.3 x 10(4) M(-1) s(-1), DeltaH# = 31 +/- 6 kJ mol(-1), and DeltaS# = -53 +/- 19 J mol(-1) K(-1), indicative of a shift from outersphere electron transfer to an innersphere mechanism. For steady state assay conditions, this shift is paralleled by a shift from saturation kinetics at pH 7.4 to first-order kinetics for H2O2 as substrate at pH 5.8. In contrast, the activation reaction between myoglobin and hydrogen peroxide was found at 25.0 degrees C to be slow and independent of pH with values of 171 +/- 7 and 196 +/- 19 M(-1) s(-1) found at physiological and meat pH, respectively, as determined by sequential stopped flow spectroscopy, from which a lower limit of k = 6 x 10(5) M(-1) s(-1) for the reaction between perferrylmyoglobin, .MbFe(IV)=O, and ABTS could be estimated. As compared to the traditional peroxidase assay, a better characterization of pseudoperoxidase activity of heme pigments and their denatured or proteolyzed forms is thus becoming possible, and specific kinetic effects on activation, substrate oxidation, or shift in rate determining steps may be detected.

New functionalization of myoglobin by chemical modification of heme-propionates

The reconstitution of myoglobin with an artificially created prosthetic group is a unique method for introducing a new chemical function into the protein. Particularly, the modification of two heme-propionates gives us an effective binding domain or binding site on the protein surface. This Account traces the design and construction of the highly ordered binding domain around the entrance of the heme pocket. The discussion includes the protein-small molecule or protein-protein recognition, electron transfer reaction within the complex, and enhancement of the chemical reactivity of the myoglobin with a substrate binding site. The synthetic approach to modifying a protein will be a new trend in engineering a novel function in naturally occurring hemoprotein.

Enzymatic properties of human hemalbumin

The binding of hemin to the primary site of human serum albumin (HSA) has been reinvestigated using UV-Vis, CD and NMR techniques. The major fraction of bound hemin contains a five-coordinated high-spin iron(III) center, but a minor fraction of the metal appears to be in a six-coordinated, low-spin state, where a 'distal' residue, possibly a second histidine residue, completes the coordination sphere. The reduced, iron(II) form of the adduct contains six-coordinated low-spin heme. The distal residue hinders the access to the iron(III) center of hemin-HSA to small anionic ligands like azide and cyanide and destabilizes the binding of neutral diatomics like dioxygen and carbon monoxide to the iron(II) form. In spite of these limitations, the hemin-HSA complex promotes hydrogen peroxide activation processes that bear the characteristics of enzymatic reactions and may have biological relevance. The complex is in fact capable of catalyzing peroxidative reactions on phenolic compounds related to tyrosine and hydrogen peroxide dismutation. Kinetic and mechanistic studies confirm that the low efficiency with which peroxidative processes occur depends on the limited rate of the reaction between hydrogen peroxide and the iron(III) center, to form the active species, and by the competitive peroxide degradation reaction.