Blue myoglobin reconstituted with an iron porphycene shows extremely high oxygen affinity

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.

Reaction of cobalt porphycene with hydride reagents: spectroscopic detection of Co–H porphycene species and formation of Co–SnR3 porphycene species

The reaction of tetrapropylporphycenatocobalt(III) with tributyltin hydride generates a cobalt(III)–hydride porphycene detectable by UV-vis spectroscopy under diluted conditions, whereas it is impossible to characterize hydride species of cobalt porphyrins. One of the reasons for the stability of the porphycene hydride species is that the porphycene ring has a lower LUMO energy level due to the decrease in the symmetry of the ligand character. However, the hydride species in a highly concentrated solution of the complex is easily converted into the cobalt(II) species via dimerization or reaction of the hydride with excess tributyltin hydride through hemolysis of the Sn–H/Co–H bonds. When the Co(III) porphycene is reacted with LiBHEt3 , the final product is the cobalt(III)–ethyl complex formed by β-rearrangement during the reaction of the hydride species and diethylborane in a solvent cage. In the reaction of tetrakistrifluoromethylporphycenatocobalt(III) with tributyltin hydride, the dominant reaction pathway includes one-electron reduction of the porphycene ring together with radical coupling of the tin reagent rather than the net hydride transfer. This finding suggests that the delicate control of the LUMO energy level influences the stability of the hydride species. The tetrapropylporphycenatocobalt(III) complex with tributyltin or triphenyltin hydride in the presence of AIBN produces the corresponding Co(III)– trialkyltin complex. This complex was characterized by 1H NMR spectroscopy.

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.

Photophysical and DNA-binding properties of cytochrome c modified with a platinum(II) complex

Cytochrome c (cyt c) derivatives modified with a platinum(II) complex at the lysine residue, cyt c(III)-[Pt(bpy)(dapap)](1) {bpy = 2,2'-bipyridine, and dapap = 3-(2,3-diaminopropionylamino)propionic acid}, have been prepared. The modified residues are Lys8, Lys13, Lys55, Lys60, Lys73, and Lys88. In the case of the cyt c(III)-[Pt(bpy)(dapap)](1) dyad, the photoexcited singlet state of (1)([Pt(bpy)(dapap)](1))* was quenched by the heme Fe(III) moiety through the intramolecular photoinduced energy-transfer reaction via a through-space mechanism. Next, in the presence of calf thymus (CT)-DNA, the DNA-responsive fluorescence properties of cyt c(III)-[Pt(bpy)(dapap)](1) isomers were investigated. The order of the obtained binding constants between the cyt c(III)-[Pt(bpy)(dapap)](1) isomer and CT-DNA in an aqueous solution suggested that the electrostatic interaction is one of the important factors to stabilize the cyt c-DNA complex. Finally, we discussed the rotational motion of the [Pt(bpy)(dapap)](2+) moiety at the surface of cyt c by fluorescence anisotropy measurement. The increase in the anisotropy parameter, r, for each cyt c isomer clearly revealed that the noncovalent recognition of the [Pt(bpy)(dapap)](2+) moiety by CT-DNA is an essential event in the formation of the cyt c-DNA complex and generation of DNA-sensitive fluorescence signals.

Palladium(0) catalyzed 2,2'-bipyrrole syntheses

Several 2-iodopyrroles are used in Pd(0) catalyzed homocoupling reactions at room temperature in the presence of water to efficiently synthesize 2,2'-bipyrroles. These 2,2'-bipyrroles are strongly luminous materials and have high fluorescence quantum yields.

Porphycene-mediated photooxidation of benzylamines by visible light

A variety of primary and secondary benzylic amines were oxidized efficiently to N-benzylidenebenzylamines and imines, respectively, using 2,7,12,17-tetrapropylporphycene (H(2)TPrPc) photocatalyst and blue light emitting diodes (LEDs). The photooxidation of 4-methoxybenzylamine in the presence of H(2)TPrPc and its tin(IV) complex Sn(TPrPc)Cl(2) was studied in detail in order to show that operating mechanisms can be different depending on the photosensitizer type. Two experiments involving solvent deuterium isotope effect and competitive quenching with DABCO provide evidence for the singlet oxygen mechanism as the major pathway in the H(2)TPrPc-catalyzed reaction and the predominance of the direct electron transfer from the photoexcited dye to the amine when the Sn(TPrPc)Cl(2) complex was used as a photocatalyst.

Syntheses and photophysical behavior of porphyrin isomer Sn(IV) complexes

2,3,6,7,12,13,16,17-Octaethylhemiporphycenato Sn(IV) chloride, [Sn(IV)(OEHPc)Cl(2)], and 2,3,6,7,12,13,16,17-octaethylporphycenato Sn(IV) chloride, [Sn(IV)(OEPc)Cl(2)], were synthesized in high yields and fully characterized by various spectroscopic methods. The X-ray crystal structures of the Sn(IV) complexes with porphycene and hemiporphycene were determined for the first time. The photophysical and photochemical properties of the singlet state of the Sn(IV) porphycene and hemiporphycene complexes, structural isomers of porphyrin, have been investigated by fluorescence and phosphorescence spectroscopies. The relatively strong emission and long fluorescence lifetime of the Sn(IV) porphycene complexes indicated by the fluorescence quantum yield and lifetime measurement were observed in the case of the Sn(IV) porphycene ([Sn(IV)(OEPc)Cl(2)], Phi(F) = 0.125, tau(s) = 2681 ps; [Sn(IV)(OEP)Cl(2)], Phi(F) = 0.010, tau(s) = 438 ps; [Sn(IV)(OEHPc)Cl(2)], Phi(F) = 0.027, tau(s) = 733 ps). The triplet state of the Sn(IV) complexes was investigated by transient absorption spectroscopy. It became clear that the triplet lifetime of the Sn(IV) porphycene (tau(T) = 49.9 mus) was longer when compared to those of the Sn(IV) porphyrin (tau(T) = 32.6 mus) and hemiporphycene complexes (tau(T) = 28.3 mus). These porphyrin isomer Sn(IV) complexes showed the high singlet oxygen generating ability, and the photo-oxidation of the 1,5-hydroxynaphthalene mediated by the Sn(IV) porphycene was the most effective among the complexes. This result is due to its more effective light absorption in the visible region and indicated that the porphycene is an excellent candidate as a photosensitizer.

Addressable DNA-myoglobin photocatalysis

Photoactivatable myoglobin containing a DNA oligonucleotide as a structural anchor was designed by using the reconstitution of artificial heme moieties containing Ru(3+) ions. This semisynthetic DNA-enzyme conjugate was successfully used for the oxidation of peroxidase substrates by using visible light instead of H(2)O(2) for the activation. The DNA anchor was utilized for the immobilization of the enzyme on the surface of magnetic microbeads. Enzyme activity measurements not only indicated undisturbed biofunctionality of the tethered DNA but also enabled magnetic separation-based enrichment and recycling of the photoactivatable biocatalyst.

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.

Analysis of heme-reconstitution of apoenzymes by means of surface plasmon resonance

Surface plasmon resonance was used to determine the kinetic parameters for heme reconstitution of apoenzymes.

Photophysical and photosensitizing properties of brominated porphycenes

A heavy atom, bromine, was directly substituted into the porphycene macrocycle to promote intersystem crossing by way of spin-orbit coupling. The singlet oxygen production ability of the porphycene is dramatically enhanced, and the highest value of 0.95 for the quantum yield of singlet oxygen generation (PhiDelta) was obtained for the dibrominated porphycene by visible light excitation.

Effect of peripheral trifluoromethyl groups in artificial iron porphycene cofactor on ligand binding properties of myoglobin

An iron porphycene, a structural isomer of iron porphyrin, with trifluoromethyl groups at the peripheral position of the framework was incorporated into sperm whale apomyoglobin. The prepared myoglobin shows the higher O(2) affinity than the native protein. However, the oxygen affinity of the reconstituted myoglobin is lower than that of the myoglobin having an iron porphycene without trifluoromethyl groups, which is mainly originated from the enhancement of the O(2) dissociation. The CO affinity of the myoglobin with the trifluoromethylated iron porphycene is similar to that observed for the reference protein having the iron porphycene without trifluoromethyl groups, although their C-O stretching frequencies are significantly different. The relationship between the electronic states of the porphycene ring and the ligand bindings is discussed.

Improved Synthesis of Functionalized 2,2‘-Bipyrroles

A series of 2,2'-bipyrroles has been efficiently synthesized using an improved synthetic approach based on Pd(0)-catalyzed homocoupling of various 2-iodopyrroles. This new synthetic approach takes place at room temperature and in the presence of water. Functional groups such as formyl, ester, and nitrile are able to survive these reaction conditions. Solvents are found to play an important role in this reaction.

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.

Biosynthetic Inorganic Chemistry

Inorganic chemistry and biology can benefit greatly from each other. Although synthetic and physical inorganic chemistry have been greatly successful in clarifying the role of metal ions in biological systems, the time may now be right to utilize biological systems to advance coordination chemistry. One such example is the use of small, stable, easy-to-make, and well-characterized proteins as ligands to synthesize novel inorganic compounds. This biosynthetic inorganic chemistry is possible thanks to a number of developments in biology. This review summarizes the progress in the synthesis of close models of complex metalloproteins, followed by a description of recent advances in using the approach for making novel compounds that are unprecedented in either inorganic chemistry or biology. The focus is mainly on synthetic "tricks" learned from biology, as well as novel structures and insights obtained. The advantages and disadvantages of this biosynthetic approach are discussed.

Ligand binding properties of two kinds of reconstituted myoglobins with iron porphycene having propionates: Effect of β-pyrrolic position of two propionate side chains in porphycene framework

An iron porphycene containing two propionate side chains at the 12th and 17th beta-pyrrolic positions of the porphycene ring was synthesized and incorporated into sperm whale apomyoglobin in order to investigate the O(2) and CO binding properties of the reconstituted ferrous myoglobin. The protein showed a slower O(2) dissociation rate by 1/20, compared to the native myoglobin, whereas the CO dissociation rates were found to be almost the same. This tendency is similar to the result of a previous study on the reconstituted myoglobin with a porphycene having the propionates at the 13th and 16th beta-pyrrolic positions. However, the present myoglobin showed a faster O(2) dissociation than the previously studied myoglobin. This finding suggests that the position of the two propionates as well as the symmetry of the porphycene framework is an important factor for obtaining a stable oxygenated iron porphycene myoglobin.

On the O2 binding of Fe–porphyrin, Fe–porphycene, and Fe–corrphycene complexes

Based on our previous study for the O2 binding of the Fe-Por complex, this study investigates the O2 binding mechanism in the Fe-porphyrin isomers, Fe-porphycene (FePc), and Fe-corrphycene (FeCor) complexes. By calculating the potential energy surface of the O2 binding, the present study explains the reason for the dramatic increase of O2 affinities observed in the FePc complex. In the case of FeCor-O2, the O2 binding process includes the intersystem crossing from a triplet to singlet state, as in the FePor-O2 complex. However, FePc-O2 uses only a singlet surface. This is because the ground state of the FePc complex in the deoxy state is a triplet state, while those of FePor and FeCor are a quintet state. Such difference originates from character of the SOMO. We estimated an equilibrium constant for the O2 binding that reasonably reproduced the trend observed in the experiments.

Electronic reorganization: Origin of sigma trans promotion effect

Binding of two ligands trans to each other by some transition metal complexes may be cooperative [Khoroshun et al., Mol Phys 2002, 100, 523]. Several interesting consequent effects include (i) inverse relationship between bond strength and binding affinity; (ii) smaller coordination barriers to formation of weaker bonds; (iii) enhancement of Lewis acidity with increased number of ligands. We describe a simple model, sigma trans promotion effect (TPE), which considers electronic reorganization between two Lewis structures, and predicts the above-mentioned effects. The applied result of present study is the unified perspective on several facts of heme chemistry. Particularly, we reiterate an important but often overlooked notion, developed previously within the spin pairing model [Drago and Corden, Acc Chem Res 1980, 13, 353], that, in hemoproteins, the proximal histidine and the distal ligand such as O2 or CO cooperate in promoting electronic reorganization. As a result, depopulation of dz2 orbital upon ligand binding contributes to the phenomenon of hemoglobin cooperativity. The presented density functional (B3LYP) calculations on realistic models, the processes of carbon monoxide binding by Fe(II) porphyrins and dinitrogen binding by triamido/triamidoamine Mo(III) complexes, particularly the evaluation of the coordination barriers due to spin-state change by location of the minima on seams of crossing, support the TPE model predictions. From a broader theoretical perspective, the present study would hopefully stimulate the development of much needed frameworks and tools for facile comparisons of wave functions and their properties between different geometries, species, and electronic states. Advancement of practical wave function comparisons may yield fresh qualitative perspectives on chemical reactivity, and promote better understanding of related concepts such as electronic reorganization.

On the reversible O2 binding of the Fe–porphyrin complex

Electronic mechanism of the reversible O(2) binding by heme was studied by using Density Functional Theory calculations. The ground state of oxyheme was calculated to be open singlet state [Fe(S =1/2) + O(2)(S = 1/2)]. The potential energy surface for singlet state is associative, while that for triplet state is dissociative. Because the ground state of the O(2)+ deoxyheme system is triplet in the dissociation limit [Fe(S = 2) + O(2)(S = 1)], the O(2) binding process requires relativistic spin-orbit interaction to accomplish the intersystem crossing from triplet to singlet states. Owing to the singlet-triplet crossing, the activation energies for both O(2) binding and dissociation become moderate, and hence reversible. We also found that the deviation of the Fe atom from the porphyrin plane is also important reaction coordinate for O(2) binding. The potential surface is associative/dissociative when the Fe atom locates in-plane/out-of-plane.

Design and engineering of metalloproteins containing unnatural amino acids or non-native metal-containing cofactors

An emerging branch of metalloprotein design and engineering is on the horizon, where unnatural amino acids or non-native metal-containing cofactors are employed in the design and engineering process. These endeavors have been shown to be quite effective in elucidating the precise roles of key residues in protein structures and functions, in providing guiding principles on protein design, in fine-tuning the protein properties to an unprecedented level, and in expanding the repertoire of protein functionalities, and thus its range of applications.