Redox chemistry of cobalamin and its derivatives

Electrocatalytic Reduction of Disulfide Bonds across Chemical Modalities

The chemical properties of disulfides are leveraged in a wide array of applications, ranging from protein-drug conjugates for cancer treatment to self-healing materials. However, disulfide reduction strategies remain severely underdeveloped despite being the key to efficiently accessing the desired targets. Specifically, no homogeneous catalyst has been reported for this reaction, and conditions that allow the use of mild and green reductants (e.g., via electrochemical reduction) are not known. Herein, we unveil a vitamin B12-catalyzed, electrochemically driven protocol for efficiently reducing disulfide bonds in various aqueous buffers over a broad pH range. This robust and simple method is suitable for disulfide reductions of substrates ranging from small molecules to large proteins. Finally, one-pot reduction and conjugation of disulfide bonds in a monoclonal antibody were demonstrated to produce antibody conjugates.

Modulation of polyaniline memristive device switching voltage by nucleotide-free analogue of vitamin B12

Memristive devices offer essential properties to become a part of the next-generation computing systems based on neuromorphic principles. Organic memristive devices exhibit a unique set of properties which makes them an indispensable choice for specific applications, such as interfacing with biological systems. While the switching rate of organic devices can be easily adjusted over a wide range through various methods, controlling the switching potential is often more challenging, as this parameter is intricately tied to the materials used. Given the limited options in the selection conductive polymers and the complexity of polymer chemical engineering, the most straightforward and accessible approach to modulate switching potentials is by introducing specific molecules into the electrolyte solution. In our study, we show polyaniline (PANI)-based device switching potential control by adding nucleotide-free analogue of vitamin B12, aquacyanocobinamide, to the electrolyte solution. The employed concentrations of this molecule, ranging from 0.2 to 2 mM, enabled organic memristive devices to achieve switching potential decrease for up to 100 mV, thus providing a way to control device properties. This effect is attributed to strong aromatic interactions between PANI phenyl groups and corrin macrocycle of the aquacyanocobinamide molecule, which was supported by ultraviolet-visible spectra analysis.

Non-native Intramolecular Radical Cyclization Catalyzed by a B12 -Dependent Enzyme

Despite the unique reactivity of vitamin B12 and its derivatives, B12 -dependent enzymes remain underutilized in biocatalysis. In this study, we repurposed the B12 -dependent transcription factor CarH to enable non-native radical cyclization reactions. An engineered variant of this enzyme, CarH*, catalyzes the formation γ- and δ-lactams through either redox-neutral or reductive ring closure with marked enhancement of reactivity and selectivity relative to the free B12 cofactor. CarH* also catalyzes an unusual spirocyclization by dearomatization of pendant arenes to produce bicyclic 1,3-diene products instead of 1,4-dienes provided by existing methods. These results and associated mechanistic studies highlight the importance of protein scaffolds for controlling the reactivity of B12 and expanding the synthetic utility of B12 -dependent enzymes.

High Reactivity of Supermolecular Nanoentities of a Vitamin B12 Derivative in Langmuir-Schaefer Films Toward Gaseous Toxins

Recently, we have described the first supermolecular nanoentities (SMEs) of a vitamin B12 derivative, viz., a monocyano form of heptabutyl cobyrinate ((CN-)BuCby), unique nanoparticles with strong noncovalent intermolecular interactions, and emerging optical and redox properties. In this work, the fast response of thin films based on the SMEs of the B12 derivative to gaseous toxins (viz., hydrogen cyanide, ammonia, sulfur dioxide, and hydrogen sulfide) particularly dangerous for humans was demonstrated. The reaction between SMEs of (CN-)BuCby in Langmuir-Schaefer (LS) films and HCN generates dicyano species and proceeds ca. 5-fold more rapidly than the process involving drop-coated films that contain (CN-)BuCby in molecular form. The highest sensitivity toward HCN was achieved by using thicker LS films. The reaction proceeds reversibly: upon exposure to air, the dicyano complex undergoes partial decyanation. The decyanated complex retains reactivity toward HCN for at least four subsequent cycles. The processes involving SMEs of (CN-)BuCby and NH3, SO2, and H2S are irreversible, and the sensitivity of the films toward these gases is lower in comparison with HCN. Presented data provides mechanistic information on the reactions involving solid vitamin B12 derivatives and gaseous toxins. In the case of NH3, deprotonation of the coordinated Co(III)-ion water molecule occurs, and the generated hydroxocyano species exhibit high air stability. After binding of SO2, a mixture of sulfito and dicyano species is produced, and the regenerated film contains aquacyano and diaqua or aquahydroxo species, which possess high reactivity toward gaseous toxins. Reaction with H2S produces a mixture of the Co(III)-dicyano form and Co(II)-species containing sulfide oxidation products, which are resistant to aerobic oxidation. Our findings can be used for the development of naked-eye, electronic optic, and chemiresistive sensors toward gaseous toxins with improved reactivity for prompt cyanide detection in air, blood, and plant samples and for analysis of exhaled gases for the diagnosis of diseases.

The adducts of cyano- and aquacobalamin with hypochlorite

Hypochlorite is known to oxidatively degrade the corrin ring of cobalamin. Here, transient reaction intermediates are described in the reaction of aqua as well as of cyano-cobalamin with hypochlorite, using stopped-flow UV-vis kinetics. For aqua-cobalamin, the intermediate is assigned as arising from substitution of the aqua ligand with hypochlorite. For cyano-cobalamin, the intermediate is proposed to arise from substitution of the benzimidazole ligand trans to the cyanide. In both cases, the intermediates would feature a new Co(III)-OCl-bond-which is also supported by density functional theory (DFT) calculations.

meso-Bromination of cyano- and aquacobalamins facilitates their processing into Co(II)-species by glutathione

Cyanocobalamin (CNCbl), a medicinal form of vitamin B12, is resistant to glutathione (GSH), and undergoes intracellular processing via reductive decyanation producing the Co(II)-form of Cbl (Cbl(II)) mediated by the CblC-protein. Alteration of the CblC-protein structure might inhibit CNCbl processing. Here, we showed that introducing a bromine atom to the C10-position of the CNCbl corrin ring facilitates its reaction with GSH leading to the formation of Cbl(II) and cyanide dissociation. In a neutral medium, the reaction between C10-Br-CNCbl and GSH proceeds via the complexation of the reactants further leading to dimethylbenzimidazole (DMBI) substitution and electron transfer from GSH to the Co(III)-ion. The reaction is accelerated upon the GSH thiol group deprotonation. The key factors explaining the higher reactivity of C10-Br-CNCbl compared with unmodified CNCbl towards GSH are increasing the electrode potential of CNCbl two-electron reduction upon meso-bromination and the substantial labilization of DMBI, which was shown by comparing their reactions with cyanide and the pKa values of DMBI protonation (pKa base-off). Aquacobalamin (H2OCbl) brominated at the C10-position of the corrin reacts with GSH to give Cbl(II) via GSH complexation and subsequent reaction of this complex with a second GSH molecule, whereas unmodified H2OCbl generates glutathionyl-Cbl, which is resistant to further reduction by GSH.

The effect of vitamin B12 on DNA adduction by styrene oxide, a genotoxic xenobiotic

Vitamin B12 (cyano- or hydroxo-cobalamin) acts, via its coenzymes, methyl- and adenosyl-cobalamin, as a partner for enzymatic reactions in humans catalysed by methionine synthase and methylmalonyl-CoA mutase. As well as its association with pernicious anaemia, human B12 deficiency may also be a risk factor for neurological illnesses, heart disease and cancer. In the present work the effect of vitamin B12 (hydroxocobalamin) on the formation of DNA adducts by the epoxide phenyloxirane (styrene oxide), a genotoxic metabolite of phenylethene (styrene), has been studied using an in vitro model system. Styrene was converted to its major metabolite styrene oxide as a mixture of enantiomers using a microsomal fraction from the livers of Sprague-Dawley rats with concomitant inhibition of epoxide hydrolase. However, microsomal oxidation of styrene in the presence of vitamin B12 gave diastereoisomeric 2-hydroxy-2-phenylcobalamins. The quantitative formation of styrene oxide-DNA adducts was investigated using 2-deoxyguanosine or calf thymus DNA in the presence or absence of vitamin B12. Microsomal incubations containing either deoxyguanosine or DNA in the absence of vitamin B12 gave 2-amino-7-(2-hydroxy-1-phenylethyl)-1,7-dihydro-6H-purin-6-one [N7-(2-hydroxy-1-phenylethyl)-guanine], and 2-amino-7-(2-hydroxy-2-phenylethyl)-1,7-dihydro-6H-purin-6-one [N7-(2-hydroxy-2-phenylethyl)guanine] as the principal adducts. With deoxyguanosine the level of formation of guanine adducts was ca. 150 adducts/106 unmodified nucleoside. With DNA the adduct level was 36 pmol/mg DNA (ca. 1 adduct/0.83 × 105 nucleotides). Styrene oxide adducts from deoxyguanosine or DNA were not detected in microsomal incubations of styrene in the presence of vitamin B12. These results suggest that vitamin B12 could protect DNA against genotoxicity due to styrene oxide and other xenobiotic metabolites. However, this potential defence mechanism requires that the 2-hydroxyalkylcobalamins derived from epoxides are not 'anti-vitamins' and ideally liberate, and therefore, recycle vitamin B12. Otherwise, depletion of vitamin B12 leading to human deficiency could increase the risk of carcinogenesis initiated by genotoxic epoxides.

Rapid, green disulphide bond formation in water using the corrin dicyanocobinamide

Peptide chemists seek rapid methods combined with facile purification when producing disulphide bonds post solid-phase synthesis. Current methods typically require long reaction times of up to two days, can result in side-products from over-oxidation and/or degradation, require organic solvents, and/or require challenging purification. Herein, we describe a rapid, green, and facile oxidation of a series of peptides with up to three disulphide bonds. The method was conducted in aqueous solution, in air, utilizing the biocompatible corrin ring-containing compound dicyanocobinamide, and offers reaction times under 1 hour with simple one step removal of the catalyst.

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Supermolecular Nanoentities of Vitamin B12 Derivative as a Link in the Evolution of the Parent Molecules During Self-Assembly at the Air-Water Interface

Nanoarchitectures with promising properties have now been formed from many important biomolecules. However, the preparation of nanoparticles of vitamin B₁₂ and its derivatives remains an ongoing research challenge. This paper describes the formation of supermolecular nanoentities (SMEs) of vitamin B₁₂ derivatives, unique nanoparticles with strong noncovalent intermolecular interactions, emerging properties, and activity. These were created by a nanoarchitectonic approach using directed assembly of layers at the air-water interface as a link in the chain of evolution of the parent molecules under specially created conditions. Such layers can be represented as a nanocosm, where, at a critical density, the assemblies act as nanoreactors in which the transformation of the original material occurs. The discovered SMEs not only replicate the functioning of vitamin B₁₂ assemblies with proteins in living organisms and act as vitamin B₁₂-depended enzymes but also demonstrate important advantages over vitamin B₁₂. They are more efficient in oxygen reduction/evolution reactions and in transformation into other forms. These SMEs, in performing advanced tasks, are an alternative to widely used materials based on noble metals for catalysis, medicine, and environment protection. Our findings open new perspectives both for the fabrication of novel SMEs of biomolecules and for a better understanding of the evolution of biomolecules in nature.

Differences in the Formation of Reactive Oxygen Species and Their Cytotoxicity between Thiols Combined with Aqua- and Cyanocobalamins

Cobalamin is an essential nutrient required for the normal functioning of cells. Its deficiency can lead to various pathological states. Hydroxocobalamin (HOCbl) and cyanocobalamin (CNCbl) are the forms of vitamin B12 that are most commonly used for supplementation. There is substantial evidence indicating that cobalamins can both suppress and promote oxidative stress; however, the mechanisms underlying these effects are poorly understood. Here, it was shown that the oxidation of thiols catalyzed by HOCbl and CNCbl is accompanied by reactive oxygen species (ROS) production and induces, under certain conditions, oxidative stress and cell death. The form of vitamin B12 and the structure of thiol play a decisive role in these processes. It was found that the mechanisms and kinetics of thiol oxidation catalyzed by HOCbl and CNCbl differ substantially. HOCbl increased the rate of oxidation of thiols to a greater extent than CNCbl, but quenched ROS in combination with certain thiols. Oxidation catalyzed by CNCbl was generally slower. Yet, the absence of ROS quenching resulted in their higher accumulation. The aforementioned results might explain a more pronounced cytotoxicity induced by combinations of thiols with CNCbl. On the whole, the data obtained provide a new insight into the redox processes in which cobalamins are involved. Our results might also be helpful in developing new approaches to the treatment of some cobalamin-responsive disorders in which oxidative stress is an important component.

Versatile Enzymology and Heterogeneous Phenotypes in Cobalamin Complementation Type C Disease

Nutritional deficiency and genetic errors that impair the transport, absorption, and utilization of vitamin B₁₂ (B₁₂) lead to hematological and neurological manifestations. The cblC disease (cobalamin complementation type C) is an autosomal recessive disorder caused by mutations and epi-mutations in the MMACHC gene and the most common inborn error of B₁₂ metabolism. Pathogenic mutations in MMACHC disrupt enzymatic processing of B₁₂, an indispensable step before micronutrient utilization by the two B₁₂-dependent enzymes methionine synthase (MS) and methylmalonyl-CoA mutase (MUT). As a result, patients with cblC disease exhibit plasma elevation of homocysteine (Hcy, substrate of MS) and methylmalonic acid (MMA, degradation product of methylmalonyl-CoA, substrate of MUT). The cblC disorder manifests early in childhood or in late adulthood with heterogeneous multi-organ involvement. This review covers current knowledge on the cblC disease, structure–function relationships of the MMACHC protein, the genotypic and phenotypic spectra in humans, experimental disease models, and promising therapies.

Application of bioorganometallic B12 in green organic synthesis

Bioorganometallic structure found in coenzyme B₁₂ is a key component in B₁₂-dependent enzymatic reactions in natural enzymes. Cleavage of a cobalt-carbon bond in organometallic B₁₂ compound provide reactive intermediate for molecular transformations. Application of the bioorganometallic B₁₂ in organic synthesis have been developed using natural vitamin B₁₂ as well as synthetic vitamin B₁₂ derivatives as a bioinspired catalyst in organic solvent. Vitamin B₁₂ derivatives composed of corrinoid structure should form stable organometallic compound having a cobalt-carbon bond. Using the unique property of the organometallic vitamin B₁₂ derivatives, various catalytic reactions have been developed in synthetic organic chemistry. The dual catalytic system of vitamin B₁₂ derivatives and photocatalyst, such as Ru(II) polypyridyl complex or titanium oxide, could construct light-driven molecular transformations. The B₁₂-dependent enzymes mimic reactions, such as the dechlorination of organic halides and the radical mediated isomerization reactions, catalytically proceed in the dual catalyst system. Electroorganic syntheses mediated by the vitamin B₁₂ derivatives have been developed as green molecular transformations. The redox active vitamin B₁₂ derivatives shows a unique catalysis in the electroorganic synthesis, such as alkene and alkyne reductions.

Bioinspired Electrolysis for Green Molecular Transformations of Organic Halides Catalyzed by B12 Complex

Naturally-occurring B₁₂ -dependent enzymes catalyze various molecular transformations that are of particular interest from the viewpoint of biological chemistry as well as synthetic organic chemistry. Inspired by the unique property of the B₁₂ -dependent enzymes, various catalytic reactions have been developed using its model complex. Among the B₁₂ model complexes, heptamethyl cobyrinate, synthesized from natural vitamin B₁₂ , is highly soluble in various organic solvents and a redox active cobalt complex with an excellent catalysis in electroorganic synthesis. The electrochemical dechlorination of pollutant organic chlorides, such as DDT, was effectively catalyzed by the B₁₂ complex. Modification of the electrode surface by the sol-gel method to immobilize the B₁₂ complex was also developed. The B₁₂ modified electrodes were effective for the dehalogenation of organic halides with high turnover numbers based on the immobilized B₁₂ complex. Electrolysis of an organic halide catalyzed by the B₁₂ complex provided dechlorinated products under anaerobic conditions, while the electrolysis under aerobic conditions afforded oxygen incorporated products, such as an ester and amide along with dechlorination. Benzotrichloride was transformed into ethylbenzoate or N,N-diethylbenzamide in the presence of ethanol or diethylamine, respectively. This amide formation was further expanded to a unique paired electrolysis. Electrochemical reductions of an alkene and alkyne were also catalyzed by the B₁₂ complex. A cobalt-hydrogen complex should be formed as a bioinspired intermediate. Using the B₁₂ complex, light-assisted electrosynthesis was also developed to save the applied energy.

Mechanism of cyanocobalamin chlorination by hypochlorous acid

Hypochlorous acid (HOCl) is a strong oxidant produced by myeloperoxidase. Previous work suggested that HOCl modifies the corrin ring of cobalamins to yield chlorinated species via mechanisms that are incompletely understood. Herein, we report a mechanistic study on the reaction between cyanocobalamin (CNCbl, vitamin B₁₂) and HOCl. Under weakly acidic, neutral and weakly alkaline conditions, the reaction produces the c-lactone derivative of CNCbl chlorinated at the C10-position of corrin ring (C10-Cl-CNCbl-c-lactone). Formation of C10-Cl-CNCbl-c-lactone was not observed at pH ≥ 9.9. The chlorination of CNCbl by HOCl proceeds via two pathways involving one and two HOCl molecules: the reaction is initiated by the very fast formation of a complex between CNCbl and HOCl, which either undergoes slow transformation to chlorinated species, or rapidly reacts with a second HOCl molecule to produce C10-Cl-CNCbl. Subsequent reaction of C10-Cl-CNCbl with HOCl proceeds rapidly toward lactone ring formation by H-atom abstraction at position C8. This work uncovered mechanisms and products of the reaction of a biologically active and therapeutically used cobalamin, CNCbl and the endogenous oxidant HOCl. Binding and reactivity studies of C10-Cl-CNCbl and C10-Cl-CNCbl-c-lactone with relevant proteins of the cobalamin pathway and with cultured cells are necessary to elucidate the potential physiological effects of these species.

Utilization of cobalamin is ubiquitous in early-branching fungal phyla

Cobalamin is a cofactor present in essential metabolic pathways in animals and one of the water-soluble vitamins. It is a complex compound synthesized solely by prokaryotes. Cobalamin dependence is scattered across the tree of life. In particular, fungi and plants were deemed devoid of cobalamin. We demonstrate that cobalamin is utilized by all non-Dikarya fungi lineages. This observation is supported by the genomic presence of both B12-dependent enzymes and cobalamin modifying enzymes. Fungal cobalamin-dependent enzymes are highly similar to their animal homologs. Phylogenetic analyses support a scenario of vertical inheritance of the cobalamin usage with several losses. Cobalamin usage was probably lost in Mucorinae and at the base of Dikarya which groups most of the model organisms and which hindered B12-dependent metabolism discovery in fungi. Our results indicate that cobalamin dependence was a widely distributed trait at least in Opisthokonta, across diverse microbial eukaryotes and was likely present in the LECA.

Cobalt(II) Complexes Based on Benzylmalonate Anions Exhibiting Field-Induced Single-Ion Magnet Slow Relaxation Behavior

The reaction of (NBu4)2Bzmal (where Bzmal2− is benzylmalonate dianion) with Co(OAc)2∙4H2O gives the [Co(Bzmal)(EtOH)(H2O)]n 2D-polymer (1). The addition of 2,2′-bipyridine (bpy) to the starting system results in the [Co(Bzmal)(bpy)2]·H2O·EtOH molecular complex (2). Their molecular and crystal structures were analyzed by single-crystal X-ray crystallography. An analysis of the static magnetic data supported by the SA-CASSCF/NEVPT2 calculations revealed the presence of easy-plane magnetic anisotropy in both complexes. The AC susceptibility data confirm that both complexes show a slow field-induced (HDC = 1000 Oe) magnetic relaxation behavior.

Genetic Analysis of Citrobacter sp.86 Reveals Involvement of Corrinoids in Chlordecone and Lindane Biotransformations

Chlordecone (Kepone®) and γ-hexachlorocyclohexane (γ-HCH or lindane) have been used for decades in the French West Indies (FWI) resulting in long-term soil and water pollution. In a previous work, we have identified a new Citrobacter species (sp.86) that is able to transform chlordecone into numerous products under anaerobic conditions. No homologs to known reductive dehalogenases or other candidate genes were found in the genome sequence of Citrobacter sp.86. However, a complete anaerobic pathway for cobalamin biosynthesis was identified. In this study, we investigated whether cobalamin or intermediates of cobalamin biosynthesis was required for chlordecone microbiological transformation. For this purpose, we constructed a set of four Citrobacter sp.86 mutant strains defective in several genes belonging to the anaerobic cobalamin biosynthesis pathway. We monitored chlordecone and its transformation products (TPs) during long-term incubation in liquid cultures under anaerobic conditions. Chlordecone TPs were detected in the case of cobalamin-producing Citrobacter sp.86 wild-type strain but also in the case of mutants able to produce corrinoids devoid of lower ligand. In contrast, mutants unable to insert the cobalt atom in precorrin-2 did not induce any transformation of chlordecone. In addition, it was found that lindane, previously shown to be anaerobically transformed by Citrobacter freundii without evidence of a mechanism, was also degraded in the presence of the wild-type strain of Citrobacter sp.86. The lindane degradation abilities of the various Citrobacter sp.86 mutant strains paralleled chlordecone transformation. The present study shows the involvement of cobalt-containing corrinoids in the microbial degradation of chlorinated compounds with different chemical structures. Their increased production in contaminated environments could accelerate the decontamination processes.

Thiolatocobalamins repair the activity of pathogenic variants of the human cobalamin processing enzyme CblC

Thiolatocobalamins are a class of cobalamins comprised of naturally occurring and synthetic ligands. Glutathionylcobalamin (GSCbl) occurs naturally in mammalian cells, and also as an intermediate in the glutathione-dependent dealkylation of methylcobalamin (MeCbl) to form cob(I)alamin by pure recombinant CblC from C. elegans. Glutathione-driven deglutathionylation of GSCbl was demonstrated both in mammalian as well as in C. elegans CblC. Dethiolation is orders of magnitude faster than dealkylation of Co-C bonded cobalamins, which motivated us to investigate two synthetic thiolatocobalamins as substrates to repair the enzymatic activity of pathogenic CblC variants in humans. We report the synthesis and kinetic characterization of cysteaminylcobalamin (CyaCbl) and 2-mercaptopropionylglycinocobalamin (MpgCbl). Both CyaCbl and MpgCbl were obtained in high purity (90-95%) and yield (78-85%). UV-visible spectral properties agreed with those reported for other thiolatocobalamins with absorbance maxima observed at 372 nm and 532 nm. Both CyaCbl and MpgCbl bound to wild type human recombinant CblC inducing spectral blue-shifts characteristic of the respective base-on to base-off transitions. Addition of excess glutathione (GSH) resulted in rapid elimination of the β-ligand to give aquacobalamin (H₂OCbl) as the reaction product under aerobic conditions. Further, CyaCbl and MpgCbl underwent spontaneous dethiolation thereby repairing the loss of activity of pathogenic variants of human CblC, namely R161G and R161Q. We posit that thiolatocobalamins could be exploited therapeutically for the treatment of inborn errors of metabolism that impair processing of dietary and supplemental cobalamin forms. While these disorders are targets for newborn screening in some countries, there is currently no effective treatment available to patients.

Not Guilty on Every Count: The "Non-Innocent" Nitrosyl Ligand in the Framework of IUPAC's Oxidation-State Formalism

Nitrosyl-metal bonding relies on the two interactions between the pair of N-O-π* and two of the metal's d orbitals. These (back)bonds are largely covalent, which makes their allocation in the course of an oxidation-state determination ambiguous. However, apart from M-N-O-angle or net-charge considerations, IUPAC's "ionic approximation" is a useful tool to reliably classify nitrosyl metal complexes in an orbital-centered approach.

Catalytic effect of riboflavin on electron transfer from NADH to aquacobalamin

Reduction of cobalamin by non-dedicated cellular reductases has been reported in earlier work, however, the sources of reducing power and the mechanisms are unknown. This study reports results of kinetic and mechanistic investigation of the reaction between aquacobalamin, H₂OCbl, and reduced β-nicotinamide adenine dinucleotide, NADH. This interaction leads to the formation of one-electron reduced cobalamin, cob(II)alamin, and proceeds via water substitution on aquacobalamin by NADH and further decomposition of NADH-Co(III) complex to cob(II)alamin and NADH^·+. Riboflavin catalyzes the reduction of aquacobalamin by NADH both in free form and with aquacobalamin bound to the cobalamin processing enzyme CblC. The rate-determining step of this catalytic reaction is the interaction between riboflavin and NADH to produce a charge transfer complex that reacts with aquacobalamin. Aquacobalamin quenches the fluorescence of NADH and riboflavin predominantly via a static mechanism.

Redox potential changes during ATP-dependent corrinoid reduction determined by redox titrations with europium(II)-DTPA

Corrinoids are essential cofactors of enzymes involved in the C₁ metabolism of anaerobes. The active, super-reduced [Co^I ] state of the corrinoid cofactor is highly sensitive to autoxidation. In O-demethylases, the oxidation to inactive [Co^II ] is reversed by an ATP-dependent electron transfer catalyzed by the activating enzyme (AE). The redox potential changes of the corrinoid cofactor, which occur during this reaction, were studied by potentiometric titration coupled to UV/visible spectroscopy. By applying europium(II)-diethylenetriaminepentaacetic acid (DTPA) as a reductant, we were able to determine the midpoint potential of the [Co^II ]/[Co^I ] couple of the protein-bound corrinoid cofactor in the absence and presence of AE and/or ATP. The data revealed that the transfer of electrons from a physiological donor to the corrinoid as the electron-accepting site is achieved by increasing the potential of the corrinoid cofactor from -530 ± 15 mV to -250 ± 10 mV (E_SHE , pH 7.5). The first 50 to 100 mV of the shift of the redox potential seem to be caused by the interaction of nucleotide-bound AE with the corrinoid protein or its cofactor. The remaining 150-200 mV had to be overcome by the chemical energy of ATP hydrolysis. The experiments revealed that Eu(II)-DTPA, which was already known as a powerful reducing agent, is a suitable electron donor for titration experiments of low-potential redox centers. Furthermore, the results of this study will contribute to the understanding of thermodynamically unfavorable electron transfer processes driven by the power of ATP hydrolysis.

Encapsulation of vitamin B12 into nanoengineered capsules and soft matter nanosystems for targeted delivery

Targeted delivery of vitamins to a desirable area is an active branch in a modern pharmacology. The most important and difficult delivery of vitamin B₁₂ is that to bone marrow and nerve cells. Herein we present a first step towards the development of two types of smart carriers, polymer capsules and lyotropic liquid-crystalline nanosystems, for vitamin B₁₂ targeted delivery and induced release. A vitamin B₁₂ encapsulation technique into nanoengineered polymeric capsules produced by layer-by-layer assembling of polymeric shells on CaCO₃ templates has been developed. The effectiveness of the process was demonstrated by optical absorption spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM) and small-angle X-ray diffraction. TEM and AFM analyses performed on capsules after their drying, confirmed the presence of the vitamin B₁₂ inside the capsules in the form of crystalline nanoaggregates, 50-300 nm in diameter. Soft lipid nanovectors consisting of amphiphilic phytantriol molecules, which in water excess spontaneously self-assembly in 3D well-ordered inverse bicontinuous cubic bulk phase, were used as alternative carriers for vitamin B₁₂. It was shown that about 30% of the vitamin added in the preparation of the soft lipid system was actually encapsulated in cubosomes and that no structural changes occurred upon loading. The Vitamin stabilizes the lipid system playing the role of its structure-forming element. The biocompatible nature, the stability and the feasibility of these systems make them good candidates as carriers for hydrophilic vitamins.

Catalytic antioxidants for therapeutic medicine

In this Review, we focus on catalytic antioxidant study based on transition metal complexes, organoselenium compounds, supramolecules and protein scaffolds.

Kinetics and mechanism of the reaction of cyanocobalamin with potassium hydroxide in non-aqueous media

The reaction of cyanocobalamin (CNCbl) with potassium hydroxide (KOH) was studied in isopropyl alcohol (iPrOH) and dimethyl sulfoxide (DMSO) under anaerobic conditions.

Hydroxycobalamin catalyzes the oxidation of diethyldithiocarbamate and increases its cytotoxicity independently of copper ions

It is known that some metals (Cu, Zn, Cd, Au) markedly increase the toxic effect of thiocarbamates. It was shown in the present study that hydroxycobalamin (a form of vitamin B12, HOCbl), which incorporates cobalt, significantly enhances the cytotoxicity of diethyldithiocarbamate (DDC), decreasing its IC50 value in tumor cells three to five times. The addition of HOCbl to aqueous DDC solutions accelerated the reduction of oxygen. No hydrogen peroxide accumulation was observed in DDC + HOCbl solutions; however, catalase slowed down the oxygen reduction rate. Catalase as well as the antioxidants N-acetylcysteine (NAC) and glutathione (GSH) partially inhibited the cytotoxic effect of DDC + HOCbl, whereas ascorbate, pyruvate, and tiron, a scavenger of superoxide anion, had no cytoprotective effect. The administration of HOCbl into DDC solutions (> 1 mM) resulted in the formation of a crystalline precipitate, which was inhibited in the presence of GSH. The data of UV and NMR spectroscopy and HPLC and Mass Spectrometry (LC/MS) indicated that the main products of the reaction of DDC with HOCbl are disulfiram (DSF) and its oxidized forms, sulfones and sulfoxides. The increase in the cytotoxicity of DDC combined with HOCbl occurred both in the presence of Cu2+ in culture medium and in nominally Cu-free solutions, as well as in growth medium containing the copper chelator bathocuproine disulfonate (BCS). The results indicate that HOCbl accelerates the oxidation of DDC with the formation of DSF and its oxidized forms. Presumably, the main cause of the synergistic increase in the toxic effect of DDC + HOCbl is the formation of sulfones and sulfoxides of DSF.

Synthesis of vitamin B12 derivatives with sodium hydroxymethanesulfinate

The reaction of cyanocobalamin (CNCbl) with sodium hydroxymethanesulfinate (HMS) was studied over a wide range of pH (4–11) under aerobic conditions. CNCbl is destroyed in the presence of HMS in aqueous solution to form uncolored substances. The accumulation of stable yellow corrinoids (SYCs) preceded these changes at pH [Formula: see text] 8. The major stable yellow corrinoid is (15R)-Co[Formula: see text], Coß — dicyano-13-dehydro-15-hydro-l5-hydroxycob(III)alamin. The yield of this SYC is 25%, and the stability of this compound decreases significantly with increasing concentrations of HMS, pH and temperature.

Learning from B12 enzymes: biomimetic and bioinspired catalysts for eco-friendly organic synthesis

Cobalamins (B₁₂) play various important roles in vivo. Most B₁₂-dependent enzymes are divided into three main subfamilies: adenosylcobalamin-dependent isomerases, methylcobalamin-dependent methyltransferases, and dehalogenases. Mimicking these B₁₂ enzyme functions under non-enzymatic conditions offers good understanding of their elaborate reaction mechanisms. Furthermore, bio-inspiration offers a new approach to catalytic design for green and eco-friendly molecular transformations. As part of a study based on vitamin B₁₂ derivatives including heptamethyl cobyrinate perchlorate, we describe biomimetic and bioinspired catalytic reactions with B₁₂ enzyme functions. The reactions are classified according to the corresponding three B₁₂ enzyme subfamilies, with a focus on our recent development on electrochemical and photochemical catalytic systems. Other important reactions are also described, with a focus on radical-involved reactions in terms of organic synthesis.

Accumulation of NO2 -cobalamin in nutrient-stressed ammonia-oxidizing archaea and in the oxygen deficient zone of the eastern tropical North Pacific

Cobalamin (vitamin B₁₂ ) is a precious resource in natural systems that is produced by select prokaryotes and required by a broad range of organisms. In this way, the production of cobalamin reinforces numerous microbial interdependencies. Here we report the accumulation of an unusual form of cobalamin, nitrocobalamin (NO₂ -cobalamin), in a marine oxygen deficient zone (ODZ), isolates of ammonia-oxidizing archaea (AOA), and an anaerobic ammonium-oxidizing (anammox) bacteria enriched bioreactor. Low oxygen waters were enriched in NO₂ -cobalamin, and AOA isolates experiencing ammonia or copper stress produced more NO₂ -cobalamin, though there is wide strain-to-strain and batch-to-batch variability. NO₂ -cobalamin has no known biochemical role. We hypothesize that AOA and anammox bacteria are a source of marine NO₂ -cobalamin in the environment via a reactive nitrogen intermediate. These findings suggest connections between cobalamin forms and nitrogen transformations, physiological stress and ocean deoxygenation.

Modified vitamin B12 derivatives with a peptide backbone for biomimetic studies and medicinal applications

This short review highlights the author’s group research on modified vitamin B[Formula: see text] derivatives with a peptide backbone as (1) inhibitors of B[Formula: see text]-dependent enzymes and as (2) models of cofactor B[Formula: see text]-protein complexes.

Characterization of the complex between native and reduced bovine serum albumin with aquacobalamin and evidence of dual tetrapyrrole binding

Serum albumin binds to a variety of endogenous ligands and drugs. Human serum albumin (HSA) binds to heme via hydrophobic interactions and axial coordination of the iron center by protein residue Tyr161. Human serum albumin binds to another tetrapyrrole, cobalamin (Cbl), but the structural and functional properties of this complex are poorly understood. Herein, we investigate the reaction between aquacobalamin (H₂OCbl) and bovine serum albumin (BSA, the bovine counterpart of HSA) using Ultraviolet-Visible and fluorescent spectroscopy, and electron paramagnetic resonance. The reaction between H₂OCbl and BSA led to the formation of a BSA-Cbl(III) complex consistent with N-axial ligation (amino). Prior to the formation of this complex, the reactants participate in an additional binding event that has been examined by fluorescence spectroscopy. Binding of BSA to Cbl(III) reduced complex formation between the bound cobalamin and free cyanide to form cyanocobalamin (CNCbl), suggesting that the β-axial position of the cobalamin may be occupied by an amino acid residue from the protein. Reaction of BSA containing reduced disulfide bonds with H₂OCbl produces cob(II)alamin and disulfide with intermediate formation of thiolate Cbl(III)-BSA complex and its decomposition. Finally, in vitro studies showed that cobalamin binds to BSA only in the presence of an excess of protein, which is in contrast to heme binding to BSA that involves a 1:1 stoichiometry. In vitro formation of BSA-Cbl(III) complex does not preclude subsequent heme binding, which occurs without displacement of H₂OCbl bound to BSA. These data suggest that the two tetrapyrroles interact with BSA in different binding pockets.