Structural basis for universal corrinoid recognition by the cobalamin transport protein haptocorrin

Vitamin B12 Metabolism: A Network of Multi-Protein Mediated Processes

The water-soluble vitamin, vitamin B12, also known as cobalamin, plays a crucial role in cellular metabolism, particularly in DNA synthesis, methylation, and mitochondrial functionality. Its deficiency can lead to hematological and neurological disorders; however, the manifestation of these clinical outcomes is relatively late. It leads to difficulties in the early diagnosis of vitamin B12 deficiency. A prolonged lack of vitamin B12 may have severe consequences including increased morbidity to neurological and cardiovascular diseases. Beyond inadequate dietary intake, vitamin B12 deficiency might be caused by insufficient bioavailability, blood transport disruptions, or impaired cellular uptake and metabolism. Despite nearly 70 years of knowledge since the isolation and characterization of this vitamin, there are still gaps in understanding its metabolic pathways. Thus, this review aims to compile current knowledge about the crucial proteins necessary to efficiently accumulate and process vitamin B12 in humans, presenting these systems as a multi-protein network. The epidemiological consequences, diagnosis, and treatment of vitamin B12 deficiency are also highlighted. We also discuss clinical warnings of vitamin B12 deficiency based on the ongoing test of specific moonlighting proteins engaged in vitamin B12 metabolic pathways.

The structure of the rat vitamin B12 transporter TC and its complex with glutathionylcobalamin

Vitamin B12 (cobalamin or Cbl) functions as a cofactor in two important enzymatic processes in human cells, and life is not sustainable without it. B12 is obtained from food and travels from the stomach, through the intestine, and into the bloodstream by three B12-transporting proteins: salivary haptocorrin (HC), gastric intrinsic factor, and transcobalamin (TC), which all bind B12 with high affinity and require proteolytic degradation to liberate Cbl. After intracellular delivery of dietary B12, Cbl in the aquo/hydroxocobalamin form can coordinate various nucleophiles, for example, GSH, giving rise to glutathionylcobalamin (GSCbl), a naturally occurring form of vitamin B12. Currently, there is no data showing whether GSCbl is recognized and transported in the human body. Our crystallographic data shows for the first time the complex between a vitamin B12 transporter and GSCbl, which compared to aquo/hydroxocobalamin, binds TC equally well. Furthermore, sequence analysis and structural comparisons show that TC recognizes and transports GSCbl and that the residues involved are conserved among TCs from different organisms. Interestingly, haptocorrin and intrinsic factor are not structurally tailored to bind GSCbl. This study provides new insights into the interactions between TC and Cbl.

The complex machinery of human cobalamin metabolism

Vitamin B₁₂ (cobalamin, Cbl) is required as a cofactor by two human enzymes, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR) and methylmalonyl-CoA mutase (MMUT). Within the body, a vast array of transporters, enzymes and chaperones are required for the generation and delivery of these cofactor forms. How they perform these functions is dictated by the structure and interactions of the proteins involved, the molecular bases of which are only now being elucidated. In this review, we highlight recent insights into human Cbl metabolism and address open questions in the field by employing a protein structure and interactome based perspective. We discuss how three very similar proteins-haptocorrin, intrinsic factor and transcobalamin-exploit slight structural differences and unique ligand receptor interactions to effect selective Cbl absorption and internalisation. We describe recent advances in the understanding of how endocytosed Cbl is transported across the lysosomal membrane and the implications of the recently solved ABCD4 structure. We detail how MMACHC and MMADHC cooperate to modify and target cytosolic Cbl to the client enzymes MTR and MMUT using ingenious modifications to an ancient nitroreductase fold, and how MTR and MMUT link with their accessory enzymes to sustainably harness the supernucleophilic potential of Cbl. Finally, we provide an outlook on how future studies may combine structural and interactome based approaches and incorporate knowledge of post-translational modifications to bring further insights.

Bioprocess Strategies for Vitamin B12 Production by Microbial Fermentation and Its Market Applications

Vitamin B₁₂ is a widely used compound in the feed and food, healthcare and medical industries that can only be produced by fermentation because of the complexity of its chemical synthesis. For this reason, finding better producer strains and optimizing their bioprocesses have been the main focus of industrial producers over the last few decades. In this review, we initially provide a historical overview of vitamin B₁₂ research and the main biosynthetic characteristics of the two microorganism families typically used for its industrial production: several strains of Propionibacterium freudenreichii and strains related to Pseudomonas denitrificans. Later, a complete summary of the current state of vitamin B₁₂ industrial production as well as the main advances and challenges for improving it is detailed, with a special focus on bioprocess optimization, which aims not only to increase production but also sustainability. In addition, a comprehensive list of the most important and relevant patents for the present industrial strains is provided. Finally, the potential applications of vitamin B₁₂ in different markets are discussed.

Loss of the Vitamin B-12 Transport Protein Tcn2 Results in Maternally Inherited Growth and Developmental Defects in Zebrafish

BACKGROUND

In humans, vitamin B-12 (cobalamin) transport involves 3 paralogous proteins: transcobalamin, haptocorrin, and intrinsic factor. Zebrafish (Danio rerio) express 3 genes that encode proteins homologous to known B-12 carrier proteins: tcn2 (a transcobalamin ortholog) and 2 atypical β-domain-only homologs, tcnba and tcnbb.

OBJECTIVES

Given the orthologous relation between zebrafish Tcn2 and human transcobalamin, we hypothesized that zebrafish carrying null mutations of tcn2 would exhibit phenotypes consistent with vitamin B-12 deficiency.

METHODS

First-generation and second-generation tcn2-/- zebrafish were characterized using phenotypic assessments, metabolic analyses, viability studies, and transcriptomics.

RESULTS

Homozygous tcn2-/- fish produced from a heterozygous cross are viable and fertile but exhibit reduced growth, which persists into adulthood. When first-generation female tcn2-/- fish are bred, their offspring exhibit gross developmental and metabolic defects. These phenotypes are observed in all offspring from a tcn2-/- female regardless of the genotype of the male mating partner, suggesting a maternal effect, and can be rescued with vitamin B-12 supplementation. Transcriptome analyses indicate that offspring from a tcn2-/- female exhibit expression profiles distinct from those of offspring from a tcn2+/+ female, which demonstrate dysregulation of visual perception, fatty acid metabolism, and neurotransmitter signaling pathways.

CONCLUSIONS

Our findings suggest that the deposition of vitamin B-12 in the yolk by tcn2-/- females may be insufficient to support the early development of their offspring. These data present a compelling model to study the effects of vitamin B-12 deficiency on early development, with a particular emphasis on transgenerational effects and gene-environment interactions.

Heme Uptake in Lactobacillus sakei Evidenced by a New Energy Coupling Factor (ECF)-Like Transport System

Lactobacillus sakei is a nonpathogenic lactic acid bacterium and a natural inhabitant of meat ecosystems. Although red meat is a heme-rich environment, L. sakei does not need iron or heme for growth, although it possesses a heme-dependent catalase. Iron incorporation into L. sakei from myoglobin and hemoglobin was previously shown by microscopy and the L. sakei genome reveals the complete equipment for iron and heme transport. Here, we report the characterization of a five-gene cluster (from lsa1836 to lsa1840 [lsa1836-1840]) encoding a putative metal iron ABC transporter. Interestingly, this cluster, together with a heme-dependent catalase gene, is also conserved in other species from the meat ecosystem. Our bioinformatic analyses revealed that the locus might correspond to a complete machinery of an energy coupling factor (ECF) transport system. We quantified in vitro the intracellular heme in the wild type (WT) and in our Δlsa1836-1840 deletion mutant using an intracellular heme sensor and inductively coupled plasma mass spectrometry for quantifying incorporated 57Fe heme. We showed that in the WT L. sakei, heme accumulation occurs rapidly and massively in the presence of hemin, while the deletion mutant was impaired in heme uptake; this ability was restored by in trans complementation. Our results establish the main role of the L. sakei Lsa1836-1840 ECF-like system in heme uptake. Therefore, this research outcome sheds new light on other possible functions of ECF-like systems.IMPORTANCELactobacillus sakei is a nonpathogenic bacterial species exhibiting high fitness in heme-rich environments such as meat products, although it does not need iron or heme for growth. Heme capture and utilization capacities are often associated with pathogenic species and are considered virulence-associated factors in the infected hosts. For these reasons, iron acquisition systems have been deeply studied in such species, while for nonpathogenic bacteria the information is scarce. Genomic data revealed that several putative iron transporters are present in the genome of the lactic acid bacterium L. sakei In this study, we demonstrate that one of them is an ECF-like ABC transporter with a functional role in heme transport. Such evidence has not yet been brought for an ECF; therefore, our study reveals a new class of heme transport system.

Vitamin B12 and its binding proteins in milk from cow and buffalo in relation to bioavailability of B12

Milk is an important source of highly bioavailable vitamin B₁₂ (cobalamin) in human nutrition. In most animal products, vitamin B₁₂ is strongly bound to various specific protein carriers. The 2 vitamin B₁₂-specific proteins, predominantly transcobalamin (TC) and haptocorrin (HC), were earlier found in milk from Holstein Friesian cows and in human or sow milk, respectively. As the type of vitamin B₁₂ binders may influence bioavailability of the vitamin, we examined vitamin B₁₂ carriers in pooled milk specimens derived from European and Indian cow and buffalo herds. The total endogenous vitamin B₁₂ concentration was comparable in all milk pools (≈3 nM), but the vitamin carriers varied considerably: TC + caseins in Danish cows, TC + HC in Indian cows and buffaloes, and mainly HC in Italian buffaloes. Danish cow milk contained half as much TC as vitamin B₁₂, and the surplus vitamin was all attached via a single coordination bond to abundantly available histidine residues of casein. The specific binding proteins in Indian cow milk (TC + HC) approximately matched the molar content of vitamin B₁₂. Milk from the 2 buffalo breeds contained more specific binders than vitamin B₁₂, and the surplus proteins included the unsaturated TC ≈ 3 nM (Indian stock), or both TC ≈ 4 nM and HC ≈ 23 nM (Italian stock). The abundant HC of the latter sample bound nearly all endogenous vitamin B₁₂. We tested (in vitro) the transfer of vitamin B₁₂ from milk proteins to human carriers, involved in the intestinal uptake. The bovine TC-vitamin B₁₂ complex rapidly dissociated at pH 2 (time of half reaction, τ_1/2 < 1 min, 37°C) and was susceptible to digestion with trypsin + chymotrypsin (pH 7.5). Transfer of vitamin B₁₂ from the precipitated bovine casein (pH 2) to human carriers proceeded with τ_1/2 ≈ 7 min (37°C) and τ_1/2 ≈ 35 min (20°C). Liberation of vitamin B₁₂ from buffalo HC was hampered because of its pH stability and slow proteolysis. Nutritional availability of vitamin B₁₂ is expected to be high in cow milk (with TC-vitamin B₁₂ and casein-vitamin B₁₂ complexes) but potentially constrained in buffalo milk (with HC-vitamin B₁₂). This especially concerns the Italian buffalo milk, where a high excess of HC was found. We speculate whether the isolated stock of Italian buffalo maintained the ancestral secretion of carriers (HC ≫ vitamin B₁₂, TC ≈ 0), whereas intensive crossbreeding of cows and buffaloes from other regions caused a change to TC ≤ vitamin B₁₂, with low or absent HC. The substitution of HC by less sturdy carriers is apparently more beneficial to human consumers as far as vitamin B₁₂ bioavailability is concerned.

Functional and phylogenetic characterization of noncanonical vitamin B12-binding proteins in zebrafish suggests involvement in cobalamin transport

In humans, transport of food-derived cobalamin (vitamin B12) from the digestive system into the bloodstream involves three paralogous proteins: transcobalamin (TC), haptocorrin (HC), and intrinsic factor (IF). Each of these proteins contains two domains, an α-domain and a β-domain, which together form a cleft in which cobalamin binds. Zebrafish (Danio rerio) are thought to possess only a single cobalamin transport protein, referred to as Tcn2, which is a transcobalamin homolog. Here, we used CRISPR/Cas9 mutagenesis to create null alleles of tcn2 in zebrafish. Fish homozygous for tcn2-null alleles were viable and exhibited no obvious developmentally or behaviorally abnormal phenotypes. For this reason, we hypothesized that previously unidentified cobalamin-carrier proteins encoded in the zebrafish genome may provide an additional pathway for cobalamin transport. We identified genes predicted to code for two such proteins, Tcn-beta-a (Tcnba) and Tcn-beta-b (Tcnbb), which differ from all previously characterized cobalamin transport proteins as they lack the α-domain. These β-domain-only proteins are representative of an undescribed class of cobalamin-carrier proteins that are highly conserved throughout the ray-finned fishes. We observed that the genes encoding the three cobalamin transport homologs, tcn2, tcnba, and tcnbb, are expressed in unique spatial and temporal patterns in the developing zebrafish. Moreover, exogenously expressed recombinant Tcnba and Tcnbb bound cobalamin with high affinity, comparable with binding by full-length Tcn2. Taken together, our results suggest that this noncanonical protein structure has evolved to fully function as a cobalamin-carrier protein, thereby allowing for a compensatory cobalamin transport mechanism in the tcn2-/- zebrafish.

Gastrointestinal Handling of Water-Soluble Vitamins

Nine compounds are classified as water-soluble vitamins, eight B vitamins and one vitamin C. The vitamins are mandatory for the function of numerous enzymes and lack of one or more of the vitamins may lead to severe medical conditions. All the vitamins are supplied by food in microgram to milligram quantities and in addition some of the vitamins are synthesized by the intestinal microbiota. In the gastrointestinal tract, the vitamins are liberated from binding proteins and for some of the vitamins modified prior to absorption. Due to their solubility in water, they all require specific carriers to be absorbed. Our current knowledge concerning each of the vitamins differs in depth and focus and is influenced by the prevalence of conditions and diseases related to lack of the individual vitamin. Because of that we have chosen to cover slightly different aspects for the individual vitamins. For each of the vitamins, we summarize the physiological role, the steps involved in the absorption, and the factors influencing the absorption. In addition, for some of the vitamins, the molecular base for absorption is described in details, while for others new aspects of relevance for human deficiency are included. © 2018 American Physiological Society. Compr Physiol 8:1291-1311, 2018.

Expression of TCN1 in Blood is Negatively Associated with Verbal Declarative Memory Performance

Memory is indispensable for normal cognitive functioning, and the ability to store and retrieve information is central to mental health and disease. The molecular mechanisms underlying complex memory functions are largely unknown, but multiple genome-wide association studies suggest that gene regulation may play a role in memory dysfunction. We performed a global gene expression analysis using a large and balanced case-control sample (n = 754) consisting of healthy controls and schizophrenia and bipolar disorder patients. Our aim was to discover genes that are differentially expressed in relation to memory performance. Gene expression in blood was measured using Illumina HumanHT-12 v4 Expression BeadChip and memory performance was assessed with the updated California Verbal Learning Test (CVLT-II). We found that elevated expression of the vitamin B12-related gene TCN1 (haptocorrin) was significantly associated with poorer memory performance after correcting for multiple testing (β = −1.50, p = 3.75e-08). This finding was validated by quantitative real-time PCR and followed up with additional analyses adjusting for confounding variables. We also attempted to replicate the finding in an independent case-control sample (n = 578). The relationship between TCN1 expression and memory impairment was comparable to that of important determinants of memory function such as age and sex, suggesting that TCN1 could be a clinically relevant marker of memory performance. Thus, we identify TCN1 as a novel genetic finding associated with poor memory function. This finding may have important implications for the diagnosis and treatment of vitamin B12-related conditions.

Cysteine-mediated decyanation of vitamin B12 by the predicted membrane transporter BtuM

Uptake of vitamin B12 is essential for many prokaryotes, but in most cases the membrane proteins involved are yet to be identified. We present the biochemical characterization and high-resolution crystal structure of BtuM, a predicted bacterial vitamin B12 uptake system. BtuM binds vitamin B12 in its base-off conformation, with a cysteine residue as axial ligand of the corrin cobalt ion. Spectroscopic analysis indicates that the unusual thiolate coordination allows for decyanation of vitamin B12. Chemical modification of the substrate is a property other characterized vitamin B12-transport proteins do not exhibit.

Uptake of vitamin B12 is essential for many prokaryotes, but in most cases the membrane proteins involved are yet to be identified. Here, the authors use X-ray crystallography and spectroscopy to characterize BtuM, a predicted bacterial substrate-modifying vitamin B12 transporter.

Functional and structural characterization of an ECF-type ABC transporter for vitamin B12

Vitamin B12 (cobalamin) is the most complex B-type vitamin and is synthetized exclusively in a limited number of prokaryotes. Its biologically active variants contain rare organometallic bonds, which are used by enzymes in a variety of central metabolic pathways such as L-methionine synthesis and ribonucleotide reduction. Although its biosynthesis and role as co-factor are well understood, knowledge about uptake of cobalamin by prokaryotic auxotrophs is scarce. Here, we characterize a cobalamin-specific ECF-type ABC transporter from Lactobacillus delbrueckii, ECF-CbrT, and demonstrate that it mediates the specific, ATP-dependent uptake of cobalamin. We solved the crystal structure of ECF-CbrT in an apo conformation to 3.4 Å resolution. Comparison with the ECF transporter for folate (ECF-FolT2) from the same organism, reveals how the identical ECF module adjusts to interact with the different substrate binding proteins FolT2 and CbrT. ECF-CbrT is unrelated to the well-characterized B12 transporter BtuCDF, but their biochemical features indicate functional convergence.

Vitamin B12

The biosynthesis of B₁₂, involving up to 30 different enzyme-mediated steps, only occurs in bacteria. Thus, most eukaryotes require an external source of B₁₂, and yet the vitamin appears to have only two functions in eukaryotes: as a cofactor for the enzymes methionine synthase and methylmalonylCoA mutase. These two functions are crucial for normal health in humans, and in particular, the formation of methionine is essential for providing methyl groups for over 100 methylation processes. Interference with the methionine synthase reaction not only depletes the body of methyl groups but also leads to the accumulation of homocysteine, a risk factor for many diseases. The syndrome pernicious anemia, characterized by lack of intrinsic factor, leads to a severe, sometimes fatal form of B₁₂ deficiency. However, there is no sharp cutoff for B₁₂ deficiency; rather, there is a continuous inverse relationship between serum B₁₂ and a variety of undesirable outcomes, including neural tube defects, stroke, and dementia. The brain is particularly vulnerable; in children, inadequate B₁₂ stunts brain and intellectual development. Suboptimal B₁₂ status (serum B₁₂<300pmol/L) is very common, occurring in 30%-60% of the population, in particular in pregnant women and in less-developed countries. Thus, many tens of millions of people in the world may suffer harm from having a poor B₁₂ status. Public health steps are urgently needed to correct this inadequacy.

5′-Vitamin B12 derivatives suitable for bioconjugation via the amide bond

Vitamin B₁₂ derivatives bearing either the amino or the carboxylic group at the 5′-position were synthesised hence enabling conjugation of therapeutics to vitamin B₁₂via the amide bond.

Organometallic DNA-B12 Conjugates as Potential Oligonucleotide Vectors: Synthesis and Structural and Binding Studies with Human Cobalamin-Transport Proteins

The synthesis and structural characterization of Co-(dN)₂₅ -Cbl (Cbl: cobalamin; dN: deoxynucleotide) and Co-(dN)₃₉ -Cbl, which are organometallic DNA-B₁₂ conjugates with single DNA strands consisting of 25 and 39 deoxynucleotides, respectively, and binding studies of these two DNA-Cbl conjugates to three homologous human Cbl transporting proteins, transcobalamin (TC), intrinsic factor (IF), and haptocorrin (HC), are reported. This investigation tests the suitability of such DNA-Cbls for the task of eventual in vivo oligonucleotide delivery. The binding of DNA-Cbl to TC, IF, and HC was investigated in competition with either a fluorescent Cbl derivative and Co-(dN)₂₅ -Cbl, or radiolabeled vitamin B₁₂ (⁵⁷ Co-CNCbl) and Co-(dN)₂₅ -Cbl or Co-(dN)₃₉ -Cbl. Binding of the new DNA-Cbl conjugates was fast and tight with TC, but poorer with HC and IF, which extends a similar original finding with the simpler DNA-Cbl, Co-(dN)₁₈ -Cbl. The contrasting affinities of TC versus IF and HC for the DNA-Cbl conjugates are rationalized herein by a stepwise mechanism of Cbl binding. Critical contributions to overall affinity result from gradual conformational adaptations of the Cbl-binding proteins to the DNA-Cbl, which is first bound to the respective β domains. This transition is fast with TC, but slow with IF and HC, with which weaker binding results. The invariably tight interaction of the DNA-Cbl conjugates with TC makes the Cbl moiety a potential natural vector for the specific delivery of oligonucleotide loads from the blood into cells.

Structure of the human transcobalamin beta domain in four distinct states

Vitamin B12 (cyanocobalamin, CNCbl) is an essential cofactor-precursor for two biochemical reactions in humans. When ingested, cobalamins (Cbl) are transported via a multistep transport system into the bloodstream, where the soluble protein transcobalamin (TC) binds Cbl and the complex is taken up into the cells via receptor mediated endocytosis. Crystal structures of TC in complex with CNCbl have been solved previously. However, the initial steps of holo-TC assembly have remained elusive. Here, we present four crystal structures of the beta domain of human TC (TC-beta) in different substrate-bound states. These include the apo and CNCbl-bound states, providing insight into the early steps of holo-TC assembly. We found that in vitro assembly of TC-alpha and TC-beta to a complex was Cbl-dependent. We also determined the structure of TC-beta in complex with cobinamide (Cbi), an alternative substrate, shedding light on the specificity of TC. We finally determined the structure of TC-beta in complex with an inhibitory antivitamin B12 (anti-B12). We used this structure to model the binding of anti-B12 into full-length holo-TC and could rule out that the inhibitory function of anti-B12 was based on an inability to form a functional complex with TC.

Conformational Change of a Tryptophan Residue in BtuF Facilitates Binding and Transport of Cobinamide by the Vitamin B12 Transporter BtuCD-F

BtuCD-F is an ABC transporter that mediates cobalamin uptake into Escherichia coli. Early in vivo data suggested that BtuCD-F might also be involved in the uptake of cobinamide, a cobalamin precursor. However, neither was it demonstrated that BtuCD-F indeed transports cobinamide, nor was the structural basis of its recognition known. We synthesized radiolabeled cyano-cobinamide and demonstrated BtuCD-catalyzed in vitro transport, which was ATP- and BtuF-dependent. The crystal structure of cobinamide-bound BtuF revealed a conformational change of a tryptophan residue (W66) in the substrate binding cleft compared to the structure of cobalamin-bound BtuF. High-affinity binding of cobinamide was dependent on W66, because mutation to most other amino acids substantially reduced binding. The structures of three BtuF W66 mutants revealed that tight packing against bound cobinamide was only provided by tryptophan and phenylalanine, in line with the observed binding affinities. In vitro transport rates of cobinamide and cobalamin were not influenced by the substitutions of BtuF W66 under the experimental conditions, indicating that W66 has no critical role in the transport reaction. Our data present the molecular basis of the cobinamide versus cobalamin specificity of BtuCD-F and provide tools for in vitro cobinamide transport and binding assays.

Photodissociation of ethylphenylcobalamin antivitamin B12

Biologically active forms of cobalamins are crucial cofactors in biochemical reactions and these metabolites can be inhibited by their structurally similar analogues known as antivitamins B₁₂.

Structural basis of transcobalamin recognition by human CD320 receptor

Cellular uptake of vitamin B12 (cobalamin) requires capture of transcobalamin (TC) from the plasma by CD320, a ubiquitous cell surface receptor of the LDLR family. Here we present the crystal structure of human holo-TC in complex with the extracellular domain of CD320, visualizing the structural basis of the TC-CD320 interaction. The observed interaction chemistry can rationalize the high affinity of CD320 for TC and lack of haptocorrin binding. The in vitro affinity and complex stability of TC-CD320 were quantitated using a solid-phase binding assay and thermostability analysis. Stable complexes with TC were also observed for the disease-causing CD320ΔE88 mutant and for the isolated LDLR-A2 domain. We also determined the structure of the TC-CD320ΔE88 complex, which revealed only minor changes compared with the wild-type complex. Finally, we demonstrate significantly reduced in vitro affinity of TC for CD320 at low pH, recapitulating the proposed ligand release during the endocytic pathway.

Cellular uptake of vitamin B12 (cobalamin) requires the binding of holo-transcobalamin (TC) from plasma by CD320. Here, the authors report the structure of a complex between CD320 and TC loaded with cyanocobalamin, alongside additional functional analysis.

Vitamin B12 Suitably Tailored for Disulfide-Based Conjugation

Vitamin B12 has been proposed to be a natural vector for the in vivo delivery of biologically active compounds. Most synthetic methodologies leading to vitamin B12 conjugates involve functionalization at the 5' position via either carbamate-based linkages or using copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), resulting in stable conjugates that are not cleaved within the cell. We have developed a novel vitamin B12 derivative suitably tailored for disulfide-based conjugation that can undergo cleavage in the presence of glutathione, the most abundant thiol in mammalian cells. This active compound is simple to prepare and allows for the direct disulfide-based attachment of therapeutic cargos.

Vitamin B-12 and Perinatal Health

Vitamin B-12 deficiency (<148 pmol/L) is associated with adverse maternal and neonatal outcomes, including developmental anomalies, spontaneous abortions, preeclampsia, and low birth weight (<2500 g). The importance of adequate vitamin B-12 status periconceptionally and during pregnancy cannot be overemphasized, given its fundamental role in neural myelination, brain development, and growth. Infants born to vitamin B-12-deficient women may be at increased risk of neural tube closure defects, and maternal vitamin B-12 insufficiency (<200 pmol/L) can impair infant growth, psychomotor function, and brain development, which may be irreversible. However, the underlying causal mechanisms are unknown. This review was conducted to examine the evidence that links maternal vitamin B-12 status and perinatal outcomes. Despite the high prevalence of vitamin B-12 deficiency and associated risk of pregnancy complications, few prospective studies and, to our knowledge, only 1 randomized trial have examined the effects of vitamin B-12 supplementation during pregnancy. The role of vitamin B-12 in the etiology of adverse perinatal outcomes needs to be elucidated to inform public health interventions.

Recent trends in the development of vitamin B12 derivatives for medicinal applications

This Feature Article highlights recent developments in the field of vitamin B12 derivatives for medicinal applications. The following topics are emphasized: (1) the development of aquacorrinoids for cyanide detection and detoxification, (2) the use of vitamin B12 conjugates and (3) antivitamins B12 for therapy and diagnosis, and (4) the design of corrinoids as activators of soluble guanylyl cyclase (sGC).

Antivitamins B12--A Structure- and Reactivity-Based Concept

B12 -antimetabolites are compounds that counteract the physiological effects of vitamin B12 and related natural cobalamins. Presented here is a structure- and reactivity-based concept of the specific 'antivitamins B12 ': it refers to analogues of vitamin B12 that display high structural similarity to the vitamin and are 'locked chemically' to prevent their metabolic conversion into the crucial organometallic B12 -cofactors. Application of antivitamins B12 to healthy laboratory animals is, thus, expected to induce symptoms of B12 -deficiency. Antivitamins B12 may, hence, be helpful in elucidating still largely puzzling pathophysiological phenomena associated with B12 -deficiency, and also in recognizing physiological roles of B12 that probably still remain to be discovered.

Antivitamins for Medicinal Applications

Antivitamins represent a broad class of compounds that counteract the essential effects of vitamins. The symptoms triggered by such antinutritional factors resemble those of vitamin deficiencies, but can be successfully reversed by treating patients with the intact vitamin. Despite being undesirable for healthy organisms, the toxicities of these compounds present considerable interest for biological and medicinal purposes. Indeed, antivitamins played fundamental roles in the development of pioneering antibiotic and antiproliferative drugs, such as prontosil and aminopterin. Their development and optimisation were made possible by the study, throughout the 20th century, of the vitamins' and antivitamins' functions in metabolic processes. However, even with this thorough knowledge, commercialised antivitamin-based drugs are still nowadays limited to antagonists of vitamins B9 and K. The antivitamin field thus still needs to be explored more intensely, in view of the outstanding therapeutic success exhibited by several antivitamin-based medicines. Here we summarise historical achievements and discuss critically recent developments, opportunities and potential limitations of the antivitamin approach, with a special focus on antivitamins K, B9 and B12 .

Basal Gnathostomes provide unique insights into the evolution of vitamin B12 binders

The uptake and transport of vitamin B12 (cobalamin; Cbl) in mammals involves a refined system with three evolutionarily related transporters: transcobalamin 1 (Tcn1), transcobalamin 2 (Tcn2), and the gastric intrinsic factor (Gif). Teleosts have a single documented binder with intermediate features to the human counterparts. Consequently, it has been proposed that the expansion of Cbl binders occurred after the separation of Actinopterygians. Here, we demonstrate that the diversification of this gene family took place earlier in gnathostome ancestry. Our data indicates the presence of single copy orthologs of the Sarcopterygii/Tetrapoda duplicates Tcn1 and Gif, and Tcn2, in Chondrichthyes. In addition, a highly divergent Cbl binder was found in the Elasmobranchii. We unveil a complex scenario forged by genome, tandem duplications and lineage-specific gene loss. Our findings suggest that from an ancestral transporter, exhibiting large spectrum and high affinity binding, highly specific Cbl transporters emerged through gene duplication and mutations at the binding pocket.

Organometallic B12-DNA conjugate: synthesis, structure analysis, and studies of binding to human B12-transporter proteins

Design, synthesis, and structural characterization of a B12-octadecanucleotide are presented herein, a new organometallic B12-DNA conjugate. In such covalent conjugates, the natural B12 moiety may be a versatile vector for controlled in vivo delivery of oligonucleotides to cellular targets in humans and animals, through the endogenous B12 transport systems. Binding of the organometallic B12 octadecanucleotide to the three important human proteins of B12 transport was studied, to examine its structural suitability for the task of eventual in vivo oligonucleotide delivery. Binding was efficient with transcobalamin (TC), but not so efficient with the homologous glycoproteins intrinsic factor and haptocorrin. Binding of the B12 octadecanucleotide to TC suggests the capacity of the B12 moiety to serve as a natural vector for specific transport of single stranded, organometallic oligonucleotide loads from the blood stream into cells.

Structural basis for universal corrinoid recognition by the cobalamin transport protein haptocorrin

Cobalamin (Cbl; vitamin B12) is an essential micronutrient synthesized only by bacteria. Mammals have developed a sophisticated uptake system to capture the vitamin from the diet. Cbl transport is mediated by three transport proteins: transcobalamin, intrinsic factor, and haptocorrin (HC). All three proteins have a similar overall structure but a different selectivity for corrinoids. Here, we present the crystal structures of human HC in complex with cyanocobalamin and cobinamide at 2.35 and 3.0 Å resolution, respectively. The structures reveal that many of the interactions with the corrin ring are conserved among the human Cbl transporters. However, the non-conserved residues Asn-120, Arg-357, and Asn-373 form distinct interactions allowing for stabilization of corrinoids other than Cbl. A central binding motif forms interactions with the e- and f-side chains of the corrin ring and is conserved in corrinoid-binding proteins of other species. In addition, the α- and β-domains of HC form several unique interdomain contacts and have a higher shape complementarity than those of intrinsic factor and transcobalamin. The stabilization of ligands by all of these interactions is reflected in higher melting temperatures of the protein-ligand complexes. Our structural analysis offers fundamental insights into the unique binding behavior of HC and completes the picture of Cbl interaction with its three transport proteins.