Conformational changes of transcobalamin induced by aquocobalamin binding. Mechanism of substitution of the cobalt-coordinated group in the bound ligand

Macro-B12 and Unexpectedly High Levels of Plasma B12: A Critical Review

A low total plasma vitamin B12 supports a clinical suspicion of B12 deficiency, while the interpretation of an unexpectedly normal/high level is marred by controversies. Here, we critically review current knowledge on B12 in blood plasma, including the presence of the so-called "macro-B12". The latter form is most often defined as the fraction of B12 that can be removed by precipitation with polyethylene glycol (PEG), a nonspecific procedure that also removes protein polymers and antibody-bound analytes. Plasma B12 includes B12 attached to transcobalamin and haptocorrin, and an increased concentration of one or both proteins almost always causes an elevation of B12. The total plasma B12 is measured by automated competitive binding assays, often incorrectly referred to as immunoassays, since the binding protein is intrinsic factor and not an antibody. An unexpectedly high level of B12 may be further explored using immunological measurements of haptocorrin and transcobalamin (optionally combined with e.g., size-exclusion chromatography). Nonspecific methods, such as PEG precipitation, are likely to give misleading results and cannot be recommended. Currently, the need for evaluation of a high B12 of unknown etiology is limited since other tests (such as measurements of methylmalonic acid) may better guide the diagnosis of B12 deficiency.

Vitamin B12 binding to mutated human transcobalamin, in-silico study of TCN2 alanine scanning and ClinVar missense mutations/SNPs

Many missense mutations/SNPs of the TCN2 gene (which yield Transcobalamin (TC)) were reported in the literature but no study is available about their effect on binding to vitamin B12(B12) at the structural level experimentally nor computationally. Predict the effect of TC missense mutations/SNPs on binding affinity to B12 and characterize their contacts to B12 at the structural level. TC-B12 binding energy difference from the wildtype (ΔΔGmut) was calculated for 378 alanine scanning mutations and 76 ClinVar missense mutations, repeated on two distinct X-ray structures of holoTC namely 2BB5 and 4ZRP. Destabilizing mutations then went through 100 ns molecular dynamics simulation to study their effect on TC-B12 binding at the structural level employing 2BB5 structure. Out of the studied 454 mutations (378 alanine mutations + 76 ClinVar mutations), 19 were destabilizing representing 17 amino acid locations. Mutation energy results show a neutral effect on B12 binding of several missense SNPs reported in the literature including I23V, G94S, R215W, P259R, S348F, L376S, and R399Q. Compared to the wildtype, all the destabilizing mutations have higher average RMSD-Ligand in the last 25% of the MD simulation trajectories and lower average hydrogen bond count while the other parameters vary. Previously reported TCN2 SNPs with an unknown effect on TC-B12 binding were found to have a neutral effect in the current study based on mutation energy calculations. Also, we reported 17 possible amino acids that destabilize TC-B12 binding upon mutation (four listed in ClinVar) and studied their structural effect computationally. Communicated by Ramaswamy H. Sarma.

Kinetic analysis of transcellular passage of the cobalamin-transcobalamin complex in Caco-2 monolayers

We suggest a novel kinetic approach to quantifying receptor–ligand interactions via the cellular transport and/or accumulation of the ligand. The system of cobalamin (Cbl, vitamin B12) transport was used as a model, because Cbl is an obligatory cofactor, taken up by animal cells with the help of a transport protein and a membrane receptor. Bovine transcobalamin (bTC) stimulated the cellular accumulation and transcytosis of radioactive [⁵⁷Co]Cbl in polarized monolayers of Caco-2 cells. The bovine protein was much more efficient than human TC. The transport was inhibited in a dose-dependent manner by the unlabeled bTC-Cbl complex, the ligand-free bTC, and the receptor-associated protein (RAP). This inhibition pattern implied the presence of a megalin-like receptor. Quantitative assessment of kinetic records by the suggested method revealed the apparent concentration of receptors in vitro (≈15 nM), as well as the dissociation constants of bTC–Cbl (K_d = 13 nM) and RAP (K_d = 1.3 nM). The data were used to estimate the effective luminal concentrations of TC-specific receptors in kidneys (3.8 µM) and intestine (50 nM), the tissues resembling polarized Caco-2 cells.

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.

Tissue distribution of oral vitamin B12 is influenced by B12 status and B12 form: an experimental study in rats

Purpose

Hydroxocobalamin (HOCbl) is the dominating Cbl form in food, whereas cyanocobalamin (CNCbl) is common in vitamin pills and oral supplements. This study compares single-dose absorption and distribution of oral HO[⁵⁷Co]Cbl and CN[⁵⁷Co]Cbl in Cbl-deficient and normal rats.

Methods

Male Wistar rats (7 weeks) were fed a 14-day diet with (n = 15) or without (n = 15) Cbl. We compared the uptakes of HO[⁵⁷Co]Cbl (free or bound to bovine transcobalamin) and free CN[⁵⁷Co]Cbl administered by gastric gavage (n = 5 in each diet group). Rats were sacrificed after 24 h. Blood, liver, kidney, brain, heart, spleen, intestines, skeletal muscle, 24-h urine and faeces were collected, and the content of [⁵⁷Co]Cbl was measured. Endogenous Cbl in tissues and plasma was analysed by routine methods.

Results

Mean endogenous plasma-Cbl was sevenfold lower in deficient vs. normal rats (190 vs. 1330 pmol/L, p < 0.0001). Cbl depletion increased endogenous Cbl ratios (tissue/plasma = k_in/k_out) in all organs except for the kidney, where the ratio decreased considerably. Twenty-four-hour accumulation of labelled Cbl showed that HOCbl > CNCbl (liver) and CNCbl > HOCbl (brain, muscle and plasma).

Conclusions

The Cbl status of rats and the administered Cbl form influence 24-h Cbl accumulation in tissues and plasma.

Electronic supplementary material

The online version of this article (doi:10.1007/s00394-017-1424-0) contains supplementary material, which is available to authorized users.

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.

Absorption and retention of free and milk protein-bound cyano- and hydroxocobalamins. An experimental study in rats

INTRODUCTION

Cobalamin/Vitamin B12 (Cbl) is an essential vitamin, supplied mainly as hydroxocobalamin (OHCbl) by animal products, including cows' milk. Cyanocobalamin (CNCbl) is the usual form in vitamin pills. The aim was to explore absorption and tissue accumulation of two Cbl forms, administered alone or bound to milk protein.

MATERIALS AND METHODS

We synthesized labeled OH[(57)Co]Cbl from commercially available CN[(57)Co]Cbl. Recombinant bovine transcobalamin (rbTC) was produced in yeast and skimmed milk obtained off the shelf. Male Wistar rats (250-300 g) received labeled Cbl by gastric gavage. First, we administered CN[(57)Co]Cbl, free or rbTC-bound (n = 15 in each group). Rats were sacrificed after two, 24, and 48 h. In the following studies, rats were sacrificed after 24 h. We compared absorption of free or rbTC-bound CN[(57)Co]Cbl added to cows' milk and analogous absorption of OH[(57)Co]Cbl, free or rbTC-bound, to absorption of free CN[(57)Co]Cbl, (n = 10 in each group). Blood, tissues, 24-h urine and feces were collected. Labeled Cbl was measured using a gamma counter. Results are expressed as percentage of administered dose.

RESULTS

Absorptions of CNCbl and OHCbl were neither influenced by rbTC-binding nor administration in milk. Absorption increased in the first 24 h with no further tissue accumulation during the subsequent 24 h. Accumulation of free CNCbl and (OHCbl) was 1.4, (4.1) (liver); 20.2, (16.4) (kidney); and 0.05, (0.02) (plasma)% 24 h after administration. Total organ accumulations were 21.6, (20.5)%. While total accumulations of CNCbl and OHCbl were equal, distributions between liver, kidney, and plasma showed significant differences (p < 0.0001; p = 0.01; p < 0.0001).

CONCLUSIONS

Cbl added to milk (spiked with rbTC) has high bioavailability matching that of free Cbl. OHCbl and CNCbl are absorbed equally well, but much more OHCbl accumulated in the liver. Benefits of oral supplementation with OHCbl compared to CNCbl should be investigated.

The extracellular heme-binding protein HbpS from the soil bacterium Streptomyces reticuli is an aquo-cobalamin binder

The extracellular protein HbpS from Streptomyces reticuli interacts with iron ions and heme. It also acts in concert with the two-component sensing system SenS-SenR in response to oxidative stress. Sequence comparisons suggested that the protein may bind a cobalamin. UV-visible spectroscopy confirmed binding (Kd = 34 μm) to aquo-cobalamin (H2OCbl(+)) but not to other cobalamins. Competition experiments with the H2OCbl(+)-coordinating ligand CN(-) and comparison of mutants identified a histidine residue (His-156) that coordinates the cobalt ion of H2OCbl(+) and substitutes for water. HbpS·Cobalamin lacks the Asp-X-His-X-X-Gly motif seen in some cobalamin binding enzymes. Preliminary tests showed that a related HbpS protein from a different species also binds H2OCbl(+). Furthermore, analyses of HbpS-heme binding kinetics are consistent with the role of HbpS as a heme-sensor and suggested a role in heme transport. Given the high occurrence of HbpS-like sequences among Gram-positive and Gram-negative bacteria, our findings suggest a great functional versatility among these proteins.

Physiological and molecular aspects of cobalamin transport

Minute doses of a complex cofactor cobalamin (Cbl, vitamin B12) are essential for metabolism. The nutritional chain for humans includes: (1) production of Cbl by bacteria in the intestinal tract of herbivores; (2) accumulation of the absorbed Cbl in animal tissues; (3) consumption of food of animal origin. Most biological sources contain both Cbl and its analogues, i.e. Cbl-resembling compounds physiologically inactive in animal cells. Selective assimilation of the true vitamin requires an interplay between three transporting proteins - haptocorrin (HC), intrinsic factor (IF), transcobalamin (TC) - and several receptors. HC is present in many biological fluids, including gastric juice, where it assists in disposal of analogues. Gastric IF selectively binds dietary Cbl and enters the intestinal cells via receptor-mediated endocytosis. Absorbed Cbl is transmitted to TC and delivered to the tissues with blood flow. The complex transport system guarantees a very efficient uptake of the vitamin, but failure at any link causes Cbl-deficiency. Early detection of a negative B12 balance is highly desirable to prevent irreversible neurological damages, anaemia and death in aggravated cases. The review focuses on the molecular mechanisms of cobalamin transport with emphasis on interaction of corrinoids with the specific proteins and protein-receptor recognition. The last section briefly describes practical aspects of recent basic research concerning early detection of B12-related disorders, medical application of Cbl-conjugates, and purification of corrinoids from biological samples.

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.

Vitamin B12 regulates photosystem gene expression via the CrtJ antirepressor AerR in Rhodobacter capsulatus

The tetrapyrroles haem, bacteriochlorophyll and cobalamin (B12 ) exhibit a complex interrelationship regarding their synthesis. In this study, we demonstrate that AerR functions as an antirepressor of the tetrapyrrole regulator CrtJ. We show that purified AerR contains B12 that is bound to a conserved histidine (His145) in AerR. The interaction of AerR to CrtJ was further demonstrated in vitro by pull down experiments using AerR as bait and quantified using microscale thermophoresis. DNase I DNA footprint assays show that AerR containing B12 inhibits CrtJ binding to the bchC promoter. We further show that bchC expression is greatly repressed in a B12 auxotroph of Rhodobacter capsulatus and that B12 regulation of gene expression is mediated by AerR's ability to function as an antirepressor of CrtJ. This study thus provides a mechanism for how the essential tetrapyrrole, cobalamin controls the synthesis of bacteriochlorophyll, an essential component of the photosystem.

Hydroxocobalamin association during cell culture results in pink therapeutic proteins

Process control of protein therapeutic manufacturing is central to ensuring the product is both safe and efficacious for patients. In this work, we investigate the cause of pink color variability in development lots of monoclonal antibody (mAb) and Fc-fusion proteins. Results show pink-colored product generated during manufacturing is due to association of hydroxocobalamin (OH-Cbl), a form of vitamin B12. OH-Cbl is not part of the product manufacturing process; however we found cyanocobalamin (CN-Cbl) in cell culture media converts to OH-Cbl in the presence of light. OH-Cbl can be released from mAb and Fc-fusion proteins by conversion with potassium cyanide to CN-Cbl, which does not bind. By exploiting the differential binding of CN-Cbl and OH-Cbl, we developed a rapid and specific assay to accurately measure B12 levels in purified protein. Analysis of multiple products and lots using this technique gives insight into color variability during manufacturing.

A single rainbow trout cobalamin-binding protein stands in for three human binders

Cobalamin uptake and transport in mammals are mediated by three cobalamin-binding proteins: haptocorrin, intrinsic factor, and transcobalamin. The nature of cobalamin-binding proteins in lower vertebrates remains to be elucidated. The aim of this study was to characterize the cobalamin-binding proteins of the rainbow trout (Oncorhynchus mykiss) and to compare their properties with those of the three human cobalamin-binding proteins. High cobalamin-binding capacity was found in trout stomach (210 pmol/g), roe (400 pmol/g), roe fluid (390 nmol/liter), and plasma (2500 nmol/liter). In all cases, it appeared to be the same protein based on analysis of partial sequences and immunological responses. The trout cobalamin-binding protein was purified from roe fluid, sequenced, and further characterized. Like haptocorrin, the trout cobalamin-binding protein was stable at low pH and had a high binding affinity for the cobalamin analog cobinamide. Like haptocorrin and transcobalamin, the trout cobalamin-binding protein was present in plasma and recognized ligands with altered nucleotide moiety. Like intrinsic factors, the trout cobalamin-binding protein was present in the stomach and resisted degradation by trypsin and chymotrypsin. It also resembled intrinsic factor in the composition of conserved residues in the primary cobalamin-binding site in the C terminus. The trout cobalamin-binding protein was glycosylated and displayed spectral properties comparable with those of haptocorrin and intrinsic factor. In conclusion, only one soluble cobalamin-binding protein was identified in the rainbow trout, a protein that structurally behaves like an intermediate between the three human cobalamin-binding proteins.

Mechanistic studies on the reaction between cob(II)alamin and peroxynitrite: evidence for a dual role for cob(II)alamin as a scavenger of peroxynitrous acid and nitrogen dioxide

Peroxynitrite/peroxynitrous acid (ONOO(-)/ONOOH; pK(a(ONOOH)) =6.8) is implicated in multiple chronic inflammatory and neurodegenerative diseases. Both mammalian B(12)-dependent enzymes are inactivated under oxidative stress conditions. We report studies on the kinetics of the reaction between peroxynitrite/peroxynitrous acid and a major intracellular vitamin B(12) form, cob(II)alamin (Cbl(II)), using stopped-flow spectroscopy. The pH dependence of the reaction is consistent with peroxynitrous acid reacting directly with Cbl(II) to give cob(III)alamin (Cbl(III)) and (.)NO(2) , followed by a subsequent rapid reaction between (.)NO(2) and a second molecule of Cbl(II) to primarily form nitrocobalamin. In support of this mechanism, a Cbl(II)/ONOO(H) stoichiometry of 2:1 is observed at pH 7.35 and 12.0. The final major Cbl(III) product observed (nitrocobalamin or hydroxycobalamin) depends on the solution pH. Analysis of the reaction products in the presence of tyrosine-a well-established (.)NO(2) scavenger-reveals that Cbl(II) reacts with (.)NO(2) at least an order of magnitude faster than tyrosine itself. Given that protein-bound Cbl is accessible to small molecules, it is likely that enzyme-bound and free intracellular Cbl(II) molecules are rapidly oxidized to inactive Cbl(III) upon exposure to peroxynitrite or (.)NO(2).

Identification of multidrug resistance protein 1 (MRP1/ABCC1) as a molecular gate for cellular export of cobalamin

Cobalamin (Cbl, vitamin B(12)) deficiency in humans is a cause of hematologic and neurologic disorders. We show here that the cellular export of Cbl, in contrast to the carrier- and receptor-dependent cellular import of Cbl, occurs by transmembrane transport of "free" Cbl. Screening of candidate transporters by cellular gene silencing showed a role in cellular Cbl efflux of the ATP-binding cassette (ABC)-drug transporter, ABCC1, alias multidrug resistance protein 1 (MRP1), which is present in the basolateral membrane of intestinal epithelium and in other cells. The ability of MRP1 to mediate ATP-dependent Cbl transport was confirmed by vesicular transport experiments, and a physiologic role of MRP1 in mammalian Cbl homeostasis is indicated by the phenotype of knockout mice with targeted disruption of MRP1. These animals have a reduced concentration of Cbl in plasma and in the storage organs liver and kidney. In contrast, Cbl accumulates in the terminal part of the intestine of these mice, suggesting a functional malabsorption because of a lower epithelial basolateral Cbl efflux. The identification of this Cbl export mechanism now allows the delineation of a coherent pathway for Cbl trafficking from food to the body cells.

Design, synthesis, and characterization of fluorescent cobalamin analogues with high quantum efficiencies

Cobalamin tethered to fluorescein or Rhodamine 6G has been synthesized and characterized. The fluorophore is conjugated to the ribose-5'-OH of cobalamin through a rigid linker to prevent the fluorophore from folding back through space and interacting with the corrin ring of cobalamin. This increases the fluorescence quantum yield. This new family of cobalamin analogues may be suitable for use as tumor markers to tag cancer cells for surgical resection.

Active B12: a rapid, automated assay for holotranscobalamin on the Abbott AxSYM analyzer

BACKGROUND

Conventional tests for vitamin B(12) deficiency measure total serum vitamin B12, whereas only that portion of vitamin B12 carried by transcobalamin (holotranscobalamin) is metabolically active. Measurement of holotranscobalamin (holoTC) may be more diagnostically accurate for detecting B(12) deficiency that requires therapy. We developed an automated assay for holoTC that can be used on the Abbott AxSYM immunoassay analyzer.

METHODS

AxSYM Active B12 is a 2-step sandwich microparticle enzyme immunoassay. In step 1, a holoTC-specific antibody immobilized onto latex microparticles captures holoTC in samples of serum or plasma. In step 2, the captured holoTC is detected with a conjugate of alkaline phosphatase and antiTC antibody.

RESULTS

Neither apoTC nor haptocorrin exhibited detectable cross-reactivity. The detection limit was < or = 0.1 pmol/L. Within-run and total imprecision (CV ranges) were 3.4%-5.1% and 6.3%-8.5%, respectively. Assay CVs were < 20% from at least 3 pmol/L to 107 pmol/L. With diluted serum samples, measured concentrations were 104%-114% of the expected values in the working range of the assay. No interference from bilirubin, hemoglobin, triglycerides, erythrocytes, rheumatoid factor, or total protein was detected at expected (abnormal) concentrations. A comparison of the AxSYM Active B12 assay with a commercial RIA for holoTC yielded the regression equation: AxSYM = 0.98RIA + 4.7 pmol/L (S(y x), 11.4 pmol/L; n = 204). Assay throughput was 45 tests/h. A 95% reference interval of 19-134 pmol/L holoTC was established with samples from 292 healthy individuals.

CONCLUSIONS

The AxSYM Active B12 assay allows rapid, precise, sensitive, specific, and automated measurement of human holoTC in serum and plasma.

Mechanisms of discrimination between cobalamins and their natural analogues during their binding to the specific B12-transporting proteins

Three proteins, intrinsic factor (IF), transcobalamin (TC), and haptocorrin (HC), all have an extremely high affinity for the cobalamins (Cbls, Kd approximately 5 fM) but discriminate these physiological ligands from Cbl analogues with different efficiencies decreasing in the following order: IF > TC > HC. We investigated interactions of these proteins with a number of ligands: Cbl, fluorescent conjugate CBC, two base-off analogues [pseudo-coenzyme B12 (pB) and adenosyl factor A (fA)], and a baseless corrinoid cobinamide. Protein-ligand encounter and the following internal rearrangements in both molecules were registered as a change in the fluorescence of CBC (alone or mixed with other ligands), a transition in absorbance of pB and fA (base-off --> on-base conversion), and alterations in the molecular mass of two split IF domains. The greater complexity of the binding kinetics followed better Cbl specificity (HC < TC < IF). On the basis of the experimental results, we propose a general binding model with three major steps: (1) initial attachment of the ligand to the high-affinity C-domain, (2) primary assembly of N- and C-domains, and (3) slow adjustments and fixation of the ligand at the domain-domain interface. Since step 3 was characteristic of highly specific TC and especially IF, we suggest its particular importance for ligand recognition. The designed models revealed the absolute Kd values for a group of analogues. Calculations show that most of them could potentially bind to the specific transporters IF and TC under physiological conditions. Implications of this finding and the protective role of HC are discussed.

X-ray structural characterization of imidazolylcobalamin and histidinylcobalamin: cobalamin models for aquacobalamin bound to the B12 transporter protein transcobalamin

The X-ray structures of imidazolylcobalamin (ImCbl) and histidinylcobalamin (HisCbl) are reported. These structures are of interest given that the recent structures of human and bovine transcobalamin prepared in their holo forms from aquacobalamin show a histidine residue of the metalloprotein bound at the beta-axial site of the cobalamin (Wuerges, J. et al. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 4386-4391). The beta-axial Co-N bond distances for ImCbl and HisCbl are 1.94(1) and 1.951(7) A, respectively. The alpha-axial Co-N bond distances to the 5,6-dimethylbenzimidazole are 2.01(1) and 1.979(8) A for ImCbl and HisCbl, respectively, and are typical for cobalamins with weak sigma-donor ligands at the beta-axial site. The corrin fold angles of 11.8(3) degrees (ImCbl) and 12.0(3) degrees (HisCbl) are smaller than those typically observed for cobalamins.

Application of a fluorescent cobalamin analogue for analysis of the binding kinetics. A study employing recombinant human transcobalamin and intrinsic factor

Fluorescent probe rhodamine was appended to 5' OH-ribose of cobalamin (Cbl). The prepared conjugate, CBC, bound to the transporting proteins, intrinsic factor (IF) and transcobalamin (TC), responsible for the uptake of Cbl in an organism. Pronounced increase in fluorescence upon CBC attachment facilitated detailed kinetic analysis of Cbl binding. We found that TC had the same affinity for CBC and Cbl (K(d) = 5 x 10(-15) m), whereas interaction of CBC with the highly specific protein IF was more complex. For instance, CBC behaved normally in the partial reactions CBC + IF(30) and CBC + IF(20) when binding to the isolated IF fragments (domains). The ligand could also assemble them into a stable complex IF(30)-CBC-IF(20) with higher fluorescent signal. However, dissociation of IF(30)-CBC-IF(20) and IF-CBC was accelerated by factors of 3 and 20, respectively, when compared to the corresponding Cbl complexes. We suggest that the correct domain-domain interactions are the most important factor during recognition and fixation of the ligands by IF. Dissociation of IF-CBC was biphasic, and existence of multiple protein-analogue complexes with normal and partially corrupted structure may explain this behaviour. The most stable component had K(d) = 1.5 x 10(-13) m, which guarantees the binding of CBC to IF under physiological conditions. The specific intestinal receptor cubilin bound both IF-CBC and IF-Cbl with equal affinity. In conclusion, the fluorescent analogue CBC can be used as a reporting agent in the kinetic studies, moreover, it seems to be applicable for imaging purposes in vivo.

Structural basis for mammalian vitamin B12 transport by transcobalamin

Cobalamin (Cbl, vitamin B(12)) serves for two essential cofactors in mammals. The pathway for its intestinal absorption, plasma transport, and cellular uptake uses cell surface receptors and three Cbl-transporting proteins, haptocorrin, intrinsic factor, and transcobalamin (TC). We present the structure determination of a member of the mammalian Cbl-transporter family. The crystal structures of recombinant human and bovine holo-TCs reveal a two-domain architecture, with an N-terminal alpha(6)-alpha(6) barrel and a smaller C-terminal domain. One Cbl molecule in base-on conformation is buried inside the domain interface. Structural data combined with previous binding assays indicate a domain motion in the first step of Cbl binding. In a second step, the weakly coordinated ligand H(2)O at the upper axial side of added H(2)O-Cbl is displaced by a histidine residue of the alpha(6)-alpha(6) barrel. Analysis of amino acid conservation on TC's surface in orthologous proteins suggests the location of the TC-receptor-recognition site in an extended region on the alpha(6)-alpha(6) barrel. The TC structure allows for the mapping of sites of amino acid variation due to polymorphisms of the human TC gene. Structural information is used to predict the overall fold of haptocorrin and intrinsic factor and permits a rational approach to the design of new Cbl-based bioconjugates for diagnostic or therapeutic drug delivery.

Characterization of a monoclonal antibody with specificity for holo-transcobalamin

Background

Holotranscobalamin, cobalamin-saturated transcobalamin, is the minor fraction of circulating cobalamin (vitamin B12), which is available for cellular uptake and hence is physiologically relevant. Currently, no method allows simple, direct quantification of holotranscobalamin. We now report on the identification and characterization of a monoclonal antibody with a unique specificity for holotranscobalamin.

Methods

The specificity and affinity of the monoclonal antibodies were determined using surface plasmon resonance and recombinant transcobalamin as well as by immobilizing the antibodies on magnetic microspheres and using native transcobalamin in serum. The epitope of the holotranscobalamin specific antibody was identified using phage display and comparison to a de novo generated three-dimensional model of transcobalamin using the program Rosetta. A direct assay for holotrnscobalamin in the ELISA format was developed using the specific antibody and compared to the commercial assay HoloTC RIA.

Results

An antibody exhibiting >100-fold specificity for holotranscobalamin over apotranscobalamin was identified. The affinity but not the specificity varied inversely with ionic strength and pH, indicating importance of electrostatic interactions. The epitope was discontinuous and epitope mapping of the antibody by phage display identified two similar motifs with no direct sequence similarity to transcobalamin. A comparison of the motifs with a de novo generated three-dimensional model of transcobalamin identified two structures in the N-terminal part of transcobalamin that resembled the motif. Using this antibody an ELISA based prototype assay was developed and compared to the only available commercial assay for measuring holotranscobalamin, HoloTC RIA.

Conclusion

The identified antibody possesses a unique specificity for holotranscobalamin and can be used to develop a direct assay for the quantification of holotranscobalamin.

Mapping the functional domains of human transcobalamin using monoclonal antibodies

Recombinant human transcobalamin (TC) was probed with 17 monoclonal antibodies (mAbs), using surface plasmon resonance measurements. These experiments identified five distinct epitope clusters on the surface of holo-TC. Western blot analysis of the CNBr cleavage fragments of TC allowed us to distribute the epitopes between two regions, which spanned either the second quarter of the TC sequence GQLA...TAAM(103-198) or the C-terminal peptide LEPA...LVSW(316-427). Proteolytic fragments of TC and the synthetic peptides were used to further specify the epitope map and define the functional domains of TC. Only one antibody showed some interference with cobalamin (Cbl) binding to TC, and the corresponding epitope was situated at the C-terminal stretch TQAS...QLLR(372-399). We explored the receptor-blocking effect of several mAbs and heparin to identify TC domains essential for the interaction between holo-TC and the receptor. The receptor-related epitopes were located within the TC sequence GQLA...HHSV(103-159). The putative heparin-binding site corresponded to a positively charged segment KRSN...RTVR(207-227), which also seemed to be necessary for receptor binding. We conclude that conformational changes in TC upon Cbl binding are accompanied by the convergence of multiple domains, and only the assembled conformation of the protein (i.e. holo-TC) has high affinity for the receptor.

Composite organization of the cobalamin binding and cubilin recognition sites of intrinsic factor

Intrinsic factor (IF(50)) is a cobalamin (Cbl)-transporting protein of 50 kDa, which can be cleaved into two fragments: the 30 kDa N-terminal peptide IF(30) and the 20 kDa C-terminal glycopeptide IF(20). Experiments on binding of Cbl to IF(30), IF(20), and IF(50) revealed comparable association rate constants (k(+)(Cbl) = 4 x 10(6), 14 x 10(6), and 26 x 10(6) M(-1) s(-1), respectively), but the equilibrium dissociation constants were essentially different (K(Cbl) = 200 microM, 0.2 microM, and <or=1 pM, respectively). The smaller fragment, IF(20), had unexpectedly high affinity for Cbl; however, efficient retention of the ligand required the presence of both fragments. Detailed schemes of the interaction of Cbl with IF(50) and with IF(30) and IF(20) are presented, where the sequential attachment of Cbl to the IF(20) and IF(30) domains plays the key role in recognition and retention of the ligand. Each isolated fragment of IF was tested for the binding to the specific receptor cubilin in the presence or absence of Cbl. Neither apo nor holo forms of IF(20) and IF(30) were recognized by the receptor. When two fragments were mixed and incubated with Cbl, they associated into a stable complex, IF(30+20).Cbl, which bound to cubilin as well as the noncleaved IF(50).Cbl complex. We suggest that formation of the cubilin recognition site on IF is caused by assembly of two distant domains, which allows the saturated protein to be recognized by the receptor. The obtained parameters for ligand and receptor binding indicate that both full-length IF(50) and the fragments may be involved in Cbl assimilation.

Cobalamin (vitamin B12) binding, phylogeny, and synteny of human transcobalamin

Selected residues in a highly conserved 15-residue region, 174SVDTAAMAGLAFTC L188 of human transcobalamin (TC), a cobalamin (Cbl: vitamin B12) binding protein, were subjected to site-directed mutagenesis. The mutant constructs were expressed in TC-deficient fibroblasts or in vitro to assess the effect of these mutations on Cbl binding. Phylogenetic analyses and protein parsimony indicated that TC evolved earlier than other mammalian Cbl-binding proteins, intrinsic factor and haptocorrins, and divergence occurred between mouse/rat and human dispersing TC gene to different chromosomes. These studies show that (a) two of the three polar residues, S174, T177, or D176 and two of the three conserved alanine residues, A179 and A184 present in the 15-residue evolutionary conserved region are essential for Cbl-binding by human TC, and (b) TC gene is transferred in a syntenic manner to different chromosomes, at least before the divergence of mouse/rat and human.

Assembly of the Intrinsic Factor Domains and Oligomerization of the Protein in the Presence of Cobalamin

Human intrinsic factor (IF) was purified from the recombinant plant Arabidopsis thaliana by affinity chromatography. Cobalamin (Cbl) saturated protein was separated by gel filtration into peaks I and II, which contained according to SDS electrophoresis the 50 kDa full-length protein IF(50) and a mixture of two fragments, respectively. Two components of peak II were identified as the 30 kDa N-terminal peptide IF(30) and the 20 kDa C-terminal glycopeptide IF(20). Measurements of M(w) under the nondenaturing conditions were conducted by static light scattering. They revealed 100 kDa IF dimers in peak I, whereas 50 kDa cleaved monomers were found in peak II. The protein devoid of Cbl dissociated to the elementary units incapable of association in the absence of Cbl. The individual proteolytic fragments bound Cbl at high concentration of the ligand; however, neither IF(30).Cbl nor IF(20).Cbl oligomerized. A mixture of two fragments IF(30) + IF(20) and Cbl produced a firm complex, IF(30+20).Cbl, which could not associate to dimers. In contrast to IF(30+20).Cbl, the saturated full-length monomers IF(50).Cbl dimerized with K(d) approximately 1 microM. We suggest a two-domain organization of the full-length protein, where two distant units, IF(30) and IF(20), can be assembled only by Cbl. They are connected by a protease-sensitive link, whose native structure is likely to be important for dimerization. However, linkage between two domains is not compulsory for Cbl binding. Advantages of the two-domain structure of IF are discussed.

Equilibrium and kinetic analyses of the interactions between vitamin B(12) binding proteins and cobalamins by surface plasmon resonance

Surface plasmon resonance biosensor analysis was used to evaluate the thermodynamics and binding kinetics of naturally occurring and synthetic cobalamins interacting with vitamin B(12) binding proteins. Cyanocobalamin-b-(5-aminopentylamide) was immobilized on a biosensor chip surface to determine the affinity of different cobalamins for transcobalamin, intrinsic factor, and nonintrinsic factor. A solution competition binding assay, in which a surface immobilized cobalamin analog competes with analyte cobalamin for B(12) protein binding, shows that only recombinant human transcobalamin is sensitive to modification of the corrin ring b-propionamide of cyanocobalamin. A direct binding assay, where recombinant human transcobalamin is conjugated to a biosensor chip, allows kinetic analysis of cobalamin binding. Response data for cyanocobalamin binding to the transcobalamin protein surface were globally fitted to a bimolecular interaction model that includes a term for mass transport. This model yields association and dissociation rate constants of k(a) = 3 x 10(7) M(-1) s(-1) and k(d) = 6 x 10(-4) s(-1), respectively, with an overall dissociation constant of K(D) = 20 pM at 30 degrees C. Transcobalamin binds cyanocobalamin-b-(5-aminopentylamide) with association and dissociation rates that are twofold slower and threefold faster, respectively, than transcobalamin binding to cyanocobalamin. The affinities determined for protein-ligand interaction, using the solution competition and direct binding assays, are comparable, demonstrating that surface plasmon resonance provides a versatile way to study the molecular recognition properties of vitamin B(12) binding proteins.

Comparative analysis of cobalamin binding kinetics and ligand protection for intrinsic factor, transcobalamin, and haptocorrin

Changes in the absorbance spectrum of aquo-cobalamin (Cbl x OH(2)) revealed that its binding to transcobalamin (TC) is followed by slow conformational reorganization of the protein-ligand complex (Fedosov, S. N., Fedosova, N. U., Nexø, E., and Petersen, T. E. (2000) J. Biol. Chem. 275, 11791-11798). Two phases were also observed for TC when interacting with a Cbl-analogue cobinamide (Cbi), but not with other cobalamins. The slow phase had no relation to the ligand recognition, since both Cbl and Cbi bound rapidly and in one step to intrinsic factor (IF) and haptocorrin (HC), namely the proteins with different Cbl specificity. Spectral transformations observed for TC in the slow phase were similar to those upon histidine complexation with Cbl x OH(2) and Cbi. In contrast to a closed structure of TC x Cbl x OH(2), the analogous IF and HC complexes revealed accessibility of Cbl's upper face to the external reagents. The binders decreased sensitivity of adenosyl-Cbl (Cbl x Ado) to light in the range: free ligand, IF x, HC x, TC x Cbl x Ado. The spectrum of TC x Cbl small middle dotAdo differed from those of IF and HC and mimicked Cbl x Ado participating in catalysis. The above data suggest presence of a histidine-containing cap shielding the Cbl-binding site in TC. The cap coordinates to certain corrinoids and, possibly, produces an incapsulated Ado-radical when Cbl small middle dotAdo is bound.

Direct Assay for Cobalamin Bound to Transcobalamin (Holo-Transcobalamin) in Serum

Background: Only cobalamin carried by transcobalamin (holo-transcobalamin) is available for cellular uptake and hence is physiologically relevant. However, no reliable or accurate methods for quantifying holo-transcobalamin are available. We report a novel holo-transcobalamin assay based on solid-phase capture of transcobalamin.

Methods: A monoclonal antibody specific for human transcobalamin with an affinity constant &gt;1010 L/mol was immobilized on magnetic microspheres to capture and concentrate transcobalamin. The cobalamin bound to transcobalamin was then released and assayed by a competitive binding radioassay. The quantification of holo-transcobalamin was accomplished using calibrators composed of recombinant, human holo-transcobalamin.

Results: The assay was specific for holo-transcobalamin and had a detection limit of 5 pmol/L. Within-run and total imprecision (CV) was 5% and 8–9%, respectively. The working range (CV &lt;20%) was 5–370 pmol/L. Dilutions of serum were linear in the assay range. The recovery of recombinant, human holo-transcobalamin added to serum was 93–108%. A 95% reference interval of 24–157 pmol/L was established for holo-transcobalamin in 105 healthy volunteers 20–80 years of age. For 72 of these sera, holo-haptocorrin and total cobalamin were also determined. Whereas holo-haptocorrin correlated well (r2 = 0.87) with total cobalamin, holo-transcobalamin correlated poorly (r2 = 0.23) with total cobalamin or holo-haptocorrin.

Conclusions: The solid-phase capture assay provides a simple, reliable method for quantitative determination of holo-transcobalamin in serum.