Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase

Chlorquinaldol Alleviates Lung Fibrosis in Mice by Inhibiting Fibroblast Activation through Targeting Methionine Synthase Reductase

Idiopathic pulmonary fibrosis (IPF) is a progressive interstitial lung disease with limited treatment options. Thus, it is essential to investigate potential druggable targets to improve IPF treatment outcomes. By screening a curated library of 201 small molecules, we have identified chlorquinaldol, a known antimicrobial drug, as a potential antifibrotic agent. Functional analyses have demonstrated that chlorquinaldol effectively inhibits the transition of fibroblasts to myofibroblasts in vitro and mitigates bleomycin-induced pulmonary fibrosis in mice. Using a mass spectrometry-based drug affinity responsive target stability strategy, we revealed that chlorquinaldol inhibited fibroblast activation by directly targeting methionine synthase reductase (MTRR). Decreased MTRR expression was associated with IPF patients, and its reduced expression in vitro promoted extracellular matrix deposition. Mechanistically, chlorquinaldol bound to the valine residue (Val-467) in MTRR, activating the MTRR-mediated methionine cycle. This led to increased production of methionine and s-adenosylmethionine, counteracting the fibrotic effect. In conclusion, our findings suggest that chlorquinaldol may serve as a novel antifibrotic medication, with MTRR-mediated methionine metabolism playing a critical role in IPF development. Therefore, targeting MTRR holds promise as a therapeutic strategy for pulmonary fibrosis.

A Noble Metal Substitution Leads to B12 Cofactor Mimicry by a Rhodibalamin

In mammals, cobalamin is an essential cofactor that is delivered by a multitude of chaperones in an elaborate trafficking pathway to two client enzymes, methionine synthase and methylmalonyl-CoA mutase (MMUT). Rhodibalamins, the rhodium analogs of cobalamins, have been described as antimetabolites due to their ability to inhibit bacterial growth. In this study, we have examined the reactivity of adenosylrhodibalamin (AdoRhbl) with two key human chaperones, MMACHC (also known as CblC) and adenosyltransferase (MMAB, also known as ATR), and with the human and Mycobacterium tuberculosis MMUT. We demonstrate that while AdoRhbl binds tightly to all four proteins, the Rh-carbon bond is resistant to homolytic (on MMAB and MMUT) as well as heterolytic (on MMACHC) rupture. On the other hand, MMAB catalyzes Rh-carbon bond formation, converting rhodi(I)balamin in the presence of ATP to AdoRhbl. We report the first crystal structure of a rhodibalamin (AdoRhbl) bound to a B12 protein, i.e., MMAB, in the presence of triphosphate, which shows a weakened but intact Rh-carbon bond. The structure provides insights into how MMAB cleaves the corresponding Co-carbon bond in a sacrificial homolytic reaction that purportedly functions as a cofactor sequestration strategy. Collectively, the study demonstrates that while the noble metal substitution of cobalt by rhodium sets up structural mimicry, it compromises chemistry, which could be exploited for targeting human and bacterial B12 chaperones and enzymes.

The Role of Cobalamin in Multiple Sclerosis: An Update

Multiple sclerosis (MS) is a neurodegenerative condition that results in axonal and permanent damage to the central nervous system, necessitating healing owing to autoimmune reactions and persistent neuroinflammation. Antioxidant and anti-inflammatory drugs are essential for the management of oxidative stress and neuroinflammation. Additionally, multivitamin supplementation, particularly vitamin B12 (cobalamin), may be beneficial for neuronal protection. Although there is no documented connection between vitamin B12 deficiency and MS, researchers have explored its potential as a metabolic cause. This review highlights the therapeutic benefits of cobalamin (Cbl) in patients with MS.

Coordination Chemistry Controls Coenzyme B12 Synthesis by Human Adenosine Triphosphate:Cob(I)alamin Adenosyltransferase

Cobalamin (or vitamin B12)-dependent enzymes and trafficking chaperones exploit redox-linked coordination chemistry to control the cofactor reactivity during catalysis and translocation. As the cobalt oxidation state decreases from 3+ to 1+, the preferred cobalamin geometry changes from six- to four-coordinate (4-c). In this study, we reveal the sizable thermodynamic gain that accrues for human adenosine triphosphate (ATP):cob(I)alamin adenosyltransferase (or MMAB) by enforcing an unfavorable 4-c cob(II)alamin geometry. MMAB-bound cob(II)alamin is reduced to the supernucleophilic cob(I)alamin intermediate during the synthesis of 5'-deoxyadenosylcobalamin. Herein, we report the first experimentally determined reduction potential for 4-c cob(II)alamin (-325 ± 9 mV), which is 180 mV more positive than for the five-coordinate (5-c) water-liganded species. The redox potential of MMAB-bound cob(II)alamin is within the range of adrenodoxin, which we demonstrate functions as an electron donor. We also show that stabilization of 5-c cob(II)alamin by a subset of MMAB patient variants compromises the reduction by adrenodoxin, explaining the underlying pathogenic mechanism.

Proteomic analysis of fatty liver induced by starvation of medaka fish larvae

When medaka fish (Oryzias latipes) larvae are grown in the absence of exogenous nutrition, the liver becomes dark and positive to Oil Red O staining from 7 days post-hatch (dph). We determined the mechanism of this starvation-induced development of fatty liver by proteomic analysis using livers obtained from larvae grown in the presence or absence of 2% glucose at 5 dph. Results showed that changes in the expression levels of enzymes involved in glycolysis or the tricarboxylic acid cycle were modest, whereas the expression levels of enzymes involved in amino acid catabolism or β-oxidation of fatty acids were significantly elevated, suggesting that they become major energy sources under starvation conditions. Expression levels of enzymes for the uptake and β-oxidation of fatty acids as well as synthesis of triacylglycerol were elevated, whereas those for the synthesis of cholesterol as well as export of cholesterol and triacylglycerol were decreased under starvation conditions, which explains the accumulation of triacylglycerol in the liver. Our results provide the basis for future research to understand how gene malfunction(s) affects the development of fatty liver, which can lead to nonalcoholic steatohepatitis and then to liver cirrhosis.Key words: amino acid catabolism, β-oxidation, triacylglycerol, cholesterol, export.

Architecture of the human G-protein-methylmalonyl-CoA mutase nanoassembly for B12 delivery and repair

G-proteins function as molecular switches to power cofactor translocation and confer fidelity in metal trafficking. The G-protein, MMAA, together with MMAB, an adenosyltransferase, orchestrate cofactor delivery and repair of B12-dependent human methylmalonyl-CoA mutase (MMUT). The mechanism by which the complex assembles and moves a >1300 Da cargo, or fails in disease, are poorly understood. Herein, we report the crystal structure of the human MMUT-MMAA nano-assembly, which reveals a dramatic 180° rotation of the B12 domain, exposing it to solvent. The complex, stabilized by MMAA wedging between two MMUT domains, leads to ordering of the switch I and III loops, revealing the molecular basis of mutase-dependent GTPase activation. The structure explains the biochemical penalties incurred by methylmalonic aciduria-causing mutations that reside at the MMAA-MMUT interfaces we identify here.

Architecture of the human G-protein-methylmalonyl-CoA mutase nanoassembly for B 12 delivery and repair

G-proteins function as molecular switches to power cofactor translocation and confer fidelity in metal trafficking. MMAA, a G-protein motor, together with MMAB, an adenosyltransferase, orchestrate cofactor delivery and repair of B ₁₂ -dependent human methylmalonyl-CoA mutase (MMUT). The mechanism by which the motor assembles and moves a >1300 Da cargo, or fails in disease, are poorly understood. Herein, we report the crystal structure of the human MMUT-MMAA nanomotor assembly, which reveals a dramatic 180° rotation of the B ₁₂ domain, exposing it to solvent. The nanomotor complex, stabilized by MMAA wedging between two MMUT domains, leads to ordering of the switch I and III loops, revealing the molecular basis of mutase-dependent GTPase activation. The structure explains the biochemical penalties incurred by methylmalonic aciduria-causing mutations that reside at the newly identified MMAA-MMUT interfaces.

The human B12 trafficking chaperones: CblA, ATR, CblC and CblD

Mammals rely on an elaborate intracellular trafficking pathway for processing and delivering vitamin B₁₂ to two client enzymes. CblC (also known as MMACHC) is postulated to receive the cofactor as it enters the cytoplasm and converts varied B₁₂ derivatives to a common cob(II)alamin intermediate. CblD (or MMADHC) reacts with CblC-bound cob(II)alamin forming an interprotein thiolato-cobalt coordination complex and, by a mechanism that remains to be elucidated, transfers the cofactor to methionine synthase. In the mitochondrion, CblB (also known as MMAB or adenosyltransferase) synthesizes AdoCbl from cob(II)alamin and ATP in the presence of an electron donor. CblA (or MMAA), a GTPase, gates cofactor loading from CblB to methylmalonyl-CoA mutase and off-loading of cob(II)alamin in the reverse direction. This chapter focuses on assays for measuring the activities of the four B₁₂ chaperones CblA-D.

Patient mutations in human ATP:cob(I)alamin adenosyltransferase differentially affect its catalytic versus chaperone functions

Human ATP:cob(I)alamin adenosyltransferase (ATR) is a mitochondrial enzyme that catalyzes an adenosyl transfer to cob(I)alamin, synthesizing 5′-deoxyadenosylcobalamin (AdoCbl) or coenzyme B₁₂. ATR is also a chaperone that escorts AdoCbl, transferring it to methylmalonyl-CoA mutase, which is important in propionate metabolism. Mutations in ATR lead to methylmalonic aciduria type B, an inborn error of B₁₂ metabolism. Our previous studies have furnished insights into how ATR protein dynamics influence redox-linked cobalt coordination chemistry, controlling its catalytic versus chaperone functions. In this study, we have characterized three patient mutations at two conserved active site residues in human ATR, R190C/H, and E193K and obtained crystal structures of R190C and E193K variants, which display only subtle structural changes. All three mutations were found to weaken affinities for the cob(II)alamin substrate and the AdoCbl product and increase K_M(ATP). ³¹P NMR studies show that binding of the triphosphate product, formed during the adenosylation reaction, is also weakened. However, although the k_cat of this reaction is significantly diminished for the R190C/H mutants, it is comparable with the WT enzyme for the E193K variant, revealing the catalytic importance of Arg-190. Furthermore, although the E193K mutation selectively impairs the chaperone function by promoting product release into solution, its catalytic function might be unaffected at physiological ATP concentrations. In contrast, the R190C/H mutations affect both the catalytic and chaperoning activities of ATR. Because the E193K mutation spares the catalytic activity of ATR, our data suggest that the patients carrying this mutation are more likely to be responsive to cobalamin therapy.

Redox-Linked Coordination Chemistry Directs Vitamin B12 Trafficking

Metals are partners for an estimated one-third of the proteome and vary in complexity from mononuclear centers to organometallic cofactors. Vitamin B₁₂ or cobalamin represents the epitome of this complexity and is the product of an assembly line comprising some 30 enzymes. Unable to biosynthesize cobalamin, mammals rely on dietary provision of this essential cofactor, which is needed by just two enzymes, one each in the cytoplasm (methionine synthase) and the mitochondrion (methylmalonyl-CoA mutase). Brilliant clinical genetics studies on patients with inborn errors of cobalamin metabolism spanning several decades had identified at least seven genetic loci in addition to the two encoding B₁₂ enzymes. While cells are known to house a cadre of chaperones dedicated to metal trafficking pathways that contain metal reactivity and confer targeting specificity, the seemingly supernumerary chaperones in the B₁₂ pathway had raised obvious questions as to the rationale for their existence.With the discovery of the genes underlying cobalamin disorders, our laboratory has been at the forefront of ascribing functions to B₁₂ chaperones and elucidating the intricate redox-linked coordination chemistry and protein-linked cofactor conformational dynamics that orchestrate the processing and translocation of cargo along the trafficking pathway. These studies have uncovered novel chemistry that exploits the innate chemical versatility of alkylcobalamins, i.e., the ability to form and dismantle the cobalt-carbon bond using homolytic or heterolytic chemistry. In addition, they have revealed the practical utility of the dimethylbenzimidazole tail, an appendage unique to cobalamins and absent in the structural cousins, porphyrin, chlorin, and corphin, as an instrument for facilitating cofactor transfer between active sites.In this Account, we navigate the chemistry of the B₁₂ trafficking pathway from its point of entry into cells, through lysosomes, and into the cytoplasm, where incoming cobalamin derivatives with a diversity of upper ligands are denuded by the β-ligand transferase activity of CblC to the common cob(II)alamin intermediate. The broad reaction and lax substrate specificity of CblC also enables conversion of cyanocobalamin (technically, vitamin B₁₂, i.e., the form of the cofactor in one-a-day supplements), to cob(II)alamin. CblD then hitches up with CblC via a unique Co-sulfur bond to cob(II)alamin at a bifurcation point, leading to the cytoplasmic methylcobalamin or mitochondrial 5'-deoxyadenosylcobalamin branch. Mutations at loci upstream of the junction point typically affect both branches, leading to homocystinuria and methylmalonic aciduria, whereas mutations in downstream loci lead to one or the other disease. Elucidation of the biochemical penalties associated with individual mutations is providing molecular insights into the clinical data and, in some instances, identifying which cobalamin derivative(s) might be therapeutically beneficial.Our studies on B₁₂ trafficking are revealing strategies for cofactor sequestration and mobilization from low- to high-affinity and low- to high-coordination-number sites, which in turn are regulated by protein dynamics that constructs ergonomic cofactor binding pockets. While these B₁₂ lessons might be broadly relevant to other metal trafficking pathways, much remains to be learned. This Account concludes by identifying some of the major gaps and challenges that are needed to complete our understanding of B₁₂ trafficking.

Mobile loop dynamics in adenosyltransferase control binding and reactivity of coenzyme B12

Cobalamin is a complex organometallic cofactor that is processed and targeted via a network of chaperones to its dependent enzymes. AdoCbl (5'-deoxyadenosylcobalamin) is synthesized from cob(II)alamin in a reductive adenosylation reaction catalyzed by adenosyltransferase (ATR), which also serves as an escort, delivering AdoCbl to methylmalonyl-CoA mutase (MCM). The mechanism by which ATR signals that its cofactor cargo is ready (AdoCbl) or not [cob(II)alamin] for transfer to MCM, is not known. In this study, we have obtained crystallographic snapshots that reveal ligand-induced ordering of the N terminus of Mycobacterium tuberculosis ATR, which organizes a dynamic cobalamin binding site and exerts exquisite control over coordination geometry, reactivity, and solvent accessibility. Cob(II)alamin binds with its dimethylbenzimidazole tail splayed into a side pocket and its corrin ring buried. The cosubstrate, ATP, enforces a four-coordinate cob(II)alamin geometry, facilitating the unfavorable reduction to cob(I)alamin. The binding mode for AdoCbl is notably different from that of cob(II)alamin, with the dimethylbenzimidazole tail tucked under the corrin ring, displacing the N terminus of ATR, which is disordered. In this solvent-exposed conformation, AdoCbl undergoes facile transfer to MCM. The importance of the tail in cofactor handover from ATR to MCM is revealed by the failure of 5'-deoxyadenosylcobinamide, lacking the tail, to transfer. In the absence of MCM, ATR induces a sacrificial cobalt-carbon bond homolysis reaction in an unusual reversal of the heterolytic chemistry that was deployed to make the same bond. The data support an important role for the dimethylbenzimidazole tail in moving the cobalamin cofactor between active sites.

Allosteric Regulation of Oligomerization by a B12 Trafficking G-Protein Is Corrupted in Methylmalonic Aciduria

Allosteric regulation of methylmalonyl-CoA mutase (MCM) by the G-protein chaperone CblA is transduced via three "switch" elements that gate the movement of the B₁₂ cofactor to and from MCM. Mutations in CblA and MCM cause hereditary methylmalonic aciduria. Unlike the bacterial orthologs used previously to model disease-causing mutations, human MCM and CblA exhibit a complex pattern of regulation that involves interconverting oligomers, which are differentially sensitive to the presence of GTP versus GDP. Patient mutations in the switch III region of CblA perturb the nucleotide-sensitive distribution of the oligomeric complexes with MCM, leading to loss of regulated movement of B₁₂ to and/or from MCM and explain the molecular mechanism of the resulting disease.

The cause of multiple sclerosis is autoimmune attack of adenosyltransferase thereby limiting adenosylcobalamin production

The pathogenesis of multiple sclerosis (MS) begins with an infection by a bacterium from the class of bacteria that produce and utilize adenosylcobalamin (AdoCbl) and possess an adenosyl transferase enzyme (ATR); these bacteria are the exogenous antigens that cause MS. Human ATR is homologous to bacterial ATR and B cells produce anti-ATR antibodies as an autoimmune response thereby reducing the concentration of ATR and thus limiting production of AdoCbl, one of the two bioactive forms of vitamin B12. The next step in MS pathogenesis is a period of subclinical AdoCbl deficiency over a period of many years resulting in production of odd-carbon-number fatty acids that are incorporated into myelin rendering it antigenic. The next step in MS pathogenesis is breach of the blood brain barrier thereby introducing leukocytes into the brain's blood supply resulting in T cell attack of antigenic myelin. All epidemiological clusters are regions wherein the major agricultural products are legumes that produce a high percentage of odd-carbon-number fatty acids and contain symbiotic rhizobia type bacteria in root nodules and in the soil. This novel etiological hypothesis is called "multiple sclerosis due to adenosylcobalamin deficiency" (MS-AdoCbl). Creation of realistic animal models based on the MS-AdoCbl hypothesis is presented. Methods for testing predictions made by the MS-AdoCbl hypothesis are described.

Hepatic Effects of Pharmacological Doses of Hydroxy-Cobalamin[c-lactam] in Mice

The vitamin B₁₂ analog hydroxy-cobalamin[c-lactam] (HCCL) impairs hepatic mitochondrial protein synthesis and function of the electron transport chain in rats. We aimed to establish an in vivo model for mitochondrial dysfunction in mice, which could be used to investigate hepatotoxicity of mitochondrial toxicants. In a first step, we performed a dose-finding study in mice treated with HCCL 0.4 mg/kg and 4 mg/kg i.p. for two to four weeks. The plasma methylmalonate concentration was strongly increased at 4 mg/kg starting at three weeks of treatment. We subsequently treated mice daily with 4 mg/kg HCCL i.p. for three weeks and characterized liver function and histology as well as liver mitochondrial function. We found an increase in liver weight in HCCL-treated mice, which was paralleled by hepatocellular accumulation of triglycerides. In liver homogenate of HCCL-treated mice, the complex I activity of the electron transport chain was reduced, most likely explaining hepatocellular triglyceride accumulation. The activity of CPT1 was not affected by methylmalonyl-CoA in isolated liver mitochondria. Despite impaired complex I activity, mitochondrial superoxide anion production was not increased and the hepatocellular glutathione (GSH) pool was maintained. Finally, the mitochondrial DNA content was not altered with HCCL treatment. In conclusion, treatment of mice with HCCL is associated with increased liver weight explained by hepatocellular triglyceride accumulation. Hepatocellular fat accumulation is most likely a consequence of impaired activity of the mitochondrial electron transport chain. The impairment of complex I activity is not strong enough to result in ROS accumulation and reduction of the GSH stores.

Cofactor Editing by the G-protein Metallochaperone Domain Regulates the Radical B12 Enzyme IcmF

IcmF is a 5'-deoxyadenosylcobalamin (AdoCbl)-dependent enzyme that catalyzes the carbon skeleton rearrangement of isobutyryl-CoA to butyryl-CoA. It is a bifunctional protein resulting from the fusion of a G-protein chaperone with GTPase activity and the cofactor- and substrate-binding mutase domains with isomerase activity. IcmF is prone to inactivation during catalytic turnover, thus setting up its dependence on a cofactor repair system. Herein, we demonstrate that the GTPase activity of IcmF powers the ejection of the inactive cob(II)alamin cofactor and requires the presence of an acceptor protein, adenosyltransferase, for receiving it. Adenosyltransferase in turn converts cob(II)alamin to AdoCbl in the presence of ATP and a reductant. The repaired cofactor is then reloaded onto IcmF in a GTPase-gated step. The mechanistic details of cofactor loading and offloading from the AdoCbl-dependent IcmF are distinct from those of the better characterized and homologous methylmalonyl-CoA mutase/G-protein chaperone system.

The Crystal Structure of the C-Terminal Domain of the Salmonella enterica PduO Protein: An Old Fold with a New Heme-Binding Mode

The two-domain protein PduO, involved in 1,2-propanediol utilization in the pathogenic Gram-negative bacterium Salmonella enterica is an ATP:Cob(I)alamin adenosyltransferase, but this is a function of the N-terminal domain alone. The role of its C-terminal domain (PduOC) is, however, unknown. In this study, comparative growth assays with a set of Salmonella mutant strains showed that this domain is necessary for effective in vivo catabolism of 1,2-propanediol. It was also shown that isolated, recombinantly-expressed PduOC binds heme in vivo. The structure of PduOC co-crystallized with heme was solved (1.9 Å resolution) showing an octameric assembly with four heme moieities. The four heme groups are highly solvent-exposed and the heme iron is hexa-coordinated with bis-His ligation by histidines from different monomers. Static light scattering confirmed the octameric assembly in solution, but a mutation of the heme-coordinating histidine caused dissociation into dimers. Isothermal titration calorimetry using the PduOC apoprotein showed strong heme binding (K d = 1.6 × 10(-7) M). Biochemical experiments showed that the absence of the C-terminal domain in PduO did not affect adenosyltransferase activity in vitro. The evidence suggests that PduOC:heme plays an important role in the set of cobalamin transformations required for effective catabolism of 1,2-propanediol. Salmonella PduO is one of the rare proteins which binds the redox-active metabolites heme and cobalamin, and the heme-binding mode of the C-terminal domain differs from that in other members of this protein family.

Biosynthesis and Use of Cobalamin (B12)

This review summarizes research performed over the last 23 years on the genetics, enzyme structures and functions, and regulation of the expression of the genes encoding functions involved in adenosylcobalamin (AdoCbl, or coenzyme B12) biosynthesis. It also discusses the role of coenzyme B12 in the physiology of Salmonella enterica serovar Typhimurium LT2 and Escherichia coli. John Roth's seminal contributions to the field of coenzyme B12 biosynthesis research brought the power of classical and molecular genetic, biochemical, and structural approaches to bear on the extremely challenging problem of dissecting the steps of what has turned out to be one of the most complex biosynthetic pathways known. In E. coli and serovar Typhimurium, uro'gen III represents the first branch point in the pathway, where the routes for cobalamin and siroheme synthesis diverge from that for heme synthesis. The cobalamin biosynthetic pathway in P. denitrificans was the first to be elucidated, but it was soon realized that there are at least two routes for cobalamin biosynthesis, representing aerobic and anaerobic variations. The expression of the AdoCbl biosynthetic operon is complex and is modulated at different levels. At the transcriptional level, a sensor response regulator protein activates the transcription of the operon in response to 1,2-Pdl in the environment. Serovar Typhimurium and E. coli use ethanolamine as a source of carbon, nitrogen, and energy. In addition, and unlike E. coli, serovar Typhimurium can also grow on 1,2-Pdl as the sole source of carbon and energy.

Autoinhibition and signaling by the switch II motif in the G-protein chaperone of a radical B12 enzyme

MeaB is an accessory GTPase protein involved in the assembly, protection, and reactivation of 5'-deoxyadenosyl cobalamin-dependent methylmalonyl-CoA mutase (MCM). Mutations in the human ortholog of MeaB result in methylmalonic aciduria, an inborn error of metabolism. G-proteins typically utilize conserved switch I and II motifs for signaling to effector proteins via conformational changes elicited by nucleotide binding and hydrolysis. Our recent discovery that MeaB utilizes an unusual switch III region for bidirectional signaling with MCM raised questions about the roles of the switch I and II motifs in MeaB. In this study, we addressed the functions of conserved switch II residues by performing alanine-scanning mutagenesis. Our results demonstrate that the GTPase activity of MeaB is autoinhibited by switch II and that this loop is important for coupling nucleotide-sensitive conformational changes in switch III to elicit the multiple chaperone functions of MeaB. Furthermore, we report the structure of MeaB·GDP crystallized in the presence of AlFx(-) to form the putative transition state analog, GDP·AlF4(-). The resulting crystal structure and its comparison with related G-proteins support the conclusion that the catalytic site of MeaB is incomplete in the absence of the GTPase-activating protein MCM and therefore unable to stabilize the transition state analog. Favoring an inactive conformation in the absence of the client MCM protein might represent a strategy for suppressing the intrinsic GTPase activity of MeaB in which the switch II loop plays an important role.

The human flavoproteome

Vitamin B2 (riboflavin) is an essential dietary compound used for the enzymatic biosynthesis of FMN and FAD. The human genome contains 90 genes encoding for flavin-dependent proteins, six for riboflavin uptake and transformation into the active coenzymes FMN and FAD as well as two for the reduction to the dihydroflavin form. Flavoproteins utilize either FMN (16%) or FAD (84%) while five human flavoenzymes have a requirement for both FMN and FAD. The majority of flavin-dependent enzymes catalyze oxidation-reduction processes in primary metabolic pathways such as the citric acid cycle, β-oxidation and degradation of amino acids. Ten flavoproteins occur as isozymes and assume special functions in the human organism. Two thirds of flavin-dependent proteins are associated with disorders caused by allelic variants affecting protein function. Flavin-dependent proteins also play an important role in the biosynthesis of other essential cofactors and hormones such as coenzyme A, coenzyme Q, heme, pyridoxal 5'-phosphate, steroids and thyroxine. Moreover, they are important for the regulation of folate metabolites by using tetrahydrofolate as cosubstrate in choline degradation, reduction of N-5.10-methylenetetrahydrofolate to N-5-methyltetrahydrofolate and maintenance of the catalytically competent form of methionine synthase. These flavoenzymes are discussed in detail to highlight their role in health and disease.

Pharmacological chaperones as a potential therapeutic option in methylmalonic aciduria cblB type

Methylmalonic aciduria (MMA) cblB type is caused by mutations in the MMAB gene. This encodes the enzyme ATP:cob(I)alamin adenosyltransferase (ATR), which converts reduced cob(I)alamin to an active adenosylcobalamin cofactor. We recently reported the presence of destabilizing pathogenic mutations that retain some residual ATR activity. The aim of the present study was to seek pharmacological chaperones as a tailored therapy for stabilizing the ATR protein. High-throughput ligand screening of over 2000 compounds was performed; six were found to enhance the thermal stability of purified recombinant ATR. Further studies using a well-established bacterial system in which the recombinant ATR protein was expressed in the presence of these six compounds, showed them all to increase the stability of the wild-type ATR and the p.Ile96Thr mutant proteins. Compound V (N-{[(4-chlorophenyl)carbamothioyl]amino}-2-phenylacetamide) significantly increased this stability and did not act as an inhibitor of the purified protein. Importantly, compound V increased the activity of ATR in patient-derived fibroblasts harboring the destabilizing p.Ile96Thr mutation in a hemizygous state to within control range. When cobalamin was coadministrated with compound V, mutant ATR activity further improved. Oral administration of low doses of compound V to C57BL/6J mice for 12 days, led to increase in steady-state levels of ATR protein in liver and brain (disease-relevant organs). These results hold promise for the clinical use of pharmacological chaperones in MMA cblB type patients harboring chaperone-responsive mutations.

Navigating the B(12) road: assimilation, delivery, and disorders of cobalamin

The reactivity of the cobalt-carbon bond in cobalamins is the key to their chemical versatility, supporting both methyl transfer and isomerization reactions. During evolution of higher eukaryotes that utilize vitamin B12, the high reactivity of the cofactor coupled with its low abundance pressured development of an efficient system for uptake, assimilation, and delivery of the cofactor to client B12-dependent enzymes. Although most proteins suspected to be involved in B12 trafficking were discovered by 2009, the recent identification of a new protein reveals that the quest for elucidating the intracellular B12 highway is still far from complete. Herein, we review the biochemistry of cobalamin trafficking.

Spectroscopic characterization of active-site variants of the PduO-type ATP:corrinoid adenosyltransferase from Lactobacillus reuteri: insights into the mechanism of four-coordinate Co(II)corrinoid formation

The PduO-type adenosine 5'-triphosphate (ATP):corrinoid adenosyltransferase from Lactobacillus reuteri (LrPduO) catalyzes the transfer of the adenosyl-group of ATP to Co(1+)cobalamin (Cbl) and Co(1+)cobinamide (Cbi) substrates to synthesize adenosylcobalamin (AdoCbl) and adenosylcobinamide (AdoCbi(+)), respectively. Previous studies revealed that to overcome the thermodynamically challenging Co(2+) → Co(1+) reduction, the enzyme drastically weakens the axial ligand-Co(2+) bond so as to generate effectively four-coordinate (4c) Co(2+)corrinoid species. To explore how LrPduO generates these unusual 4c species, we have used magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopic techniques. The effects of active-site amino acid substitutions on the relative yield of formation of 4c Co(2+)corrinoid species were examined by performing eight single-amino acid substitutions at seven residues that are involved in ATP-binding, an intersubunit salt bridge, and the hydrophobic region surrounding the bound corrin ring. A quantitative analysis of our MCD and EPR spectra indicates that the entire hydrophobic pocket below the corrin ring, and not just residue F112, is critical for the removal of the axial ligand from the cobalt center of the Co(2+)corrinoids. Our data also show that a higher level of coordination among several LrPduO amino acid residues is required to exclude the dimethylbenzimidazole moiety of Co(II)Cbl from the active site than to remove the water molecule from Co(II)Cbi(+). Thus, the hydrophilic interactions around and above the corrin ring are more critical to form 4c Co(II)Cbl than 4c Co(II)Cbi(+). Finally, when ATP analogues were used as cosubstrate, only "unactivated" five-coordinate (5c) Co(II)Cbl was observed, disclosing an unexpectedly large role of the ATP-induced active-site conformational changes with respect to the formation of 4c Co(II)Cbl. Collectively, our results indicate that the level of control exerted by LrPduO over the timing for the formation of the 4c Co(2+)corrinoid intermediates is even more exquisite than previously anticipated.

Spectroscopic and computational studies of glutathionylcobalamin: nature of Co-S bonding and comparison to Co-C bonding in coenzyme B12

Glutathionylcobalamin (GSCbl) is a unique, biologically relevant cobalamin featuring an axial Co-S bond that distinguishes it from the enzymatically active forms of vitamin B(12), which possess axial Co-C bonds. GSCbl has been proposed to serve as an intermediate in cobalamin processing and, more recently, as a therapeutic for neurological disorders associated with oxidative stress. In this study, GSCbl and its close relative cysteinylcobalamin (CysCbl) were investigated using electronic absorption, circular dichroism, magnetic circular dichroism, and resonance Raman spectroscopies. The spectroscopic data were analyzed in the framework of density functional theory (DFT) and time-dependent DFT computations to generate experimentally validated electronic structure descriptions. Although the change in the upper axial ligand from an alkyl to a thiol group represents a major perturbation in terms of the size, basicity, and polarizability of the coordinating atom, our spectroscopic and computational results reveal striking similarities in electronic structure between methylcobalamin (MeCbl) and GSCbl, especially with regard to the σ donation from the alkyl/thiol ligand and the extent of mixing between the cobalt 3d and the ligand frontier orbitals. A detailed comparison of Co-C and Co-S bonding in MeCbl and GSCbl, respectively, is presented, and the implications of our results with respect to the proposed biological roles of GSCbl are discussed.

Loss of allostery and coenzyme B12 delivery by a pathogenic mutation in adenosyltransferase

ATP-dependent cob(I)alamin adenosyltransferase (ATR) is a bifunctional protein: an enzyme that catalyzes the adenosylation of cob(I)alamin and an escort that delivers the product, adenosylcobalamin (AdoCbl or coenzyme B(12)), to methylmalonyl-CoA mutase (MCM), resulting in holoenzyme formation. Failure to assemble holo-MCM leads to methylmalonic aciduria. We have previously demonstrated that only 2 equiv of AdoCbl bind per homotrimer of ATR and that binding of ATP to the vacant active site triggers ejection of 1 equiv of AdoCbl from an adjacent site. In this study, we have mimicked in the Methylobacterium extorquens ATR, a C-terminal truncation mutation, D180X, described in a patient with methylmalonic aciduria, and characterized the associated biochemical penalties. We demonstrate that while k(cat) and K(M)(Cob(I)) for D180X ATR are only modestly decreased (by 3- and 2-fold, respectively), affinity for the product, AdoCbl, is significantly diminished (400-fold), and the negative cooperativity associated with its binding is lost. We also demonstrate that the D180X mutation corrupts ATP-dependent cofactor ejection, which leads to transfer of AdoCbl from wild-type ATR to MCM. These results suggest that the pathogenicity of the corresponding human truncation mutant results from its inability to sequester AdoCbl for direct transfer to MCM. Instead, cofactor release into solution is predicted to reduce the capacity for holo-MCM formation, leading to disease.

Functional and structural analysis of five mutations identified in methylmalonic aciduria cblB type

ATP:cob(I)alamin adenosyltransferase (ATR, E.C.2.5.1.17) converts reduced cob(I)alamin to the adenosylcobalamin cofactor. Mutations in the MMAB gene encoding ATR are responsible for the cblB type methylmalonic aciduria. Here we report the functional analysis of five cblB mutations to determine the underlying molecular basis of the dysfunction. The transcriptional profile along with minigenes analysis revealed that c.584G>A, c.349-1G>C, and c.290G>A affect the splicing process. Wild-type ATR and the p.I96T (c.287T>C) and p.R191W (c.571C>T) mutant proteins were expressed in a prokaryote and a eukaryotic expression systems. The p.I96T protein was enzymatically active with a K(M) for ATP and K(D) for cob(I)alamin similar to wild-type enzyme, but exhibited a 40% reduction in specific activity. Both p.I96T and p.R191W mutant proteins are less stable than the wild-type protein, with increased stability when expressed under permissive folding conditions. Analysis of the oligomeric state of both mutants showed a structural defect for p.I96T and also a significant impact on the amount of recovered mutant protein that was more pronounced for p.R191W that, along with the structural analysis, suggest they might be misfolded. These results could serve as a basis for the implementation of pharmacological therapies aimed at increasing the residual activity of this type of mutations.

Genetic disorders of vitamin B₁₂ metabolism: eight complementation groups--eight genes

Vitamin B12 (cobalamin, Cbl) is an essential nutrient in human metabolism. Genetic diseases of vitamin B12 utilisation constitute an important fraction of inherited newborn disease. Functionally, B12 is the cofactor for methionine synthase and methylmalonyl CoA mutase. To function as a cofactor, B12 must be metabolised through a complex pathway that modifies its structure and takes it through subcellular compartments of the cell. Through the study of inherited disorders of vitamin B12 utilisation, the genes for eight complementation groups have been identified, leading to the determination of the general structure of vitamin B12 processing and providing methods for carrier testing, prenatal diagnosis and approaches to treatment.

Characterization of the PduS cobalamin reductase of Salmonella enterica and its role in the Pdu microcompartment

Salmonella enterica degrades 1,2-propanediol (1,2-PD) in a coenzyme B12 (adenosylcobalamin, AdoCbl)-dependent fashion. Salmonella obtains AdoCbl by assimilation of complex precursors, such as vitamin B12 and hydroxocobalamin. Assimilation of these compounds requires reduction of their central cobalt atom from Co3+ to Co2+ to Co+, followed by adenosylation to AdoCbl. In this work, the His6-tagged PduS cobalamin reductase from S. enterica was produced at high levels in Escherichia coli, purified, and characterized. The anaerobically purified enzyme reduced cob(III)alamin to cob(II)alamin at a rate of 42.3±3.2 μmol min(-1) mg(-1), and it reduced cob(II)alamin to cob(I)alamin at a rate of 54.5±4.2 nmol min(-1) mg(-1) protein. The apparent Km values of PduS-His6 were 10.1±0.7 μM for NADH and 67.5±8.2 μM for hydroxocobalamin in cob(III)alamin reduction. The apparent Km values for cob(II)alamin reduction were 27.5±2.4 μM with NADH as the substrate and 72.4±9.5 μM with cob(II)alamin as the substrate. High-performance liquid chromatography (HPLC) and mass spectrometry (MS) indicated that each monomer of PduS contained one molecule of noncovalently bound flavin mononucleotide (FMN). Genetic studies showed that a pduS deletion decreased the growth rate of Salmonella on 1,2-PD, supporting a role in cobalamin reduction in vivo. Further studies demonstrated that the PduS protein is a component of the Pdu microcompartments (MCPs) used for 1,2-PD degradation and that it interacts with the PduO adenosyltransferase, which catalyzes the terminal step of AdoCbl synthesis. These studies further characterize PduS, an unusual MCP-associated cobalamin reductase, and, in conjunction with prior results, indicate that the Pdu MCP encapsulates a complete cobalamin assimilation system.

Allelic expression imbalance at high-density lipoprotein cholesterol locus MMAB-MVK

Genome-wide association studies (GWAS) have identified numerous loci associated with various complex traits for which the underlying susceptibility gene(s) remain unknown. In a GWAS for high-density lipoprotein-cholesterol (HDL-C) level, one strongly associated locus contains at least two biologically compelling candidates, methylmalonic aciduria cblB type (MMAB) and mevalonate kinase (MVK). To detect evidence of cis-acting regulation at this locus, we measured relative allelic expression of transcribed SNPs in five genes using human hepatocyte samples heterozygous for the transcribed SNP. If an HDL-C-associated SNP allele differentially regulates mRNA level in cis, samples heterozygous both for a transcribed SNP and an HDL-C-associated SNP should display allelic expression imbalance (AEI) of the transcribed SNP. We designed statistical tests to detect AEI in a comprehensive set of linkage disequilibrium (LD) scenarios between the transcribed SNP and an HDL-C-associated SNP (rs7298565) in phase unknown samples. We observed significant AEI of 22% in MMAB (P = 1.4 x 10(-13), transcribed SNP rs11067231), and the allele associated with lower HDL-C level was associated with greater MMAB transcript level. The same rs7298565 allele was also associated with higher MMAB mRNA level (P = 0.0081) and higher MMAB protein level (P = 0.0020). In contrast, MVK, UBE3B, KCTD10 and ACACB did not show significant AEI (P > or = 0.05). These data suggest MMAB is the most likely gene influencing HDL-C levels at this locus and demonstrate that measuring AEI at loci containing more than one candidate gene can prioritize genes for functional studies.

Dihydroflavin-driven adenosylation of 4-coordinate Co(II) corrinoids: are cobalamin reductases enzymes or electron transfer proteins?

The identity of the source of the biological reductant needed to convert cobalamin to its biologically active form adenosylcobalamin has remained elusive. Here we show that free or protein-bound dihydroflavins can serve as the reductant of Co(2+)Cbl bound in the active site of PduO-type ATP-dependent corrinoid adenosyltransferase enzymes. Free dihydroflavins (dihydroriboflavin, FMNH(2), and FADH(2)) effectively drove the adenosylation of Co(2+)Cbl by the human and bacterial PduO-type enzymes at very low concentrations (1 microm). These data show that adenosyltransferase enzymes lower the thermodynamic barrier of the Co(2+) --> Co(+) reduction needed for the formation of the unique organometalic Co-C bond of adenosylcobalamin. Collectively, our in vivo and in vitro data suggest that cobalamin reductases identified thus far are most likely electron transfer proteins, not enzymes.

Cobalamin uptake and reactivation occurs through specific protein interactions in the methionine synthase-methionine synthase reductase complex

Human methionine synthase reductase (MSR), a diflavin enzyme, restores the activity of human methionine synthase through reductive methylation of methionine synthase (MS)-bound cob(II)alamin. Recently, it was also reported that MSR enhances uptake of cobalamin by apo-MS, a role associated with the MSR-catalysed reduction of exogenous aquacob(III)alamin to cob(II)alamin [Yamada K, Gravel RA, TorayaT & Matthews RG (2006) Proc Natl Acad Sci USA103, 9476-9481]. Here, we report the expression and purification of human methionine synthase from Pichia pastoris. This has enabled us to assess the ability of human MSR and two other structurally related diflavin reductase enzymes (cytochrome P450 reductase and the reductase domain of neuronal nitric oxide synthase) to: (a) stimulate formation of holo-MS from aquacob(III)alamin and the apo-form of MS; and (b) reactivate the inert cob(II)alamin form of MS that accumulates during enzyme catalysis. Of the three diflavin reductases studied, cytochrome P450 reductase had the highest turnover rate (55.5 s(-1)) for aquacob(III)alamin reduction, and the reductase domain of neuronal nitric oxide synthase elicited the highest specificity (k(cat)/K(m) of 1.5 x 10(5) m(-1) s(-1)) and MSR had the lowest K(m) (6.6 microm) for the cofactor. Despite the ability of all three enzymes to reduce aquacob(III)alamin, only MSR (the full-length form or the isolated FMN domain) enhanced the uptake of cobalamin by apo-MS. MSR was also the only diflavin reductase to reactivate the inert cob(II)alamin form of purified human MS (K(act) of 107 nm) isolated from Pichia pastoris. Our work shows that reactivation of cob(II)alamin MS and incorporation of cobalamin into apo-MS is enhanced through specific protein-protein interactions between the MSR FMN domain and MS.

Cobalamin- and corrinoid-dependent enzymes

This chapter reviews the literature on cobalamin- and corrinoid-containing enzymes. These enzymes fall into two broad classes, those using methylcobalamin or related methylcorrinoids as prosthetic groups and catalyzing methyl transfer reactions, and those using adenosylcobalamin as the prosthetic group and catalyzing the generation of substrate radicals that in turn undergo rearrangements and/or eliminations.

Kinetic and spectroscopic studies of the ATP:corrinoid adenosyltransferase PduO from Lactobacillus reuteri: substrate specificity and insights into the mechanism of Co(II)corrinoid reduction

The PduO-type ATP:corrinoid adenosyltransferase from Lactobacillus reuteri ( LrPduO) catalyzes the formation of the essential Co-C bond of adenosylcobalamin (coenzyme B 12) by transferring the adenosyl group from cosubstrate ATP to a transient Co (1+)corrinoid species generated in the enzyme active site. While PduO-type enzymes have previously been believed to be capable of adenosylating only Co (1+)cobalamin (Co (1+)Cbl (-)), our kinetic data obtained in this study provide in vitro evidence that LrPduO can in fact also utilize the incomplete corrinoid Co (1+)cobinamide (Co (1+)Cbi) as an alternative substrate. To explore the mechanism by which LrPduO overcomes the thermodynamically challenging reduction of its Co (2+)corrinoid substrates, we have examined how the enzyme active site alters the geometric and electronic properties of Co (2+)Cbl and Co (2+)Cbi (+) by using electronic absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopic techniques. Our data reveal that upon binding to LrPduO that was preincubated with ATP, both Co (2+)corrinoids undergo a partial ( approximately 40-50%) conversion to distinct paramagnetic Co (2+) species. The spectroscopic signatures of these species are consistent with essentially four-coordinate, square-planar Co (2+) complexes, based on a comparison with the results obtained in our previous studies of related enzymes. Consequently, it appears that the general strategy employed by adenosyltransferases for effecting Co (2+) --> Co (1+) reduction involves the formation of an "activated" Co (2+)corrinoid intermediate that lacks any significant axial bonding interactions, to stabilize the redox-active, Co 3d z (2) -based molecular orbital.

Structural characterization of a human-type corrinoid adenosyltransferase confirms that coenzyme B12 is synthesized through a four-coordinate intermediate

ATP:cob(I)alamin adenosyltransferases (ACAs) catalyze the transfer of the 5'-deoxyadenosyl moiety from ATP to the upper axial ligand position of cobalamin in the synthesis of coenzyme B 12. For the ACA-catalyzed reaction to proceed, cob(II)alamin must be reduced to cob(I)alamin in the enzyme active site. This reduction is facilitated through the generation of a four-coordinate cob(II)alamin intermediate on the enzyme. We have determined the high-resolution crystal structure of a human-type ACA from Lactobacillus reuteri with a four-coordinate cob(II)alamin bound in the enzyme active site and with the product, adenosylcobalamin, partially occupied in the active site. The assembled structures represent snapshots of the steps in the ACA-catalyzed formation of the cobalt-carbon bond of coenzyme B 12. The structures define the corrinoid binding site and provide visual evidence for a base-off, four-coordinate cob(II)alamin intermediate. The complete structural description of ACA-mediated catalysis reveals the molecular features of four-coordinate cob(II)alamin stabilization and provides additional insights into the molecular basis for dysfunction in human patients suffering from methylmalonic aciduria.

Restricted role for methionine synthase reductase defined by subcellular localization

Methionine synthase reductase (MSR; gene name MTRR) is responsible for the reductive activation of methionine synthase. Cloning of the MTRR gene had revealed two major transcription start sites which, by alternative splicing, allows for two potential translation products of 698 and 725 amino acids. While the shorter protein was expected to target the cytosol where methionine synthase is located, the additional sequence in the longer protein was consistent with a role as a mitochondrial leader sequence. The possibility that MSR might target mitochondria was also suggested by the work of Leal et al. [N.A. Leal, H. Olteanu, R. Banerjee, T.A. Bobik, Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase, J. Biol. Chem. 279 (2004) 47536-47542.] who showed that it can act as the reducing enzyme in combination with MMAB (ATP:Cob(I)alamin adenosyltransferase) to generate adenosylcobalamin from cob(II)alamin in vitro. Here we examined directly whether MSR protein is found in mitochondria. We show that, while two transcripts are produced by alternative splicing, the N-terminal segment of the putative mitochondrial form of MSR fused to GFP does not contain a sufficiently strong mitochondrial leader sequence to direct the fusion protein to the mitochondria of human fibroblasts. Further, antibodies to MSR protein localized MSR to the cytosol, but not to the mitochondria of human fibroblasts or the human hepatoma line Huh-1, as determined by Western blot analysis and immunofluorescence of cells in situ. These data confirm that MSR protein is restricted to the cytosol but, based on the Leal study, suggest that a similar protein may interact with MMAB to reduce the mitochondrial cobalamin substrate in the generation of adenosylcobalamin.

Functional characterization and mutation analysis of human ATP:Cob(I)alamin adenosyltransferase

ATP:cob(I)alamin adenosyltransferase catalyzes the final step in the conversion of vitamin B 12 into the active coenzyme, adenosylcobalamin. Inherited defects in the gene for the human adenosyltransferase (hATR) result in methylmalonyl aciduria (MMA), a rare but life-threatening illness. In this study, we conducted a random mutagenesis of the hATR coding sequence. An ATR-deficient strain of Salmonella was used as a surrogate host to screen for mutations that impaired hATR activity in vivo. Fifty-seven missense mutations were isolated. These mapped to 30 positions of the hATR, 25 of which had not previously been shown to impair enzyme activity. Kinetic analysis and in vivo tests for enzyme activity were performed on the hATR variants, and mutations were mapped onto a hATR structural model. These studies functionally defined the hATR active site and tentatively implicated three amino acid residues in facilitating the reduction of cob(II)alamin to cob(I)alamin which is a prerequisite to adenosylation.

Structure of ATP-bound human ATP:cobalamin adenosyltransferase

Mutations in the gene encoding human ATP:cobalamin adenosyltransferase (hATR) can result in the metabolic disorder known as methylmalonic aciduria (MMA). This enzyme catalyzes the final step in the conversion of cyanocobalamin (vitamin B12) to the essential human cofactor adenosylcobalamin. Here we present the 2.5 A crystal structure of ATP bound to hATR refined to an Rfree value of 25.2%. The enzyme forms a tightly associated trimer, where the monomer comprises a five-helix bundle and the active sites lie on the subunit interfaces. Only two of the three active sites within the trimer contain the bound ATP substrate, thereby providing examples of apo- and substrate-bound-active sites within the same crystal structure. Comparison of the empty and occupied sites indicates that twenty residues at the enzyme's N-terminus become ordered upon binding of ATP to form a novel ATP-binding site and an extended cleft that likely binds cobalamin. The structure explains the role of 20 invariant residues; six are involved in ATP binding, including Arg190, which hydrogen bonds to ATP atoms on both sides of the scissile bond. Ten of the hydrogen bonds are required for structural stability, and four are in positions to interact with cobalamin. The structure also reveals how the point mutations that cause MMA are deficient in these functions.

Effects of dietary supplements of folic acid and vitamin B12 on metabolism of dairy cows in early lactation

The present experiment was undertaken to determine the effects of dietary supplements of folic acid and vitamin B12 given from 3 wk before to 8 wk after calving on lactational performance and metabolism of 24 multiparous Holstein cows assigned to 6 blocks of 4 cows each according to their previous milk production. Supplementary folic acid at 0 or 2.6 g/d and vitamin B12 at 0 or 0.5 g/d were used in a 2 x 2 factorial arrangement. Supplementary folic acid increased milk production from 38.0 +/- 0.9 to 41.4 +/- 1.0 kg/d and milk crude protein yield from 1.17 +/- 0.02 to 1.25 +/- 0.03 kg/d. It also increased plasma Gly, Ser, Thr, and total sulfur AA, decreased Asp, and tended to increase plasma Met. Supplementary B12 decreased milk urea N, plasma Ile, and Leu and tended to decrease Val but increased homocysteine, Cys, and total sulfur AA. Liver concentration of phospholipids was higher in cows fed supplementary B12. Plasma and liver concentrations of folates and B12 were increased by their respective supplements, but the increase in plasma folates and plasma and liver B12 was smaller for cows fed the 2 vitamins together. In cows fed folic acid supplements, supplementary B12 increased plasma glucose and alanine, tended to decrease plasma biotin, and decreased Km of the methylmalonyl-coenzyme A mutase in hepatic tissues following addition of deoxyadenosylcobalamin, whereas it had no effect when cows were not fed folic acid supplements. There was no treatment effect on plasma nonesterified fatty acids as well as specific activity and gene expression of Met synthase and methylmalonyl-coenzyme A mutase in the liver. Ingestion of folic acid supplements by cows fed no supplementary B12 increased total lipid and triacylglycerols in liver, whereas these supplements had no effect in cows supplemented with B12. The increases in milk and milk protein yields due to folic acid supplements did not seem to be dependent on the vitamin B12 supply. However, when vitamin B12 was given in combination with folic acid, utilization of the 2 vitamins seems to be increased, probably more so in extrahepatic tissues. Metabolic efficiency seems also to be improved as suggested by similar lactational performance and dry matter intake for cows fed supplementary folic acid but increased plasma glucose and decreased hepatic lipids in cows fed folic acid and vitamin B12 together.

Spectroscopic studies of the corrinoid/iron-sulfur protein from Moorella thermoacetica

Methyl transfer reactions are important in a number of biochemical pathways. An important class of methyltransferases uses the cobalt cofactor cobalamin, which receives a methyl group from an appropriate methyl donor protein to form an intermediate organometallic methyl-Co bond that subsequently is cleaved by a methyl acceptor. Control of the axial ligation state of cobalamin influences both the mode (i.e., homolytic vs heterolytic) and the rate of Co-C bond cleavage. Here we have studied the axial ligation of a corrinoid iron-sulfur protein (CFeSP) that plays a key role in energy generation and cell carbon synthesis by anaerobic microbes, such as methanogenic archaea and acetogenic bacteria. This protein accepts a methyl group from methyltetrahydrofolate forming Me-Co(3+)CFeSP that then donates a methyl cation (Me) from Me-Co(3+)CFeSP to a nickel site on acetyl-CoA synthase. To unambiguously establish the binding scheme of the corrinoid cofactor in the CFeSP, we have combined resonance Raman, magnetic circular dichroism, and EPR spectroscopic methods with computational chemistry. Our results clearly demonstrate that the Me-Co3+ and Co2+ states of the CFeSP have an axial water ligand like the free MeCbi+ and Co(2+)Cbi+ cofactors; however, the Co-OH2 bond length is lengthened by about 0.2 angstroms for the protein-bound cofactor. Elongation of the Co-OH2 bond of the CFeSP-bound cofactor is proposed to make the cobalt center more "Co1+-like", a requirement to facilitate heterolytic Co-C bond cleavage.

S-adenosylmethionine and its products

S-adenosylmethionine is involved in many processes, mainly methylation, polyamine synthesis and radical-based catalysis. It is synthesised through the catalysis of differently regulated enzyme forms. When it is used, the compounds formed are reutilized in different ways: in case of methylation, its end product is homocysteine, which can be remethylated to methionine, give rise to cysteine in the so-called transsulphuration pathway, or be released; in the case of polyamine synthesis, the methylthioadenosine formed is cleaved and gives rise to compounds which can be reutilized; during radical-based catalysis, 5-deoxyadenosine is formed and this, too, is cleaved and reutilized.

Mitochondrial vitamin B12-binding proteins in patients with inborn errors of cobalamin metabolism

Inborn errors of vitamin B12 (cobalamin, Cbl) metabolism are autosomal recessive disorders and have been classified into nine distinct complementation classes (cblA-cblH and mut). Disorders affecting methylcobalamin metabolism cause megaloblastic anemia, which may be accompanied by leukopenia and thrombocytopenia, and a variety of neurological problems. Disorders affecting adenosylcobalamin cause methylmalonic acidemia and metabolic acidosis. Previous studies have shown that cobalamin binds to two enzymes in humans: methylmalonyl-CoA mutase in mitochondria and methionine synthase in the cytosol. In this study, cobalamin binding patterns were analyzed in crude mitochondrial fractions obtained from both control and patient fibroblasts that had been incubated with [57Co]cyanocobalamin. Crude mitochondrial fractions from control fibroblasts confirmed that the majority of [57Co]Cbl eluted with methylmalonyl-CoA mutase. However, in six of the nine disorders, at least one previously unidentified mitochondrial cobalamin binding protein was observed to bind [57Co]Cbl. The proportion of [57Co]Cbl that binds, is increased compared to controls when a deficiency in either adenosylcobalamin synthesis or utilization prevents binding to methylmalonyl-CoA mutase. Furthermore, unique cobalamin binding profiles emerged demonstrating how known mutations in these patients affect cobalamin binding to as yet unidentified proteins.

Structural characterization of the active site of the PduO-type ATP:Co(I)rrinoid adenosyltransferase from Lactobacillus reuteri

The three-dimensional crystal structure of the PduO-type corrinoid adenosyltransferase from Lactobacillus reuteri (LrPduO) has been solved to 1.68-A resolution. The functional assignment of LrPduO as a corrinoid adenosyltransferase was confirmed by in vivo and in vitro evidence. The enzyme has an apparent Km(ATP) of 2.2 microM and Km(Cobalamin) of 0.13 microM and a kcat of 0.025 s(-1). Co-crystallization of the enzyme with Mg-ATP resulted in well-defined electron density for an N-terminal loop that had been disordered in other PduO-type enzyme structures. This newly defined N-terminal loop makes up the lower portion of the enzyme active site with the other half being contributed from an adjacent subunit. These results provide the first detailed description of the enzyme active site for a PduO-type adenosyltransferase and identify a unique ATP binding motif at the protein N terminus. The molecular architecture at the active site offers valuable new insight into the role of various residues responsible for the human disease methylmalonic aciduria.

Polymorphisms of methionine synthase and methionine synthase reductase and risk of lung cancer: a case-control analysis

Although tobacco is the major lung cancer risk factor, folate deficiency has also been implicated as a risk. Methionine synthase (MS; gene symbol, MTR) and methionine synthase reductase (MSR; gene symbol, MTRR) play important roles in the folate metabolism pathway. It was hypothesized that polymorphisms of MTR and MTRR are associated with lung cancer risk and interact with dietary intake of folate-related nutrients in lung cancer etiology. In a hospital-based, case-control study of 1,035 lung cancer cases and 1,148 controls of non-Hispanic whites, frequency matched by age, sex, ethnicity and smoking status, the MTR 2756A>G and MTRR 66A>G polymorphisms were genotyped. It was found that the MTRRG allele was associated with a significantly increased lung cancer risk [adjusted odd ratio (OR)=1.34, 95% confidence interval (CI)=1.06-1.70 for the AG genotype and OR=1.39, 95% CI=1.08-1.78 for the GG genotype compared to the AA genotype]. Further analysis suggested some evidence of gene-diet interactions between the MTRR 66A>G polymorphism and dietary intake of total folate and vitamin B12. When the two polymorphisms were evaluated together by the number of the variant alleles (i.e. the MTR2756G and MTRR66A), lung cancer risk was significantly increased in a dose-dependent manner (Ptrend=0.045). The risk of lung cancer was 1.29 (0.98-1.69) for one variant allele, and 1.36 (1.04-1.77) for two or more variant alleles compared to the wild-type (0 variant allele) genotype. In conclusion, our data provide evidence supporting the association between the MTR 2756A>G and MTRR 66A>G polymorphisms and lung cancer risk, which may be modulated by dietary nutrient intake.

B12 trafficking in mammals: A for coenzyme escort service

Many coenzymes are vitamins that are assimilated in mammals into their active form from precursors obtained from the diet. They are often both rare and reactive rendering the likelihood low that the cell uses a collision-based strategy for their delivery to dependent enzymes. In humans, there are only two known B12 or cobalamin-dependent enzymes: methionine synthase and methylmalonyl-CoA mutase. However, the pathway for intracellular assimilation and utilization of this cofactor is complex as revealed by careful clinical analyses of fibroblasts from patients with disorders of cobalamin metabolism. In the recent past, six of the eight human genes involved in the B12 pathway have been identified and these have yielded important insights into their roles. The recent literature on the encoded proteins is reviewed, and a model for intracellular B12 trafficking is proposed in which B12 is escorted to its target proteins in the cytoplasmic and mitochondrial compartments in complex with chaperones, thereby averting problems of dilution and adventitious side reactions.

Purification and initial biochemical characterization of ATP:Cob(I)alamin adenosyltransferase (EutT) enzyme of Salmonella enterica

ATP:cob(I)alamin adenosyltransferase (EutT) of Salmonella enterica was overproduced and enriched to approximately 70% homogeneity, and its basic kinetic parameters were determined. Abundant amounts of EutT protein were produced, but all of it remained insoluble. Soluble active EutT protein (approximately 70% homogeneous) was obtained after treatment with detergent. Under conditions in which cobalamin (Cbl) was saturating, Km(ATP) = 10 microm, kcat = 0.03 s(-1), and Vmax = 54.5 nm min(-1). Similarly, under conditions in which MgATP was saturating, Km(Cbl) = 4.1 microm, kcat = 0.06 s(-1), and Vmax = 105 nm min(-1). Unlike other ATP:co(I)rrinoid adenosyltransferases in the cell (i.e. CobA and PduO), EutT activity was > or =50-fold higher with ATP versus GTP, and EutT retained 80% of its activity with ADP substituted for ATP and was completely inactive with AMP as substrate, indicating that the enzyme requires the beta-phosphate group of the nucleotide substrate. The data suggest that the amino group of adenine might play a role in nucleotide recognition and/or binding. Unlike the housekeeping CobA enzyme, EutT was not inhibited by inorganic tripolyphosphate (PPPi). Results from 31P NMR spectroscopy studies identified PPi and Pi as by-products of the EutT reaction. In the absence of Cbl, EutT cleaved ATP into adenosine and PPPi, suggesting that PPPi is broken down into PPi and Pi. Electron transfer protein partners for EutT were not encoded by the eut operon. EutT-dependent activity was detected in cell-free extracts of cobA strains enriched for EutT when FMN and NADH were used to reduce cob(III)alamin to cob(I)alamin.

Impact of cblB mutations on the function of ATP:cob(I)alamin adenosyltransferase in disorders of vitamin B12 metabolism

ATP:cob(I)alamin adenosyltransferase (MMAB protein; methylmalonic aciduria type B) is an enzyme of vitamin B(12) metabolism that converts reduced cob(I)alamin to the adenosylcobalamin co-factor required for the functional activity of methylmalonyl-CoA mutase. Mutations in the human MMAB gene result in a block in adenosylcobalamin synthesis and are responsible for the cblB complementation group of inherited vitamin B(12) disorders. In this study, we examined the impact of several mutations, previously identified in cblB patients and clustered within a small, highly conserved region in MMAB. We confirmed mitochondrial expression of MMAB in human cells and showed that two mutations, R186W and E193K, were associated with absent protein by Western blot, while one, R191W, coupled with another point mutation, produced a protein in patient fibroblasts. Wild type MMAB and all four mutant proteins were stably expressed at high level as GST-fusion proteins, but only the R191W protein was enzymatically active. It showed an elevated K(m) of 320 microM (vs 6.8 microM for wild type enzyme) for ATP and 60 microM (vs 3.7 microM) for cob(I)alamin, with a reduction in k(cat) for both substrates. Circular dichroism spectroscopy revealed that three mutant proteins examined retained a alpha-helical structure as for the wild type protein. Characterization of MMAB will contribute to our understanding of cobalamin processing in mammalian cells and of disease mechanisms in the genetic disorders.

Genetic and genomic systems to study methylmalonic acidemia

Methylmalonic acidemia (MMAemia) is the biochemical hallmark of a group of genetic metabolic disorders that share a common defect in the ability to convert methylmalonyl-CoA into succinyl-CoA. This disorder is due to either a mutant methylmalonyl-CoA mutase apoenzyme or impaired synthesis of adenosylcobalamin, the cofactor for this enzyme. In this article, we will provide an overview of the pathways disrupted in these disorders, discuss the known metabolic blocks with a particular focus on molecular genetics, and review the use of selected model organisms to study features of methylmalonic acidemia.