Stimulation in vitro of vitamin B12-dependent methionine synthase by polyamines.
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ABSTRACT:
SUBMITTER: Kenyon SH
PROVIDER: S-EPMC1217398 | biostudies-other | 1996 Jun
REPOSITORIES: biostudies-other
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Project description:BackgroundNutrients involved in one-carbon metabolism may play a key role in pancreatic carcinogenesis. The aim of this study was to examine the association between pancreatic cancer risk and intake or blood levels of vitamins B6, B12 and methionine via meta-analysis.MethodsA systematic search was performed in PubMed, Web of Knowledge and Chinese National Knowledge Infrastructure (CNKI) up to April 2020 to identify relevant studies. Risk estimates and their 95% confidence intervals (CIs) were retrieved from the studies and combined by a random-effect model.ResultsA total of 18 studies were included in this meta-analysis on the association of vitamin B6, B12 and methionine with pancreatic cancer risk. The combined risk estimate (95% CI) of pancreatic cancer for the highest vs lowest category of vitamin B6 intake and blood pyridoxal 5'-phosphate (PLP, active form of vitamin B6) levels was 0.63 (0.48-0.79) and 0.65 (0.52-0.79), respectively. The results indicated a non-linear dose-response relationship between vitamin B6 intake and pancreatic risk. Linear dose-response relationship was found, and the risk of pancreatic cancer decreased by 9% for every 10 nmol/L increment in blood PLP levels. No significant association were found between pancreatic cancer risk and vitamin B12 intake, blood vitamin B12 levels, methionine intake and blood methionine levels.ConclusionOur study suggests that high intake of vitamin B6 and high concentration of blood PLP levels may be protective against the development of pancreatic cancer. Further research are warranted to confirm the results.
| S-EPMC7534168 | biostudies-literature
Project description:Adenyl cobamide (commonly known as pseudovitamin B12) is synthesized by intestinal bacteria or ingested from edible cyanobacteria. The effect of pseudovitamin B12 on the activities of cobalamin-dependent enzymes in mammalian cells has not been studied well. This study was conducted to investigate the effects of pseudovitamin B12 on the activities of the mammalian vitamin B12-dependent enzymes methionine synthase and methylmalonyl-CoA mutase in cultured mammalian COS-7 cells to determine whether pseudovitamin B12 functions as an inhibitor or a cofactor of these enzymes. Although the hydoroxo form of pseudovitamin B12 functions as a coenzyme for methionine synthase in cultured cells, pseudovitamin B12 does not activate the translation of methionine synthase, unlike the hydroxo form of vitamin B12 does. In the second enzymatic reaction, the adenosyl form of pseudovitamin B12 did not function as a coenzyme or an inhibitor of methylmalonyl-CoA mutase. Experiments on the cellular uptake were conducted with human transcobalamin II and suggested that treatment with a substantial amount of pseudovitamin B12 might inhibit transcobalamin II-mediated absorption of a physiological trace concentration of vitamin B12 present in the medium.
| S-EPMC7396987 | biostudies-literature
Project description:BackgroundEpidemiological studies evaluating the association of vitamin B6, vitamin B12 and methionine with breast cancer risk have produced inconsistent results.MethodsPertinent studies were identified by a search in PubMed and Web of Knowledge. Random-effect model was used. Dose-response relationship was assessed by restricted cubic spline.ResultsThe combined relative risk (95% confidence interval) of breast cancer for the highest vs lowest category of serum pyridoxal 5'-phosphate (PLP, active form of vitamin B6) levels and dietary methionine intake was 0.80 (0.66-0.98, P=0.03) and 0.94 (0.89-0.99, P=0.03), respectively, and the associations of breast cancer with higher serum PLP levels and dietary methionine intake were significant among post-menopausal women, but not among pre-menopausal women. The inverse association between breast cancer risk and dietary vitamin B6 intake, serum vitamin B12 levels and dietary vitamin B12 intake was not significant overall. Linear dose-response relationship was found, and the risk of breast cancer decreased by 23% (P<0.00) for every 100?pmol?ml(-1) increment in PLP levels and 4% (P=0.05) for every 1?g per day increment in dietary methionine intake, respectively.ConclusionSerum PLP levels and methionine intake might be inversely associated with breast cancer risk, especially among postmenopausal women, which need to be confirmed.
| S-EPMC3790153 | biostudies-literature
Project description:The cobalamin-dependent methionine synthase (MetH) from Escherichia coli is a modular enzyme that catalyzes a methyl group transfer from methyltetrahydrofolate to homocysteine via a methylcob(III)alamin (MeCbl) intermediate, generating tetrahydrofolate and methionine (Met). Once every approximately 2000 turnovers, the cobalamin cofactor is converted to the inactive cob(II)alamin (Co(2+)Cbl) form, from which MeCbl has to be recovered for MetH to re-enter the catalytic cycle. A particularly puzzling aspect of this reactivation process is that it requires the reduction of the Co(2+)Cbl species to cob(I)alamin (Co(1+)Cbl) by flavodoxin, a reaction that would appear to be endergonic on the basis of the corresponding reduction potentials. To explore how MetH may overcome this apparent thermodynamic challenge, we have prepared the I690C/G743C variant of a C-terminal fragment of MetH (MetH(CT)) to lock the enzyme into the activation conformation without perturbing any of the residues in the vicinity of the active site. A detailed spectroscopic characterization of this species and the I690C/G743C/Y1139F MetH(CT) triple mutant reveals that the strategy employed by MetH to activate Co(2+)Cbl for Co(2+) --> Co(1+) reduction likely involves (i) an axial ligand switch to generate a five-coordinate species with an axially coordinated water molecule and (ii) a significant lengthening, or perhaps complete rupture, of the Co-OH(2) bond of the cofactor, thereby causing a large stabilization of the Co 3d(z(2))-based "redox-active" molecular orbital. The lengthening of the Co-OH(2) bond is mediated by the Y1139 active-site residue and becomes much more dramatic when the S-adenosylmethionine substrate is present in the enzyme active site. This substrate requirement provides MetH a means to suppress deleterious side reactions involving the transiently formed Co(1+)Cbl "supernucleophile".
| S-EPMC3101771 | biostudies-literature
Project description:The industrial solvent trichloroethylene (TCE) produces a marked formic aciduria in male and female F344 rats and in male C57Bl mice following single or multiple dosing. The two major metabolites of TCE formed by cytochromes P450 metabolism also produce formic aciduria. The quantity of formic acid excreted was about 2-fold higher following trichloroacetic acid (TCA) compared to trichloroethanol (TCE-OH) or TCE, at similar doses of 16mg/kg/day for 3 days. Prior treatment of male F344 rats with 1-aminobenzotriazole a cytochrome P450 inhibitor, followed by TCE, completely prevented the formic aciduria but had no effect on formic acid excretion produced by TCA, suggesting TCA is the proximate metabolite producing this response. Metabolism of formic acid is largely controlled by the vitamin B12 –dependent methionine salvage pathway. Transcriptomic analysis on the liver of rats dosed with 16mg/kg/day TCE for three days when compared to control liver showed nine differentially expressed genes, of particular interest was the down regulation of LMBRD1 involved in the conversion of vitamin B12 into one of two molecules, methylcobalamin (CH3Cbl) or S-adenosylcobalamin (AdoCbl). Administration of CH3Cbl or hydroxocobalamin for 3 days to rats given a single dose of TCE, lead to a reduction in formic acid in their urine. Similarly, rats given TCE followed by L-methionine for 3 days excreted less formic acid in their urine. These findings suggest an effect on the vitamin B12 –dependent methionine salvage pathway. This was supported by the finding that hepatic methionine synthase, which converts homocysteine to methionine, was inhibited following three large daily dose of TCE. We propose that TCE metabolites interact with the vitamin B12 -dependent methionine salvage pathway leading to tetrahydrofolate deficiency and increased excretion of formic acid in rat urine.
2020-11-20 | GSE122664 | GEO
Project description:Cobalamin-dependent methionine synthase (MetH) is a modular protein that catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine to produce methionine and tetrahydrofolate. The cobalamin cofactor, which serves as both acceptor and donor of the methyl group, is oxidized once every approximately 2,000 catalytic cycles and must be reactivated by the uptake of an electron from reduced flavodoxin and a methyl group from S-adenosyl-L-methionine (AdoMet). Previous structures of a C-terminal fragment of MetH (MetH(CT)) revealed a reactivation conformation that juxtaposes the cobalamin- and AdoMet-binding domains. Here we describe 2 structures of a disulfide stabilized MetH(CT) ((s-s)MetH(CT)) that offer further insight into the reactivation of MetH. The structure of (s-s)MetH(CT) with cob(II)alamin and S-adenosyl-L-homocysteine represents the enzyme in the reactivation step preceding electron transfer from flavodoxin. The structure supports earlier suggestions that the enzyme acts to lower the reduction potential of the Co(II)/Co(I) couple by elongating the bond between the cobalt and its upper axial water ligand, effectively making the cobalt 4-coordinate, and illuminates the role of Tyr-1139 in the stabilization of this 4-coordinate state. The structure of (s-s)MetH(CT) with aquocobalamin may represent a transient state at the end of reactivation as the newly remethylated 5-coordinate methylcobalamin returns to the 6-coordinate state, triggering the rearrangement to a catalytic conformation.
| S-EPMC2765455 | biostudies-literature
Project description:Coastal Antarctic marine ecosystems play an important role in carbon cycling due to their highly productive seasonal phytoplankton blooms. Southern Ocean microbes are primarily limited by light and iron (Fe) and can be co-limited by cobalamin (vitamin B12 ). Micronutrient limitation is a key driver of ecosystem dynamics and influences the composition of blooms, which are often dominated by either diatoms or the haptophyte Phaeocystis antarctica, each with varied impacts on carbon cycling. However, the vitamin requirements and ecophysiology of the keystone species P. antarctica remains poorly characterized. Using cultures, physiological analysis, and comparative ’omics we examined the response of P. antarctica to a matrix of Fe-B12 conditions. We show that P. antarctica is not auxotrophic for B12 , as previously suggested, and report new mechanistic insights of its B12 response in cultures of predominantly solitary and colonial cells. Proteomics coupled with proteogenomics detected a B12 -independent methionine synthase fusion protein (MetE-fusion) that is expressed under vitamin limitation and is interreplaced with the B12 -dependent isoform (MetH) in replete conditions. Database searches returned homologs of the MetE-fusion protein in multiple Phaeocystis species and in a wide range of marine microbes, including other photosynthetic eukaryotes with polymorphic life cycles and also bacterioplankton. Furthermore, MetE-fusion homologs were found to be expressed in metaproteomic and metatranscriptomic field samples in polar and more geographically widespread regions. As climate change impacts micronutrient availability in the coastal Southern Ocean, our finding that P. antarctica has a flexible B12 metabolism has implications for its relative fitness compared to B12 -auxotrophic diatoms.
2023-02-27 | PXD031524 | Pride
Project description:Cobalamin-dependent methionine synthase (MetH) of Escherichia coli is a large, modular enzyme that uses a cobalamin prosthetic group as a donor or acceptor in three separate methyl transfer reactions. The prosthetic group alternates between methylcobalamin and cob(I)alamin during catalysis as homocysteine is converted to methionine using a methyl group derived from methyltetrahydrofolate. Occasional oxidation of cob(I)alamin to cob(II)alamin inactivates the enzyme. Reductive methylation with flavodoxin and adenosylmethionine returns the enzyme to an active methylcobalamin state. At different points during the reaction cycle, the coordination state of the cobalt of the cobalamin changes. The imidazole side chain of His759 coordinates to cobalamin in a "His-on" state and dissociates to produce a "His-off" state. The His-off state has been associated with a conformation of MetH that is poised for reactivation of cobalamin by reductive methylation rather than catalysis. Our studies on cob(III)alamins bound to MetH, specifically aqua-, methyl-, and n-propylcobalamin, show a correlation between the accessibility of the reactivation conformation and the order of the established ligand trans influence. The trans influence also controls the affinity of MetH in the cob(III)alamin form for flavodoxin. Flavodoxin, which acts to shift the conformational equilibrium toward the reactivation conformation, binds less tightly to MetH when the cob(III)alamin has a strong trans ligand and therefore has less positive charge on cobalt. These results are compared to those for cob(II)alamin MetH, illustrating that access to the reactivation conformation is governed by the net charge on the cobalt as well as the trans influence in cob(III)alamins.
| S-EPMC2548328 | biostudies-literature
Project description:BackgroundIt is controversial whether B12 deficiency causes dementia or B12 treatment can prevent dementia.ObjectiveTo assess associations between low plasma (P-)B12 levels, B12 treatment, and risk of Alzheimer's disease (AD; primary outcome) and all-cause or vascular dementia (secondary outcomes).MethodsWe conducted a population-based cohort study using Danish registry data to assess associations between low P-B12 levels, high-dose injection or oral B12 treatment, and risk of dementia (study period 2000-2013). The primary P-B12 cohort included patients with a first-time P-B12 measurement whose subsequent B12 treatment was recorded. The secondary B12 treatment cohort included patients with a first-time B12 prescription and P-B12 measurement within one year before this prescription. For both cohorts, patients with low P-B12 levels (<200 pmol/L) were propensity score-matched 1:1 with patients with normal levels (200-600 pmol/L). We used multivariable Cox regression to compute 0-15-year hazard ratios for dementia.ResultsFor low P-B12 and normal P-B12 level groups, we included 53,089 patients in the primary P-B12 cohort and 13,656 patients in the secondary B12 treatment cohort. In the P-B12 cohort, hazard ratios for AD centered around one, regardless of follow-up period or treatment during follow-up. In the B12 treatment cohort, risk of AD was unaffected by low pre-treatment P-B12 levels, follow-up period and type of B12 treatment. Findings were similar for all-cause and vascular dementia.ConclusionWe found no associatio1n between low P-B12 levels and dementia. Associations were unaffected by B12 treatment. Results do not support routine screening for B12 deficiency in patients with suspected dementia.
| S-EPMC7990402 | biostudies-literature
Project description:We observed vitamin B(12)-mediated growth inhibition of Mycobacterium tuberculosis strain CDC1551. The B(12) sensitivity was mapped to a polymorphism in metH, encoding a coenzyme B(12)-dependent methionine synthase. Vitamin B(12)-resistant suppressor mutants of CDC1551 containing mutations in a B(12) riboswitch upstream of the metE gene, which encodes a B(12)-independent methionine synthase, were isolated. Expression analysis confirmed that the B(12) riboswitch is a transcriptional regulator of metE in M. tuberculosis.
| S-EPMC1855906 | biostudies-other