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PubMedDate not supplied

[Flavin pool in Propionibacterium shermanii and the formation of corrinoids by bacterial cultures].

The content of flavins and vitamin B12 was studied in the cultures of Propionibacterium shermanii. The limited ability of propionic bacteria to synthesize 5,6-dimethylbenzimidazole, the nucleotide base in the true forms of vitamin B12, was found to be caused by a deficiency of the biogenetic precursor (riboflavin). Exogenous 5,6-dimethylbenzimidazole had no effect on the processes of flavinogenesis. In the absence of the nucleotide base, the yield of the true forms of vitamin B12 increased upon aeration of cultures which were grown for four days on an iron-deficient medium in the anaerobic conditions.

Benzimidazoles↗
PubMed1991

Effect of the cobalt-N coordination on the cobamide recognition by the human vitamin B12 binding proteins intrinsic factor, transcobalamin and haptocorrin.

The binding of several corrinoids to the binding site of human intrinsic factor, transcobalamin or haptocorrin was investigated, p-Cresolyl cobamide and 2-amino-vitamin B12 are complete corrinoids, whose nucleotide at the lower face of the corrin ring is not coordinated to the cobalt. These corrinoids were greater than or equal to 10(3) times less efficiently recognized by intrinsic factor or transcobalamin than vitamin B12, which contains a Co-coordinated nucleotide. Pseudovitamin B12, with a weak Co-N coordination bond, revealed only moderate affinity to intrinsic factor. From these findings it is concluded that the cobamide binding to intrinsic factor and transcobalamin is strongly affected by the Co-N coordination bonds of their lower cobalt nucleotide ligands. We suggest that the Co-N coordination bond positions the nucleotide at a critical distance to the corrin ring, which is recognized by the binding proteins. Human haptocorrin, however, disclosed to distinctive selectivity regarding the different corrinoid structures. The protein bound all corrinoids with similar efficiency, independent of the strength of their Co-N coordinations, or the structures of their lower Co alpha ligands. Hence, the corrin ring, rather than a structural feature induced by the Co-N coordination, has to be considered responsible for the corrinoid binding to haptocorrin.

Binding Sites↗
PubMed1997

Methylthiol:coenzyme M methyltransferase from Methanosarcina barkeri, an enzyme of methanogenesis from dimethylsulfide and methylmercaptopropionate.

During growth on acetate, Methanosarcina barkeri expresses catabolic enzymes for other methanogenic substrates such as monomethylamine. The range of substrates used by cells grown on acetate was further explored, and it was found that cells grown on acetate also converted dimethylsulfide (DMS) and methylmercaptopropionate (MMPA) to methane. Cells or extracts of cells grown on trimethylamine or methanol did not utilize either DMS or MMPA. During growth on acetate, cultures demethylated MMPA, producing methane and mercaptopropionate. Extracts of acetate-grown cells possessed DMS- and MMPA-dependent coenzyme M (CoM) methylation activities. The activity peaks of CoM methylation with either DMS or MMPA coeluted upon gel permeation chromatography of extracts of acetate-grown cells consistent with an apparent molecular mass of 470 kDa. A 480-kDa corrinoid protein, previously demonstrated to be a CoM methylase but otherwise of unknown physiological function, was found to methylate CoM with either DMS or MMPA. MMPA was demethylated by the purified 480-kDa CoM methylase, consuming 1 mol of CoM and producing 1 mol of mercaptopropionate. DMS was demethylated by the purified protein, consuming 1 mol of CoM and producing 1 mol of methanethiol. The methylthiol:CoM methyltransferase reaction could be initiated only with the enzyme-bound corrinoid in the methylated state. CoM could demethylate, and DMS and MMPA could remethylate, the corrinoid cofactor. The monomethylamine corrinoid protein and the A isozyme of methylcobamide:CoM methyltransferase (proteins homologous to the two subunits comprising the 480-kDa CoM methylase) did not catalyze CoM methylation with methylated thiols. These results indicate that the 480-kDa corrinoid protein functions as a CoM methylase during methanogenesis from DMS or MMPA.

Acetates↗
PubMed1987

Mössbauer, EPR, and optical studies of the corrinoid/iron-sulfur protein involved in the synthesis of acetyl coenzyme A by Clostridium thermoaceticum.

We have purified to homogeneity the 88-kDa corrinoid protein from Clostridium thermoaceticum which acts as a methyl carrier in the synthesis of acetyl-CoA. As shown here, this protein contains a [4Fe-4S]1+/2+ cluster in addition to a corrinoid. The corrinoid is 5-methoxybenzimidazolylcobamide, with an OH- group probably present as the upper axial ligand. Co+ is present in the reduced form, Co2+ in the as-isolated form, and Co3+ in the methylated form of the protein. The as-isolated corrinoid/Fe-S protein exhibits a Co2+ EPR signal lacking nitrogen superhyperfine splittings, indicating that the benzimidazole base is uncoordinated ("base-off") in the Co2+ state. Optical studies suggest that the Co3+-CH3 corrinoid is also base-off. In the as-isolated and methylated forms, the iron-sulfur cluster is diamagnetic, with quadrupole splittings and isomer shifts characteristic of [4Fe-4S]2+ clusters. The protein can be reduced by CO and CO dehydrogenase in the absence of ferredoxin. The EPR spectra of the reduced cluster exhibit two components: one with principal g-values at 2.07, 1.93, and 1.82 and the other at 2.02, 1.94, and 1.86. The Mössbauer data show that these signals result from [4Fe-4S]1+ clusters. Chemical analysis shows that the iron:cobalt atomic ratio is close to 4:1, suggesting that a single [4Fe-4S]1+ cluster occurs in two distinct S = 1/2 spin states in the reduced state. Treatment with 1-2.5 M urea converts the two cluster forms into a single one, with EPR and Mössbauer spectra of typical [4Fe-4S]1+ clusters. A 27-kDa corrinoid protein (Ljungdahl, L.G., LeGall, J., and Lee, J.P. (1973) Biochemistry 12, 1802-1808) also was purified and found to be inactive in the synthesis of acetyl-CoA, contrary to the suggestion of Ljungdahl et al. (1973).

Acetyl Coenzyme A↗
PubMed1993

Identification of a cysteine involved in the interaction between carbon monoxide dehydrogenase and corrinoid/Fe-S protein from Clostridium thermoaceticum.

In Clostridium thermoaceticum, the synthesis of acetyl-CoA from methyl tetrahydrofolate occurs via a series of enzymatic reactions involving methyl transferase, corrinoid/Fe-S protein (corrinoid), carbon monoxide dehydrogenase (CODH) and ferredoxin. We have investigated the possibility of one or more of these proteins existing as multi-enzyme complexes in vivo with higher catalytic activity. A protein complex consisting of CODH and corrinoid was isolated from the cell-free extracts of Clostridium thermoaceticum. The acetyl-CoA synthesis was found to be approximately 1.8-fold higher with the complex than that observed with the isolated protein components. HPLC gel filtration analyses of the native and DTE reduced complex suggested that the CODH:corrinoid complex is held together primarily by an inter disulfide bond. By differential labeling of thiols with [14C]N-ethylmaleimide it was found that Cys-506 of the alpha subunit of CODH was involved in the disulfide linkage with the corrinoid of the complex.

Acetyl Coenzyme A↗
PubMed1996

Activation mechanism of methanol:5-hydroxybenzimidazolylcobamide methyltransferase from Methanosarcina barkeri.

Methanol:5-hydroxybenzimidazolylcobamide methyltransferase (MT1) is the first of two enzymes involved in the transmethylation reaction from methanol to 2-mercaptoethanesulfonic acid in Methanosarcina barkeri. MT1 only binds the methyl group of methanol when the cobalt atom of its corrinoid prosthetic groups is present in the highly reduced Co(I) state. Formation of this redox state requires H2, hydrogenase, methyltransferase activation protein, and ATP. Optical and electron paramagnetic resonance spectroscopy studies were employed to determine the oxidation states and coordinating ligands of the corrinoids of MT1 during the activation process. Purified MT1 contained 1.7 corrinoids per enzyme with cobalt in the fully oxidized Co(III) state. Water and N-3 of the 5-hydroxybenzimidazolyl base served as the upper and lower ligands, respectively. Reduction to the Co(II) level was accomplished by H2 and hydrogenase. The cob(II)amide of MT1 had the base coordinated at this stage. Subsequent addition of methyltransferase activation protein and ATP resulted in the formation of base-uncoordinated Co(II) MT1. The activation mechanism is discussed within the context of a proposed model and compared to those described for other corrinoid-containing methyl group transferring proteins.

Adenosine Triphosphate↗
PubMed1989

Identification of phenolyl cobamide from the homoacetogenic bacterium Sporomusa ovata.

Phenolyl cobamide was isolated from cyanide extractions of the anaerobic eubacterium Sporomusa ovata. The proposed corrinoid structure [Co alpha,Co beta-(monocyano,monoaquo)-phenolyl cobamide] has been deduced from 1H NMR, fast-atom-bombardment mass spectroscopy and ultraviolet/visible spectroscopy data. The complete corrinoid resembled p-cresolyl cobamide [Co alpha,Co beta-(monocyano,monoaquo)-p-cresolyl cobamide], which recently has been obtained from cyanide extractions of the same bacterium. The structures and chemical properties of both cobamides with uncoordinated nucleotides differed significantly from those of vitamin B12 [Co alpha-[alpha-(5,6-dimethylbenzimidazolyl)]-Co beta-cyanocobamide]. Sporomusa synthesized coenzymes of phenolyl cobamide and p-cresolyl cobamide in considerable amounts of 400 nmol/g and 1700 nmol/g dry cells, respectively. More than 90% of the complete corrinoid pool of the homoacetogenic bacterium consisted of these two corrinoids, indicating that they are physiologically important coenzymes of the bacterial metabolism.

Bacteria↗
PubMed1994

N5-methyltetrahydromethanopterin:coenzyme M methyltransferase from Methanobacterium thermoautotrophicum. Catalytic mechanism and sodium ion dependence.

N5-Methyltetrahydromethanopterin:coenzyme M methyltransferase from methanogenic Archaea is a membrane associated, corrinoid-containing enzyme complex which uses a methyl-transfer reaction to drive an energy-conserving sodium ion pump. The purified methyltransferase from Methanobacterium thermoautotrophicum (strain Marburg) exhibited a rhombic EPR signal indicative of a base-on cob(II)amide. In this form, the enzyme was almost completely inactive. Upon addition of Ti(III)citrate, which is a one-electron reductant known to reduce corrinoids to the cob(I)amide form, the EPR signal was completely quenched. In the reduced form, the enzyme was active. When the purified complex was incubated in the presence of both Ti(III) and N5-methyltetrahydromethanopterin (CH3-H4MPT), enzyme-bound Co-methyl-5'-hydroxybenzimidazolyl cob(III)amide was formed. Upon incubation of the methylated enzyme with either tetrahydromethanopterin or coenzyme M, the enzyme was demethylated with the concomitant formation of CH3-H4MPT and methylcoenzyme M, respectively. Enzyme demethylation, in contrast to enzyme methylation, was not dependent on the presence of Ti(III). Methyl transfer from the methylated enzyme to coenzyme M was essentially irreversible. These results are interpreted to that the purified enzyme complex is active only when the enzyme-bound corrinoid is in the reduced cob(I)amide form, and that methyl transfer from CH3-H4MPT to coenzyme M proceeds via nucleophilic attack of the cobalt(I) on the N5-methyl substituent of CH3-H4MPT, forming an enzyme-bound CH3-corrinoid as intermediate. Methyl-coenzyme M formation from CH3-H4MPT and coenzyme M, as catalyzed by the purified methyltransferase, was stimulated by sodium ions, half-maximal activity being obtained at approximately 50 microM Na+. We therefore infer that the methyltransferase, as isolated, is capable of vectorial sodium ion translocation.

Catalysis↗
PubMed1994

Enzymatic catalysis of mercury methylation by Desulfovibrio desulfuricans LS.

The recently defined role of methylcobalamin in Hg2+ methylation by Desulfovibrio desulfuricans LS enabled us to reexamine the question of whether the principal source of methylmercury is spontaneous transmethylation or an enzymatically catalyzed process. In cell extracts of D. desulfuricans LS, over 95% of the 57Co label was associated with macromolecules rather than with free cobalamin. Both gel filtration and electrophoresis of cell extracts identified a single corrinoid protein of 40 kDa in size. This finding, in combination with the previously reported light-reversible propyl iodide inhibition of the Hg2+ methylation process, led us to propose that this 40-kDa corrinoid protein is the in vivo methyl donor in D. desulfuricans LS. Under reducing conditions, cell extracts containing the corrinoid protein produced 14CH3Hg+ from Hg2+ and 5-14CH3-tetrahydrofolate with a maximum specific activity of 0.73 nmol min-1 mg of cell protein-1. The sequence of methyl transfer was from methyltetrahydrofolate to the corrinoid protein to Hg2+. The rate of methylation versus the Hg2+ concentration followed Michaelis-Menten kinetics, with an apparent Km of 0.87 mM HgCl2. The activity was oxygen sensitive, and Hg2+ methylation was optimal at 35 degrees C and pH 6.5. The observation of saturation kinetics and the 600-fold-higher rate of Hg2+ methylation (at pH 7.0) by cell extracts, compared with transmethylation by free methylcobalamin, proved that in vivo Hg2+ methylation is an enzymatically catalyzed process.

Bacterial Proteins↗
PubMed1987

Substitution of Co alpha-(5-hydroxybenzimidazolyl)cobamide (factor III) by vitamin B12 in Methanobacterium thermoautotrophicum.

Methanobacterium thermoautotrophicum grown on mineral medium contains 120 nmol of Co alpha-(5-hydroxybenzimidazolyl)cobamides (derivatives of factor III) per g of dry cell mass as the sole cobamide. The bacterium assimilated several corrinoids and benzimidazole bases during autotrophic growth. The corrinoids were converted into factor III; however, after three transfers in 5,6-dimethylbenzimidazole (200 microM)-supplemented mineral medium, derivatives of factor III were completely replaced by derivatives of vitamin B12, which is atypical for methanogens. The total cobamide content of these cells and their growth rate were not affected compared with factor III-containing cells. Therefore, the high cobamide content rather than a particular type of cobamide is required for metabolism of methanogens. Derivatives of factor III are not essential cofactors of cobamide-containing enzymes from methanogenic bacteria, but they are the result of a unique biosynthetic ability of these archaebacteria. The cobamide biosynthesis include unspecific enzymes, which made it possible either to convert non-species-derived corrinoids into derivatives of factor III or to synthesize other types of cobamides than factor III. The cobamide biosynthesis is regulated by its end product. In addition, the uptake of extracellular cobamides is controlled, and the assimilated corrinoids regulate cellular cobamide biosynthesis.

Benzimidazoles↗
PubMed1998

Clustered genes encoding the methyltransferases of methanogenesis from monomethylamine.

Coenzyme M (CoM) is methylated during methanogenesis from monomethyamine in a reaction catalyzed by three proteins. Using monomethylamine, a 52-kDa polypeptide termed monomethylamine methyltransferase (MMAMT) methylates the corrinoid cofactor bound to a second polypeptide, monomethylamine corrinoid protein (MMCP). Methylated MMCP then serves as a substrate for MT2-A, which methylates CoM. The genes for these proteins are clustered on 6.8 kb of DNA in Methanosarcina barkeri MS. The gene encoding MMCP (mtmC) is located directly upstream of the gene encoding MMAMT (mtmB). The gene encoding MT2-A (mtbA) was found 1.1 kb upstream of mtmC, but no obvious open reading frame was found in the intergenic region between mtbA and mtmC. A single monocistronic transcript was found for mtbA that initiated 76 bp from the translational start. Separate transcripts of 2.4 and 4.7 kb were detected, both of which carried mtmCB. The larger transcript also encoded mtmP, which is homologous to the APC family of cationic amine permeases and may therefore encode a methylamine permease. A single transcriptional start site was found 447 bp upstream of the translational start of mtmC. MtmC possesses the corrinoid binding motif found in corrinoid proteins involved in dimethylsulfide- and methanol-dependent methanogenesis, as well as in methionine synthase. The open reading frame of mtmB was interrupted by a single in-frame, midframe, UAG codon which was also found in mtmB from M. barkeri NIH. A mechanism that circumvents UAG-directed termination of translation must operate during expression of mtmB in this methanogen.

Amino Acid Sequence↗
PubMedDate not supplied

Analogues, ageing and aberrant assimilation of vitamin B12 in Alzheimer's disease.

Vitamin B12 assimilation might be disrupted in patients with Alzheimer's disease. We therefore measured B12 carrier protein saturation and inactive B12 'analogues' in patients compared with healthy elderly individuals in a prospective case-controlled survey. Twenty-three patients, aged 60 or over, with features compatible with DSM-IV criteria for primary degenerative dementia of the Alzheimer type were recruited together with 18 cognitively intact age-matched control subjects. Total vitamin B12 (active corrinoids), holo- and apo-haptocorrin and transcobalamin were measured in serum. B12 analogues (inactive corrinoids) were estimated from the difference between R-binder-determined corrinoids and an intrinsic factor based B12 assay. Alzheimer patients had significantly lower active corrinoid than control subjects and the analogue/corrinoid ratio was significantly higher in the Alzheimer group. The inter-relationship between age, analogues and transcobalamin polarised patients into two distinct groups. Two disparate mechanisms might exist for the development of cerebral B12 deficiency in Alzheimer's disease, although both imply a disruption of selective B12 assimilation and analogue elimination in such patients.

Aged↗