Methyl cobalamine

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

Fig. 13. Mechanism for the synthesis of dimethylarsine from arsenate and methyl-cobalamin...

One other aspect of the photolysis of coordinate spin labeled derivatives is of interest. Nitroxides are good free radical scavengers (123). As a result, when methyl-cobalamin is photolyzed in the presence of a nitroxide, the methyl radical generated will react with the free nitroxide and cause disappearance of the ESR spectrum (123). However, once the nitroxide is coordinated it is no longer susceptible to attack by free radicals. Thus the nitroxyl function is quite well protected from approach by other species. 

Fig. 31 shows 220 MHz spectra for methyl cobalamin and methyl cobinamide. Most of the features of the PMR work to be discussed below can be illustrated with these two spectra as references. (See Fig. 1 for references to nomenclature.)... 

Fig. 31. 220 MHz NMR spectrum of methyl cobalamin in DaO and 220 MHz NMR spectrum of methyl cobinamide in D2O...

Assignment of the remaining peaks in the cobalamin or cobinamide spectra is less straightforward. The most detailed assignments made thus far have been made by Brodie and Poe (130). These are outlined in Table 2 for methyl cobalamin. 

When the pD of a D2O solution of methyl cobalamin is lowered, the resonance position for the C(20)-methyl shifts progressively to lower field and the intensity of the peak remains constant (50). Thus rather than observing two C(20)-methyl resonances corresponding to the base-on... 

This may be of significance in connetion with the biosynthesis of acetate from carbon dioxide, because the next step, the fixation of carbon monoxide, was demonstrated by B. Krautler. He irradiated methyl cobalamin under Co at 30 atm and obtained the acyl cobalamin as the product. Interestingly, a radical mechanism was iproposed, involving the reaction of methyl radicals with CO to give acyl radicals, which then recombine with the cobalt complex /55/. 

Methyl-Cobalamine and Related Compounds. There are three major types of cobalamine in animals and man, the methyl and hydroxo derivatives and the deoxyadenosyl derivative (coenzyme B12). The unique metal-carbon bond in the methyl derivative and in B12 is extremely photolabile, and their photolyses have been widely studied. However, radiolytic processes have not been widely studied. 

Use of CD30D or methyl tetrahydrofuran solvents to encourage electron capture, resulted in a complex set of reactions for methyl cobalamine. Initial addition occurred into the w corrin orbital, but on annealing a cobalt centred radical was obtained, the e.s.r. spectrum of which was characteristic of an electron in a d z.y orbital (involving the corrin ring) rather than the expected d2z orbital. However, the final product was the normal Co species formed by loss of methyl. Formally, this requires loss of CH3 , but this step seems highly unlikely. Some form of assisted loss, such as protonation, seems probable. 

Ashby and Craig72 reported that MeSn3+ and small amounts of Me2Sn2+ are also produced when a baker s yeast (Saccharomyces cerevisiae) is incubated with tin(II) compounds including tin(II) oxalate, tin(II) sulfide and various tin amino acid complexes. Tin(II) chloride and tin(II) amino acid complexes were methylated by methyl-cobalamin, under conditions of chloride ion concentrations and pH relevant to the natural environment73. The main identified product of all reactions was monomethyltin. 

Solid Pb02 and solid Pb304 react with 1 (41). When l4C-labeled methyl-cobalamin is used, the labeled carbon becomes an unidentified, volatile... 

As a model study of methyl cobalamine (methyl transfer) in living bodies, a methyl radical, generated by the reduction of the /s(dimethylglyoximato)(pyridine)Co3+ complex to its Co1+ complex, reacts on the sulfur atom of thiolester via SH2 to generate an acyl radical and methyl sulfide. The formed methyl radical can be trapped by TEMPO or activated olefins [8-13]. As a radical character of real vitamin B12, the addition of zinc to a mixture of alkyl bromide (5) and dimethyl fumarate in the presence of real vitamin B12 at room temperature provides a C-C bonded product (6), through the initial reduction of Co3+ to Co1+ by zinc, reaction of Co1+ with alkyl bromide to form R-Co bond, its homolytic bond cleavage to form an alkyl radical, and finally the addition of the alkyl radical to diethyl fumarate, as shown in eq. 11.4 [14]. 

Figure 21-3. The methionine synthase reaction. Methionine synthase catalyzes the remethylation of homocysteine to methionine. In the first half reaction (1), a methyl group is transferred from 5-methyl tetrahydrofolate (5-MTHF) to the reduced form of cobalamin [Cob(I)], generating methyl-cobalamin [Methyl-Cob(III)] and tetrahydrofolate (THF). During the second half reaction (2), the methyl group is transferred from methylcobalamin to homocysteine, generating methionine. During the catalytic reaction, Cob(I) occasionally becomes oxidized, producing an inactive form of cobalamin, cob(II)alamin [Cob(II)]. The enzyme methionine synthase reductase (MTRR) then reactivates Cob(II) through reductive methylation, producing methyl-Cob(III). SAM, 5-adenosylmethionine SAH, 5-adeno-sylhomocysteine.

The ligand attached to the cobalt atom determines the activity of vitamin B12 in human enzymatic reactions. The two active coenzyme forms are methyl-cobalamin and 5 -adenosylcobalamin, the primary form of vitamin B12 in tissues. Cyanocobalamin, the therapeutic form of vitamin B12 contained in vitamin supplements, is produced by the cleavage of the unstable fink... 

Methyl cobalamin is usually described as a Co(II) compound, which changes to Co(III) on dissociation of CH3. Describe the probable electronic structure (splitting of d levels and number of unpaired electrons) of the cobalt in both cases. 

The 5-deoxyadenosyl cobalamin and methyl cobalamin function as coenzyme forms and are required for the action of several enzymes. Methyl malonyl CoA mutase uses 5-deoxyadenosyl cobalamin as coenzyme. Methyl cobalamin functions as a carrier of methyl group to homocysteine and convert it to methionine... 

The coenzyme form of pantothenic acid is coenzyme A and is represented as CoASH. The thiol group acts as a carrier of acyl group. It is an important coenzyme involved in fatty acid oxidation, pyruvate oxidation and is also biosynthesis of terpenes. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl carrier protein (BCCP or biocytin) and serve as an intermediate carrier of C02. Acetyl CoA pyruvate and propionyl carboxylayse require the participation of BCCP. The coenzyme form of folic acid is tetrahydro folic acid. It is associated with one carbon metabolism. The oxidised and reduced forms of lipoic acid function as coenzyme in pyruvate and a-ketoglutarate dehydrogenase complexes. The 5-deoxy adenosyl and methyl cobalamins function as coenzyme forms of vitamin B12. Methyl cobalamin is involved in the conversion of homocysteine to methionine. 

Vitamin B12 is a mixture of cobalamins. Dietary vitamin B12 is converted to the active forms, methyl-cobalamin (mecobalamin) and adenosylcobalamin. The Average Requirement of total cobalamins in adults is 1.0 microgram/day and the Population Reference Intake is 1.4 micrograms/day. The Lowest Threshold Intake is 0.6 micrograms/day. Hydroxocobalamin (rINN vitamin Bi2a) and cyanocobalamin (rINN) have been used therapeutically. 

The rate of methylmercury formation by this process is largely determined by the concentration of methyl cobalamine compounds, inorganic mercuric ions, and the oxygen concentration of the water, with the rate... 

The most reduced coenzyme is 5-methyl tetrahydrofolate poly glutamate. It is the source of the methyl group added to homocysteine regenerating methionine and tetrahydrofolate ready to accept a one-carbon unit from formate or serine. This last reaction is where folic acid and vitamin come together (Figs. 8.49, 8.52, and 8.53). The implications of this reaction and how folic acid can mask pernicious anemia are discussed in the seetion on vitamin Big (cyanocobalamin). Note that the formation of 5-methyl-THF nor-mdly is not reversible. Tetrahydrofolate can be regenerated only if there is adequate methyl cobalamin coenzyme. 

Homocysteine can be recycled back to methionine either by transfer of a methyl group from betaine catalyzed by betaine-homocysteine methyltransferase, or from N -methyltetrahydrofolate(N -methyl-FH4)catalyzedbyN -methyl-FH4-methyltransferase, which requires methyl cobalamin ... 

Mg Methylmalonic aciduria homocystinuria Formation of both adenosyl and methyl cobalamins... 

Adenosyl-cobalamine catalyzes hydrogen shifts as a special isomerisation reaction. With exception of reduction of ribonucleotides the H-shift occurs intramolecularly. Methyl-cobalamine and tetrahydrofolic add are the coenzymes in methylating homocysteine to methionine. 

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