Methylation, by methylcobalamin

Heterolytic cleavage of the Co—C bond can occur from the attack of electrophiles, including metals. The environmental methylation of Hg by methylcobalamin in a methanogenic bacterium is a well-known example. In this case, the carbanion is transferred to Hg leaving the aquacob(llI)alamin. A similar carbanion pathway holds for the methylation by methylcobalamin of lead(IV), thallium(III), palladium(II) and selenium. Dealkylations by Co", Cr" and Sn" involve radical reactions. 

A complex is Formed with and cysteine. This complex is methylated by methylcobalamin in the presence of methionine in two steps, to yield methylmer-cury thiomethyl. Vonk and Kaars Sispesteijn (1973, 1974) found that in certain fungi the toxicity of mercury(II) chloride is greatly increased by methionine. According to these authors, this is due to the synthesis of CHjSHg— or (CHjS)jHg compounds. Synthetic methylthiomercury compounds are actually more toxic because their stronger lyophilic character enables easier penetration of the cell membrane. 

The first two of these are mediated by 5 -deoxyadenosylcobalamin, whereas methyl transfers are effected by methylcobalamin. The mechanism of ribonucleotide reductase is discussed in Chapter 27. Methyl group transfers that employ tetrahydrofolate as a coenzyme are described later in this chapter. 

FIGURE 8.2 Methylation of inorganic mercury by methylcobalamine (from Crosby 1998). 

Enzymatic methylation of homocysteine (HSCHjCHjCHNHjCOOH) by methylcobalamin to give methionine (CH3SCH2CH2CHNH2COOH) was discovered in 1962 by Woods and co-workers, who also noticed the occurrence of a much slower, nonenzymatic reaction giving the same products. Methylcobinamide showed the same activity as the cobalamin in both the enzymatic and nonenzymatic reactions (72, 7/). It was subsequently discovered that HS, MeS , PhS , and w-BuS will dealkylate a variety of methyl complexes [DMG, DMG-BF2, DPG, G, salen, (DO)(DOH)pn, cobalamin] and even ethyl-Co(DMG)2 complexes to give the thioethers, and it was suggested that the reaction involved transfer of the carbonium ion to the attacking thiolate 161, 164), e.g.,... 

Early work in fractionated extracts of both Escherichia coli and liver indicated the participation of a vitamin Bi2-containing protein in the reaction of homocysteine with 5-methyltetrahydrofolate to form methionine and tetrahydrofolate. ° The reaction was stimulated by SAM, although SAM was not the stoichiometric methyl donor. Methylcobalamin was not the primary methyl donor however, it could serve as a methyl donor in the absence of 5-methyltrahydrofolate. It was suggested that methylcobalamin could be a catalytic intermediate in methyl transfer from 5-methyltetrahydrofolate. The role of SAM remained obscure until recent years, when it was found to preserve enzyme activity by maintaining cobalamin as methylcobalamin in the Bi2-protein. 

Bloalkylatlon and Bloilealkylation Methylcobalamin is important in certain bacteria. In some cases it has been found that Hg(II) in the sea can be methylated by these bacteria to give MeHg. This water-soluble organo-metallic species can be absorbed by shellfish, which can then become toxic... 

The complex [Co(dmg)2(py)(X)] can be reduced in two steps (X = Cl) like aqua-cobalamine (vide supra) to finally yield [Co(dmg)2(py)] , a powerful nucleophile that can be methylated by CH3I. The methylated derivative obtained, [Co(dmg)(py)(CH3)] is a model of methylcobalamine. 

Homocysteine is the amino acid that results after S-adenosyl methionine donates its methyl group, and then the product S-adenosyl homocysteine is hydrolyzed by a molecule of water. In mammals, homocysteine can be converted to cysteine so that the latter is not an essential amino acid, or it can be converted back to methionine by methylation with a compound which serves as a source of one carbon fragments (methyl, formyl, etc.) in biological systems tetrahydrofolic acid. In some bacteria, homocysteine can be converted back to methionine by methylation with methylcobalamin (the methyl derivative of vitamin B12), in the presence of other required compounds or cofactors. This latter methylation reaction is of interest because it can occur, at a reduced rate, in the absence of any enzymes, and a simpler model system has been developed to mimic this reaction (see details in Chapter 6). 

The transfer of methyl from methylcobalamin to HgClg has been shown to involve Me transfer and initially MeHg+. The activation energy for the reaction was determined as 52.4 kJ mol, with 49.9, A5a —58.7 kJ mol . Alkylation of tin by alkylcobaloximes has also been studied in aqueous HCl-NaCl, when one equivalent of an oxidizing agent [Fe i or aquocobalaminfm) suffice] is required. The stoicheiometry with aquocobalamin is shown in equation (10). 

Methylcobalamin is involved in a critically important physiological transformation, namely the methylation of homocysteine (8) to methionine (9) (eq. 2) catalyzed by A/ -methyltetrahydrofolate homocysteine methyltransferase. The reaction sequence involves transfer of a methyl group first from... 

The primary step in the photolysis of methylcobalamin is homolytic fission to give the Co(II) cobalamin and methyl radicals. Recombination can occur, i.e., the reaction is reversed, unless the radicals and/or Co(II) are removed by further reactions ... 

The photochemically active bands of methylcobalamin have been identified as the intense hands due to -n—n transitions within the conjugated corrin ring, and the following quantum yields (< ) were obtained A = 490 nm, Similar quantum yields ( = 0.3-0.5) were also obtained for the photolysis of methylcobalamin in acid, where the base has been displaced and protonated, and the complex is present as a mixture of the methylaquo and five coordinate methyl complexes (/40). The effect of varying the second axial ligand on the rate of photolysis by white light has also been studied (134). 

However, more recent work has shown that the reaction of thioglycollic acid (HSCH2COOH) with methylcobalamin to give the methyl thioether requires oxygen and shows an induction period [which can be reduced by increasing the pH, the thiol concentration, or the partial pressure of O2, and eliminated by the addition of the Co(II) complex], followed by a steady-state reaction [ whose rate increases with pH, the concentration of the CH3C0 and Co(II) complexes and the partial pressure of O2, but is independent of thiol concentration]. Neither the induction period nor the steady-state... 

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