Transfer of the methyl group

Catechol O-methyltransferase (COMT) is a widespread enzyme that catalyzes the transfer of the methyl group of S-adenosyl-l-methionine (AdoMet) to one of the phenolic group of the catechol substrate (Fig. 1). High COMT activity is found in the liver, kidney and gut wall... 

The 1,3,4-substituted benzene is reacted with an N,N-disubstituted-N -methyl-amine resulting in transfer of the methyl group to the 2-position of the aromatic ring, thereby creating a crowded 1,2,3,4-substituted aromatic [127]. No details on the nature of the substituents and the presence of a solvent are given, as the process is proprietary. The temperature in the micro reactor was set to 0 °C. 

By contrast, the addition of 2equiv. of ZnMe2, Scheme 66, to the cyclodiarsazane m-[Cy(H)NAs(/x-NCy)2AsN(H)Cy] did not deprotonate the arsenic compound. The reaction proceeded instead with the transfer of the methyl group from zinc to arsenic and the rupture of the (As-N)2 ring.145... 

An increase in absorbance at 351 nm and a concomitant decrease in absorbance at 380 nm in the ultraviolet visible spectrum of methylcobalamin during the abiotic transfer of the methyl group to Hg2+ are characteristic for the loss of the methyl group and formation of aquocobalamin. In experiments monitored by both analytical techniques, gas chromatographic measurements of methylmercury formation were in good agreement with the spectropho-tometric measurement of aquocobalamin formation from methylcobalamin at 351 nm. Aerobic versus anaerobic reaction conditions had no measurable effect on either the methyl transfer rates, the stability of the reactants, or on the reaction products. 

Vinyl cuprates.17 Vinyl cuprates can be prepared conveniently by in situ trans-metallation of vinylstannanes, available by hydrostannylation of alkynes, with a cuprate such as (CH3)2Cu(CN)Li2 (equation I). Reagents prepared in this way effect conjugate addition of the vinyl group to an enone with essentially no transfer of the methyl group. 

It is the role of jV5-methyl THF which is key to understanding the involvement of cobalamin in megaloblastic anaemia. The metabolic requirement for N-methyl THF is to maintain a supply of the amino acid methionine, the precursor of S-adenosyl methionine (SAM), which is required for a number of methylation reactions. The transfer of the methyl group from jV5-methyl THF to homocysteine is cobalamin-dependent, so in B12 deficiency states, the production of SAM is reduced. Furthermore, the reaction which brings about the formation of Ns-methyl THF from N5,N10-methylene THF is irreversible and controlled by feedback inhibition by SAM. Thus, if B12 is unavailable, SAM concentration falls and Ah -methyl THF accumulates and THF cannot be re-formed. The accumulation of AT-methyl THF is sometimes referred to as the methyl trap because a functional deficiency of folate is created. 

Many NRPs such as cyclosporin, complestatin, actinomycin, and chondramide contain N-methyl amides. M-Methyl transferase (N-MT) domains utilize S-adenosylmethionine (SAM) as a cofactor to catalyze the transfer of the methyl group from SAM to the a-amine of an aminoacyl-S-PCP substrate. The presence of M-methylamides in NRPs is believed to protect the peptide from proteolysis. Interestingly, N-MT domains are incorporated into the A domains of C-A-MT-PCP modules, between two of the core motifs (A8 and A9). MT domains contain three sequence motifs important for catalysis. ° 0-Methyl transferase domains are also found in NRPSs and likewise use the SAM cofactor. For instance, cryptophycin and anabaenopeptilide synthetases contain 0-MT domains for the methylation of tyrosine side chains. These 0-MT domains lack one of the three core motifs described for N-MT domains. ... 

Compared with other vitamins, the chemical structures of both folic acid and B12 are complex. They are prosthetic groups for the enzymes that catalyse the transfer of the methyl group (-CH3) between compounds (one-carbon metabolism). The -CH3 group is chemically unreactive, so that the chemistry for the transfers is difficult, requiring complex structures for catalysis. 

Enzymatic O-methylation of flavonoids, which is catalyzed by O-methyltransferases (E.C. 2.1.1.6-) involves the transfer of the methyl group of an activated methyl donor, S -adenosyl-L-methionine, to the hydroxyl group of a flavonoid acceptor with the formation of the corresponding methylether and S -adenosyl-L-homocysteine. The latter product is, in... 

Vitamin B12 consists of a porphyrin-like ring structure, with an atom of Co chelated at its centre, linked to a nucleotide base, ribose and phosphoric acid (6.34). A number of different groups can be attached to the free ligand site on the cobalt. Cyanocobalamin has -CN at this position and is the commercial and therapeutic form of the vitamin, although the principal dietary forms of B12 are 5 -deoxyadenosylcobalamin (with 5 -deoxyadeno-sine at the R position), methylcobalamin (-CH3) and hydroxocobalamin (-OH). Vitamin B12 acts as a co-factor for methionine synthetase and methylmalonyl CoA mutase. The former enzyme catalyses the transfer of the methyl group of 5-methyl-H4 folate to cobalamin and thence to homocysteine, forming methionine. Methylmalonyl CoA mutase catalyses the conversion of methylmalonyl CoA to succinyl CoA in the mitochondrion. 

Transfer of the methyl group from 5,-adenosylmethi-onine to an acceptor yields S -adenosylhomocysteine (Fig. 18-18, step (2)), which is subsequently broken down to homocysteine and adenosine (step (3)). Methionine is regenerated by transfer of a methyl group to homocysteine in a reaction catalyzed by methionine synthase (step (4)), and methionine is reconverted to 5-adenosyl-methionine to complete an activated-methyl cycle. 

The methyl group of methyl-THF is incorporated into methionine by the vitamin B12-dependent methionine synthase which is discussed in Chapter 16. Matthews suggested that methionine synthase may also make use of the 5-methyl-quinonoid-THF of Eq. 15-45. An initial reduction step would precede transfer of the methyl group. Methyl-THF is a precursor to acetate in... 

Kauss (39) has shown that the esterification of the carboxyl groups in the D-galacturonic acid chain takes place by a transfer of the methyl groups from S-adenosyl-L-methionine, analogous to the case in which the 4-methyl ether groups are transferred to D-glucuronic acid of hemi-cellulose (40). 

FUdRP inhibits the enzyme that catalyses the transfer of the methyl group. The fluorine group blocks the transfer to the 5 position on the purine nucleus. 

This is another rare inherited disorder of vitamin B12 metabolism in which both coenzyme forms, adenosylcobalamin and methylcobalamin, are affected. Methylcobalamin is required for the transfer of the methyl group of 5-methyltetrahydrofolate to homocysteine to give methionine. Lack of methylcobalamin results in deficient activity of 2V5-methyltetrahydrofolate-homo-cysteine methyltransferase, resulting in a reduced ability to methylate homocysteine. A failure of methionine synthetase would produce a similar result. 

The exact mechanism of the formation of the catalytically active species is not established it is possible that at the initial stages, MAO abstracts the methyl group from zirconium yielding cationic zirconium species that is subsequently stabilized by transfer of the methyl group from aluminum... 

As shown in Figure 10.9, the overall reaction of methionine synthetase is the transfer of the methyl group from methyl-tetrahydrofolate to homocysteine. However, the enzyme also requires S-adenosyl methionine and a flavoprotein reducing system in addition to the cobalamin prosthetic group. A common polymorphism of methionine synthetase, in which aspartate is replaced by glycine, is associated with elevated plasma homocysteine in some cases, although it is less important than methylene-tetrahydrofolate reductase polymorphisms (Section 10.3.2.1 Harmon etal., 1999). 

Cobalt accepts a methyl group from methyl-tetrahydrofolate, forming methyl Co +-cobalamin. Transfer of the methyl group onto homocysteine results in the formation of Co+-cobalamin, which can accept a methyl group from methyl-tetrahydrofolate to reform methyl Co +-cobalamin. However, except under strictly anaerobic conditions, demethylated Co+-cobalamin is susceptible to oxidation to Co +-cobalamin, which is catalyticaUy inactive. Reactivation of the enzyme requires reductive methylation, with S-adenosyl methionine as the methyl donor, and a flavoprotein linked to NADPH. For this reductive reactivation to occur, the dimethylbenzimidazole group of the coenzyme must be displaced from the cobalt atom by a histidine residue in the enzyme (Ludwig and Matthews, 1997). 

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