Methyl groups group transfer

Evidently, this reaction may take place wherever primary or secondary carbonium ions occur in the presence of secondary or tertiary methyl groups. The transfer of larger carbanions may, of course, also be found under appropriate circumstances but cannot be treated quantitatively at present. 

In the first step of the reductive branch of this metabolic pathway three out of four methyl groups are transferred from methanol to CoM-SH (13) by methyl transferases, with formation of methyl-S-CoM (21) (Scheme 1) [21]. The transformation of 21 and CoB-SH (15) into methane and CoB-S-S-CoM (22) is catalyzed by the methyl-CoM reductase. Again, reductive cleavage of 22 is mediated by the heterodisulfide reductase [22]. The oxidative part involves oxidation of... 

The presence of small quantities of methylated purine and pyrimidine bases in nucleic acid of micro-organisms and animals has been widely documented. The methyl group is transferred from N-adenosylmethionine to these bases, and methylation occurs at the polynucleotide level rather than by transfer to the acid-... 

CP/MAS NMR study of CH4 activation on [(=SiO)2Ta(H)J shows the formation even at 150 °C of methyUdene and methyUdyne species by an a-H elimination process on several sites that should correspond to the tris-hydride on other sites a methyl group is transferred to the surface, leading to the formation of (=Si-Me) and of [(=SiO)3Ta]. Correlation with EXAFS suggests that the tris-hydride should exist on surface sites (=20%) quite distant from siloxy bridges whereas methyl transfer to the surface should happen on the specific sites (=80%) close to the siloxy bridges. The latter, which are formally 10 electrons species, exhibit a moderate to weak activity in methane C-H activation. To the best of our knowledge, this is the first observation of methyl group transfer on a surface (Scheme 2.18). 

Notice that whereas in Eq. 16-43 the methyl group is transferred as CH3+ by nucleophilic displacement on a carbon atom, the transfer to Hg2+ in Eq. 16-44 is that of a carbanion, CH3, with no valence change occurring in the cobalt. However, it is also possible that transfer occurs as a methyl radical.420 Methyl corrinoids are able to undergo this type of reaction nonenzymatically, and the ability to transfer a methyl anion is a property of methyl corrinoids not shared by other transmethy-... 

In the following discussion, we shall classify, for the sake of convenience, the various mechanisms according to the nature of the methyl group being transferred. 

Type 1. The methyl group is transferred as an anion to the electrophilic metal substrate. 

Figure 5 Proposed catalytic cycle of MCR. Methyl CoM binds first to the active site pocket, followed by CoB, which plugs the channel. The methyl group is transferred from CoM to nickel (step 1). The donor for the hydron that is taken up is suggested here to be a tyrosyl residue in the active site. The Ni(III)-methyl group is a strong oxidizing species and oxidizes the CoM-SH to a thiyl radical CoM-S" (step 2). Release of the methyl group takes place by hydronation of the methyl-Ni(II), while the CoM forms a disulfide with CoB, with electron transfer to the Ni(I) these two processes are combined in step 3. Repulsion of the sulfonate group by the Ni(I) would favor the release of the product disulfide from the active site (step 4). (Adapted from Ref. 119.)...

The heteroaurate(I) [Au 2,6-(CH2NMe3)2C6H3 I]I is obtained in a different transfer reaction, a methyl group is transferred from Mel to the nitrogen atom of the ortho amino ligand, at the same time the quantitative precipitation... 

The major (salvage) pathways for the formation of phosphatidylcholine and ethanolamine are illustrated in Figure 19.16. Free (dietary) choline and etha-nolamine are converted to their CDP derivatives, which then react with diacyl-glycerol to form phosphatidylcholine and ethanolamine. In the lungs, another pathway forms dipalmitoyl phosphatidylcholine, a powerful surfactant. Phos-phatidylethanolamine may be methylated by S-adenosylmethionine (SAM see Chapter 20) to yield phosphatidylcholine. The reaction is catalyzed by two enzymes the first methyl group is transferred via phosphatidylethanolamine N-methyltransferase I. The other two methyl groups are transferred by phosphatidylethanolamine N-methyltransferase II. Some authorities believe that the two enzymes are identical. It has also been proposed that methylation of phospha-... 

It is probable that methyl transferase reactions proceed by nucleophilic attack on S-adenosylmethionine94 and may involve an SN2-like transition state.95 Thus, inversion of configuration as observed in indolmycin biosynthesis indicates that an odd number of nucleophilic displacements occurs and suggests that the methyl group is transferred directly from donor to (112), i.e. without generation of a methylated-enzyme intermediate.92 The combined results are summarized in Scheme 11. 

Dimethoxy tellurium tetrafluoride methylates pyridine2,3, imidazole2, potassium phthal-imide3, and dimethylformamide3. When alkoxy methoxy tellurium tetrafluorides were reacted with pyridine, only the methyl group was transferred to the base3. 

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 positive inductive effect of a 3-methyl group is transferred to the adjacent 2-position more effectively than that of a 2-methyl to the more remote 5-position. Compared with the rate of a-deuterium exchange for selenophene, itself, the rate is about 0.1 for 5-methyl-2-deuterioselenophene and 0.05 for 3-methyl-2-deuterioselenophene in the presence of strong bases, whereas in acidic media it is about 100 and 1000, respectively.79 The same effect of a methyl group was found... 

Methane bacteria have been shown to catalyze reactions in which the active methyl group is transferred to acceptors such as arsenate or mercury. When extracts are incubated in a hydrogen atmosphere with methylcobalamin, arsenate, and ATP, a volatile arsine derivative is formed (20). Arsines are difficult and dangerous to work with they are extremely poisonous and are oxidized rapidly in air. Fortunately they have an intense garhc odor so the investigator is warned of their presence. 

The first reaction in creatinine formation is the transfer of the amido (or amidine) group of arginine to glycine, forming guanidinoacetate. Subsequently, a methyl group is transferred from the ubiquitous 1-carbon-donor S-adenosylmethionine to guanidinoacetate to produce creatine (from which phosphocreatine is formed), some of which spontaneously cyclizes to creatinine, and is eliminated in the urine. 

Fig. 3. Farnesylation and further processing of proteins containing a C-terminal CaaX-motif. Farnesyl transferase catalyses the transfer of a farnesyl moiety from farnesyl pyrophosphate FPP) to the cysteine residue in the CaaX-motif, where C = Cys, a = usually aliphatic amino acids, and X = Met, Ser Cys, Ala, Gin. The three C-terminal amino acids (aaX) are then cleaved off, and a methyl group is transferred from S-adenosyl methionine to the now C-terminal cysteine residue...

One of the more unusual examples of hydrocarbon activation was reported by Wayland involving an example of radical homolytic cleavage of the C-H bond of methane (Fig. lb). In this reaction, the methyl group is transferred to one porphyrin metal center and the hydrogen to a second metal center. The reaction follows termolecular kinetics, which suggests a linear transition state for the cleavage (Eq. 21) [88]. In addition to methane, only the benzylic C-H bonds of tolu-... 

The methyl group is transferred to appropriate acceptors by specific methyltransferases with production of S-adenosylhomocysteine (Figure 17-15), which is hydrolyzed to homocysteine and adenosine by adenosylho-mocysteinase ... 

Methylation is rarely of quantitative importance in the metabolism of xenobiotics. The methyl group is transferred from the nucleotide S-adenosyl-L-methionine (SAM) by means of a methyltransferase. The functional groups that undergo methylation include primary, secondary and tertiary amines, pyridines, phenols, catechols, thiophenols. The aza-heterocycle pyridine is metabolized to the A-methylpyridin-ium ion, which is more toxic than pyridine itself (Figure 33.18). The binding properties of the ionized metabolite are disturbed by the loss of its hydrophobic feature, resulting from the polarity inversion. 

---