Methyltransferase reactions catalyzed

With the characterized mechanism, the next key question is the origin of its catalytic power. A prerequisite for this investigation is to reliably compute free energy barriers for both enzyme and solution reactions. By employing on-the-fly Born-Oppenheimer molecular dynamics simulations with the ab initio QM/MM approach and the umbrella sampling method, we have determined free energy profiles for the methyl-transfer reaction catalyzed by the histone lysine methyltransferase SET7/9... 

Important pathways requiring SAM include synthesis of epinephrine and of the 7-methylgua-nine cap on eukaryotic mRNA, Synthesis of SAM from methionine is shown in Figure T17-3. After donating the methyl group, SAM is converted to homocysteine and remethylated in a reaction catalyzed by N-methyl THF-homocysteine methyltransferase requirii both vitamin Bj2 and N-meth d-THF. The methionine produced is once again used to make SAM. 

Methyltransferases that utilize S-adenosyl-L-methionine as the methyl donor (and thus generating S-adenosyl-L-homocysteine) catalyze (a) A-methylation (e.g., norepinephrine methyltransferase, histamine methyltransferase, glycine methyltransferase, and DNA-(adenine-A ) methyltransferase), (b) O-methylation (e.g., acetylsero-tonin methyltransferase, catechol methyltransferase, and tRNA-(guanosine-0 ) methyltransferase), (c) S-methyl-ation (e.g., thiopurine methyltransferase and methionine S-methyltransferase), (d) C-methylation (eg., DNA-(cy-tosine-5) methyltransferase and indolepyruvate methyltransferase), and even (e) Co(II)-methylation during the course of the reaction catalyzed by methionine syn-thase. ... 

Since preliminary studies showed that 6-hydroxymellein-O-methyl-transferase activity was appreciably inhibited in the presence of the reaction products, the mode of product inhibition of the enzyme was studied in detail in order to understand the regulatory mechanism of in vivo methyltransfer. It is well known that S-adenosyl-Z.-homocysteine (SAH), which is a common product of many O-methyltransferases that use SAM as methyl donor, is usually a potent inhibitor of such enzymes. In the 6-hydroxymellein-Omethyltransferase catalyzing reaction another product of this enzyme, 6-methoxymellein, has pronounced inhibitory activity, in addition to SAH. Since the specific product of the transferase reaction, 6-methoxymellein, is capable of inhibiting transferase activity [88], this observation suggests that activity of the transferase is specifically regulated in response to increases in cellular concentrations of its reaction products in carrot cells. It has been also found that 6-methoxymellein inhibits transferase activity with respect not only to 6-hydroxymellein but also to SAM, competitively. This competitive inhibition was also found in SAH as a function of the co-substrates of the enzyme [89]. It follows that the reaction catalyzed by 6-hydroxymellein-O-methyltransferase proceeds by a sequential bireactant mechanism in which the entry of the co-substrates to form the enzyme-substrate complexes and the release of the co-products to generate free enzyme take place in random order [Fig. (7)]. This result also implies that 6-methoxymellein and SAH have to associate with the free transferase protein to exhibit their inhibitory activities, and cannot work as the inhibitors after the enzyme forms complexes with the the substrate. If, therefore, 6-hydroxymellein-O-methyltransferase activity is controlled in vivo by its specific product 6-methoxymellein, this compound should... 

SCHEME la.—Reaction Catalyzed by S-Adenosylmethionine Erythromycin C O-Methyltransferase. (D = desosamine.)... 

Methylcobalamin is completely different from adenosylcobalamin because it is essentially a conduit for synthetic reactions catalyzed by methyltransferases, illustrated in Scheme 2 for the case of methionine. These reactions depend on the supemucle-ophilicity of cob(I)alamin. In one case, this species removes a methyl group from A -methyltetrahydrofolate with the formation of methylcobalamin, and then transfers this group to the acceptor homocysteine, which results in the synthesis of methionine (see Scheme 2). 

Out of more than 150 different reactions catalyzed by the methyltransferases more than 90 are carried on small molecules. The methyl groups serve here as important chemical building blocks required for the construction of essential cellular components and metabolites. A-methylation of the smallest amino acid glycine is responsible for regulating AdoMet/AdoHcy ratio in the eukaryotic cells (5). Catechol O-methyltransferase (COMT) is responsible for dopamine and other catecholamine... 

Methionine can be regenerated by the transfer of a methyl group to homocysteine fromTV -methyltetrahydrofolate, a reaction catalyzed by methionine synthase (also known as homocysteine methyltransferase). 

Two models have been proposed for this methyl-transfer reaction, (i) Recent evidence from Kengen et al. suggests that the methyl group of CH3-H4MPT is first transferred to a corrinoid protein called methyltransferase a (MTa) yielding protein-bound CH3-B12 HBI (Reaction 19) which acts as the methyl-donor to CoM in a reaction catalyzed by methyltransferase MT2, a non-corrinoid protein (Reaction 20) [157] ... 

CoM, because the purified methyl reductase in methanogens is specific for methyl CoM and not methylcobalamin. It appears that methylcobalamin reacts with coenzyme M, 2-mercaptoethanesul-fonic acid, to give methylcoenzyme M in a reaction catalyzed by a methyltransferase enzyme. Finally, the methyl CoM is reduced to give methane (equations 41 and 42). It is noteworthy that methylcobalamin once more reacts with a thiol. The methyl coenzyme M reductase which catalyzes reaction (42) has a nickel-containing prosthetic group, factor F430, which is discussed in Section 62.1.7.2. 

S-adenosyl-L-methionine (SAM)-dependent methyl-ation was briefly discussed under Thiomethylation (see Figure 14). Other functional groups that are methylated by this mechanism include aliphatic and aromatic amines, N-heterocyclics, monophenols, and polyphenols. The most important enzymes involved in these methylation reactions with xenobiotics are catechol O-methyltransferase, histamine N-methylt-ransferase, and indolethylamine N-methyltransferase - each catalyzes the transfer of a methyl group from SAM to phenolic or amine substrates (O- and N-methyltransferases, respectively). Methylation is not a quantitatively important metabolic pathway for xenobiotics, but it is an important pathway in the intermediary metabolism of both N- and O-contain-ing catechol and amine endobiotics. 

The catecholamines epinephrine, norepinephrine, and dopamine are inactivated by oxidation reactions catalyzed by monoamine oxidase (MAO) (Figure 15.10). Because MAO is found within nerve endings, catecholamines must be transported out of the synaptic cleft before inactivation. (The process by which neurotransmitters are transported back into nerve cells so that they can be reused or degraded is referred to as reuptake.) Epinephrine, released as a hormone from the adrenal gland, is carried in the blood and is catabolized in nonneural tissue (perhaps the kidney). Catecholamines are also inactivated in methylation reactions catalyzed by catechol-O-methyltransferase (COMT). These two enzymes (MAO and COMT) work together to produce a large variety of oxidized and methylated metabolites of the catecholamines. 

Figure 25 Soluble catechol-O-methyltransferase (S-COMT). (a) Inhibitor 3,5-dinitrocatechol (114). (b) Reaction catalyzed by S-COMT.

Figure 8. The reaction catalyzed by the enzyme catechol O-methyltransferase. The methyl group is transferred from 5-adenosylmethionine to the catechol molecule.

The brief duration of bronchodllation is a result of facile metabolic Inactivation. Upon reaching systemic circulation isoproterenol is rapidly accumulated into extraneuronal cells, perhaps by an uptake-2 process (13), where, except in the gut, it is inactivated in a reaction catalyzed by catechol 0-methyltransferase (COMT) which methylates the meta-OH group (14). Isoproterenol s lack of activity following oral administration is a consequence of its metabolic conversion into readily excreted meta- or para-ethereal sulfates by sulfoklnases in the Intestine (15). 

Fig. I. Reaction catalyzed by the O -methylguanine-DNA methyltransferase (MGMT) transferase from T. kodakaraensis (Tk MGMT).

In the proposed biosynthesis pathway of wybutosine, the acp-transfer step is catalyzed by Tyw2, which has similarity to methyltransferases that catalyze nucleophilic methyl-transfer reactions (Figure 1.11). 

All eukaryotic nascent mRNAs, except for some eukaryotic viral RNAs, contain 5 -end cap structures. The cap consists of an N-methylguanine nucleotide connected to the S end (nucleotide N) of the RNA by three phosphate groups, i.e., m G(5 )ppp(5 )N h The cap is added to the free 5 ends of mRNAs in the nucleus before the polymerase has transcribed more than 20 nt. The biogenesis of the 5 -end cap involves a series of enzymatic reactions catalyzed by nucleotide phosphorylase (pppN ppN -I- Pj), RNA guanylyltransferase (ppN -I- GTP GpppN -I- PPj), and RNA methyltransferase (GpppN -P AdoMet - m GpppN -P AdoHcy). 

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