3-0-Methyltransferases

Methyltransferases are responsible for the methylation a variety of nucleophiles, typically using SAM as the carbon donor. For example, enzymic synthesis of methyl halides through this mechanism has been discovered in cell extracts of Phdlinus promaceus (a white fungus), Endocladia muricata (a marine red algae) and 

Methyl transfer for unactivated substrates often takes place under radical mechanism using methylcobalamin. For example, the key step in the biosynthesis of 

Nature often uses two pathways to produce oxygenated molecules. One is oxidative fictionalization of substrates through enzymatic hydration, monohydroxylation, dihydroxylation or epoxidation. Alternatively, oxygenated compounds can be generated by C—C formation catalyzed by aldolase, transketolase, oxynitrilase and related enzymes, where chiral centers are created simultaneously without an overall change in oxidation states [78]. 

NADP(H) [83, 84]. In addition to dioxygenation, non-heme iron oxygenases can perform monohydroxylation as in the synthesis of L-DOPA from L-tyrosine in mammalian species [85]. 

Enzymatic hydroxylation activation has perhaps the highest potential of all enzyme-catalyzed transformations for synthetic applications. Currently, whole-cell processes are used and the outcomes are often unpredictable. The discovery of new oxygenases and efficient hosts for protein expression remain keys to further expanding the synthetic applications of biocatalytic C—H activation [89, 90]. 

N-Methylation. Several enzymes are known that catalyze (V-methylation reactions. They include histamine (V-methyltransferase, a highly specific enzyme that occurs in 

O-Methylation. Catechol O-mcthy I transferase occurs in the soluble fraction of several tissues and has been purified from rat liver. The purified form has a molecular weight 23,000 daltons, requires S-adenosyIrnclhioninc and Mg+, and catalyzes the methylation of epinephrine, norepinephrine, and other catechol derivatives. There is evidence that this enzyme exists in multiple forms. 

A microsomal O -methyltransI erase that methylates a number of alkyl-, methoxy-, and halophenols has been described from rabbit liver and lungs. These methylations are inhibited by SKF-525, A-ethyl-maleimide and /- -chloromercuribenzoate. A hydroxyin-dole O-methyItransferase, which methylates A -acetyl-serotonin to melatonin and, to a lesser extent, other 5-hydroxyindoles and 5,6-dihydroxyindoles, has been described from the pineal gland of mammals, birds, reptiles, amphibians, and fish. 

The thiol group can then be methylated to yield the methylthio derivative of the original xenobiotic. 

Biomethylation of Elements. The biomethylation of elements is carried out principally by microorganisms and is important in environmental toxicology, particularly in the case of heavy metals, because the methylated compounds are absorbed through the membranes of the gut, the blood-brain barrier, and the placenta more readily than are the inorganic forms. For example, inorganic mercury can be methylated first to monomethylmercury and subsequently, to dimethylmercury  

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Photoluminescent spectra for methyltetrahydrofolate and the enzyme methyltransferase. When methyltetrahydrofolate and methyltransferase are mixed, the enzyme is no longer photoluminescent, but the photoluminescence of methyltetrahydrofolate is enhanced. (Spectra courtesy of Dave Roberts, DePauw University.)... 

Metabolism. MetaboHsm of histamine occurs via two principal enzymatic pathways (Fig. 1). Most (50 to 70%) histamine is metabolized to /V-methylhistamine by A/-methyltransferase, and some is metabolized further by monoamine oxidase to /V-methy1imidazo1eacetic acid and excreted in the urine. The remaining 30 to 40% of histamine is metabolized to imidazoleacetic acid by diamine oxidase, also called histaminase. Only 2 to 3% of histamine is excreted unchanged in the urine. 

Histamine AND histamine antagonists). It is formed from histidine by the enzyme L-histidine decarboxylase. In the periphery, histamine is stored ia mast cells, basophils, cells of the gastric mucosa, and epidermal cells. In the CNS, histamine is released from nerve cells and acts as a neurotransmitter. The actions of histamine ate terrninated by methylation and subsequent oxidation via the enzymes histamine-/V-methyltransferase and monoamine oxidase. 

Modulation of second-messenger pathways is also an attractive target upon which to base novel antidepressants. Rolipram [61413-54-5] an antidepressant in the preregistration phase, enhances the effects of noradrenaline though selective inhibition of central phosphodiesterase, an enzyme which degrades cycHc adenosiae monophosphate (cAMP). Modulation of the phosphatidyl iaositol second-messenger system coupled to, for example, 5-HT,, 5-HT,3, or 5-HT2( receptors might also lead to novel antidepressants, as well as to alternatives to lithium for treatment of mania. Novel compounds such as inhibitors of A-adenosyl-methionine or central catechol-0-methyltransferase also warrant attention. 

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... 

Isoproterenol is given sublingually or by iv. It is metabolized by monoamine oxidase and catechol-0-methyltransferase in brain, Hver, and other adrenergically innervated organs. The pharmacological effects of isoproterenol are transient because of rapid inactivation and elimination. About 60% is excreted unchanged. Adverse effects using isoproterenol therapy include nervousness, hypotension, weakness, dizziness, headache, and tachycardia (86). 

Fig. 2. Biosynthetic pathway for epinephrine, norepinephrine, and dopamine. The enzymes cataly2ing the reaction are (1) tyrosine hydroxylase (TH), tetrahydrobiopterin and O2 are also involved (2) dopa decarboxylase (DDC) with pyridoxal phosphate (3) dopamine-P-oxidase (DBH) with ascorbate, O2 in the adrenal medulla, brain, and peripheral nerves and (4) phenethanolamine A/-methyltransferase (PNMT) with. Cadenosylmethionine in the adrenal...

The farnesylation and subsequent processing of the Ras protein. Following farnesylation by the FTase, the carboxy-terminal VLS peptide is removed by a prenyl protein-specific endoprotease (PPSEP) in the ER, and then a prenylprotein-specific methyltransferase (PPSMT) donates a methyl group from S-adenosylmethionine (SAM) to the carboxy-terminal S-farnesylated cysteine. Einally, palmitates are added to cysteine residues near the C-terminus of the protein. 

The reaction mechanism of the DNA (cytosine-5)-methyltransferase-catalyzed cytosine methylation was investigated at the MP2 and DFT levels [98JA12895]. This system has been modeled by 1-methylcytosine 117, methylthiolate, and trimethylsulfonium. The cytosine methylation is initiated by an attack of the anionic methylthiolate at Cg of the cytosine ring (Scheme 77). The formation of the methylthiolate adduct 118 of the neutral 117 was found to be endothermic in the gas phase and in solution. However, the MP2 and DFT results differ... 

Uroporphyrinogen I (16c), a constitutional isomer of uroporphyrinogen III, also plays no direct role in porphyrin and corrin biosynthesis, but this unnatural substrate is methylated to give 17c10c f in the presence of SAM by the methyl transferase of some bacteria. A constitutional type I dihydroisobacteriochlorin can be obtained by methylation of uroporphyrinogen I with methylase Ml. Methyltransferase M1 is able to methylate the unnatural precorrin once more to give the trimethylpyrrocorphin type I.IOc 1... 

Sherman et al. have analyzed the function of the MitM methyltransferase in more detail [113]. They knocked out the MitM gene and found that mitomytin C production was abolished, and were able to isolate a small quantity of 9a-demethyl mitomycin A (62) from this mutant (Figure 11.9). MitM was expected to act as an O-methyltransferase, but surprisingly recombinant MitM converted 62 into 9-epi-mitomydn B (63), hence acting as an aziridine N-methyltransferase. Mitomy-... 

Figure 11.9 Role of the MitM methyltransferase in mitomycin C biosynthesis.

Together with dopamine, adrenaline and noradrenaline belong to the endogenous catecholamines that are synthesized from the precursor amino acid tyrosine (Fig. 1). In the first biosynthetic step, tyrosine hydroxylase generates l-DOPA which is further converted to dopamine by the aromatic L-amino acid decarboxylase ( Dopa decarboxylase). Dopamine is transported from the cytosol into synaptic vesicles by a vesicular monoamine transporter. In sympathetic nerves, vesicular dopamine (3-hydroxylase generates the neurotransmitter noradrenaline. In chromaffin cells of the adrenal medulla, approximately 80% of the noradrenaline is further converted into adrenaline by the enzyme phenylethanolamine-A-methyltransferase. 

Decitabine (5-aza-deoxycytosine) is an analog of the nucleoside 2 -deoxycytidine. It is believed to exert its antineoplastic effects after phosphorylation and direct incorporation into DNA and by inhibition of the enzyme DNA methyltransferase, causing hypomethylation of DNA and cellular differentiation or apoptosis. DNA hypomethylation is achieved at concentrations below those required to significantly inhibit DNA synthesis, which may promote restoration of function to genes associated with control of cellular differentiation and proliferation. Cytotoxicity in rapidly dividing cells may also result from covalent adducts between DNA methyltransferase and decitabine. 

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... 

Catechol-O-Methyltransferase. Figure. 1 The basic function of COMT. Enzymatic O-methylation of the catechol substrate to 3-methoxy (major route) or 4-methoxy (minor route) products in the presence of Mg2+ and S-adenosyl-methionine (AdoMet). 

Catechol-O-Methyltransferase. Figure 2 Some substrates of COMT. 

Catechol-O-Methyltransferase. Figure 4 Rat model of Parkinson s disease. Comparison of entacapone, tolcapone and CGP 28014 in the rat turning model of Parkinson s disease [4]. 

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