A/-methyltransferases

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. 

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...

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. 

Trace Amines. Figure 1 The main routes of trace amine metabolism. The trace amines (3-phenylethylamine (PEA), p-tyramine (TYR), octopamine (OCT) and tryptamine (TRP), highlighted by white shading, are each generated from their respective precursor amino acids by decarboxylation. They are rapidly metabolized by monoamine oxidase (MAO) to the pharmacologically inactive carboxylic acids. To a limited extent trace amines are also A/-methylated to the corresponding secondary amines which are believed to be pharmacologically active. Abbreviations AADC, aromatic amino acid decarboxylase DBH, dopamine b-hydroxylase NMT, nonspecific A/-methyltransferase PNMT, phenylethanolamine A/-methyltransferase TH, tyrosine hydroxylase. 

Neural cells convert tyrosine to epinephrine and norepinephrine (Figure 31—5). While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes. Dopa decarboxylase, a pyridoxai phosphate-dependent enzyme, forms dopamine. Subsequent hydroxylation by dopamine P-oxidase then forms norepinephrine. In the adrenal medulla, phenylethanolamine-A -methyltransferase uti-hzes S-adenosyhnethionine to methylate the primary amine of norepinephrine, forming epinephrine (Figure 31-5). Tyrosine is also a precursor of triiodothyronine and thyroxine (Chapter 42). 

Figure 13.4 Histamine synthesis, metabolism and receptors. Current knowledge does not justify presentation of a schematic histaminergic synapse. (1) Histidine decarboxylase (2) histamine-A-methyltransferase (3) mono amine oxidase (MAOb)...

The mechanism of tellurium resistance has been investigated using genetic manipulation similar to that of Se (see above) and cellular oxidant capacity apparently plays an important role.144,206 A few tellurite determinants - both chromosomal and plasmid encoded - have been identified in bacte-ria.113,147 192 207 208 Recent studies have focused on the role of methyltransf-erases in Te resistance. Liu et a/.111 determined that the E. coli gene tehB uses S-adenosyl methionine and a methyltransferase in tellurite detoxification, but while no methylated tellurium compounds (see below) were observed, a loss of tellurite was observed in tellurite-amended cultures and Te complexation was inferred.191... 

A minor pathway to synthesize PC, which is mainly active in liver cells, utilizes the enzyme phosphatidylethanolamine-A-methyltransferase (PEMT), which converts phosphatidylethanolamine (PE) to PC by the subsequent transfer of three methyl groups from S-adenosylmethionine (Vance et al, 1997). The PEMT pathway, which links PE synthesis to PC, was found to be critical for PC homeostasis in the Uver dining choline deficiency (Walkey et al, 1997). 

Studying the sequences of farnesylated proteins indicated that all lipidated proteins bear a cysteine residue near the C-terminus revealing the CAAX-motif, where C is a cysteine, A stands for an aliphatic amino acid, and X can be any amino acid. Database searches resulted in more prenylated proteins, all bearing the CAAX-motif, in systems from lower eukaryotes to mammals. A closer look at the mature proteins revealed that prenylation was only the first step of processing of the CAAX-motif-encoded proteins. After transfer of the isoprene unit, the last three amino acids are cleaved proteolytically by an endoprotease and the C-terminal cysteine is carboxymethylated by a methyltransferase. ... 

Cap formation This process is the addition of a single guanine base to the 5 end of the RNA molecule. The guanine is attached to the terminal nucleotide via a triphosphate link. The guanine is methylated in position 7 of the base, catalysed by a methyltransferase. The cap plays a role in translation by facilitation of the binding of mRNA to the ribosome (see below). 

DeLuca and co-workers have reached somewhat different conclusions. In support of their pathway they have isolated an A-methyltransferase which catalyzes the N -methylation of 16-methoxy-2,3-dihydro-3-hy-droxytabersonine (104) to 4-deacetoxyvindoline (43) (2/6). The Af-methyl-transferase activity was found to co-chromatograph with chlorophyll on Sephadex 100, but subsequent centrifugation gave a supernatant which possessed the A-methyltransferase activity and a pellet which contained the chlorophyll. Highest substrate specificity was shown by 105 (0.07 pkat/100 (jlI enzyme), followed by 2,3-dihydrotabersonine (106). 4-0-Deacetyl-Aa-demethylvindoline (98) was not a substrate, but, unfortunately, 16-0,Aa-bisdemethyl-4-0-deacetylvindoline (107), which is a key intermediate in Stockigt s proposed pathway, was not tested (2/6). 

Note that in nature, these are all enzyme-catalysed reactions. This makes the reactions totally specific. It means possible competing Sn2 reactions involving attack at either of the two methylene carbons in SAM are not encountered. It also means that where the substrate contains two or more potential nucleophiles, reaction occurs at only one site, dictated by the enzyme. The enzymes are usually termed methyltransferases. Thus, in animals an A-methyltransferase is responsible for SAM-dependent A-methylation of noradrenaline (norepinephrine) to adrenaline (epinephrine), whereas an O-methyltransferase in plants catalyses esterification of salicylic acid to methyl salicylate. 

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. ... 

Glycine A-methyltransferase is also reported to have an ordered binding mechanism with SAM binding first to the enzyme, there being no metal-ion dependency. Cooperative behavior is observed with SAM binding. The cooperative nature can be eliminated by the tryptic hydrolysis of the N-terminal eight amino acid residues. 

This enzyme [EC 2.1.1.28], also known as phenylethanol-amine A -methyltransferase, catalyzes the reaction of S-adenosyl-L-methionine with phenylethanolamine to produce 5-adenosyl-L-homocysteine and A -methylphenyl-ethanolamine. The enzyme will act on a number of phe-nylethanolamines and will catalyze the conversion of noradrenalin (or norepinephrine) into adrenalin (or epinephrine). 

The electrophilic potency of the positively charged SAM allows unusual acceptors. In the biosynthesis of myxothiazol A to myxothiazol Z, a methyltransferase catalyzes the esterification of the corresponding carboxylic acid (Scheme 7.19) [74]. 

One of the striking features of enzymatic methylation is its exquisite regjoselec-tivity. For example, in the biosynthesis of novobiocin, a methyltransferase NovO catalyzes the methylation of only one of the three phenolic carbons and none of the three hydroxyl groups are methylated (Scheme 7.20). The exquisite regioselec-tivity is also compatible with substrate promiscuity. For example, the methyltransferase CouO from coumermycin A1 synthase catalyzes the methylation of both the mono- and bis-amides [75, 76]. 

Hibi et al. " have reported on putrescine A-methyltransferase isolated from the nicotine biosynthetic pathway coded by cDNA. Recent advances in cell and molecular biology of alkaloid biosynthesis have heightened awareness of the genetic importance. 

In a similar way, this lowering may facilitate an enzymatic reaction when the non-protonated species is a better substrate for an enzyme than the protonated one. The rate of the methylation of the jS,jS-difluoroethylamine by the A/-methyltransferase of rabbit lung is increased by a factor 4 [36]. 

Winz, R. A. and Baldwin, I. T. (2001). Molecular interactions between the specialist herbivore Manduca sexta (Lepidoptera, Sphingidae) and its natural host Nicotiana attenuata. IV. Insect-induced ethylene reduces jasmonate-induced nicotine accumulation by regulating putrescine A-methyltransferase transcripts. Plant Physiology 125 2189-2202. 

Figure 19-5 Schematic representation of an important chemotactic system of E. coli, S. typhimurium, and other bacteria. The transmembrane receptor activates the autokinase CheA, which transfers its phospho group to proteins CheY and CheB to form CheY-P and CheB-P. CheY-P regulates the direction of rotation of the flagella, which are distributed over the bacterial surface. CheR is a methyltransferase which methylates glutamate carboxyl groups in the receptor and modulates the CheA activity. CheZ is a phosphatase and CheB-P a methylesterase.

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