Transferases methyltransferases

EC2 Transferases methyltransferase, aspartate carbamoyltransferase, etc.) EC2.3 Acvltransferases (amino-acid N-acetyltransferase, ATP citrate synthase, etc.) luciferasa ... 

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... 

Although the pathway has not been established, relatively high yields of trimethyltin from inorganic tin have been observed in yeast concomitant with the degradation of butyltin compounds (Errecalde et al. 1995). Exceptionally, methionine transferase may carry out the methylation of Hg in Neurospora crassa (Landner 1971) and thiopurine methyltransferase the methylation of inorganic Se in Escherichia coli (Ranjard et al. 2003). 

A common way to benefit from the ability to combine different molecular orbital methods in ONIOM is to combine a DFT or ab-initio description of the reactive region with a semi-empirical treatment of the immediate protein environment, including up to 1000 atoms. Due to the requirement for reliable semi-empirical parameters, as discussed in Section 2.2.1, this approach has primarily been used for non-metal or Zn-enzymes. Examples include human stromelysin-1 [83], carboxypeptidase [84], ribonucleotide reductase (substrate reaction) [85], farnesyl transferase [86] and cytosine deaminase [87], Combining two ab-initio methods of different accuracy is not common in biocatalysis applications, and one example from is an ONIOM (MP2 HF) study of catechol O-methyltransferase [88],... 

Figure 6.1 Major branch pathways of flavonoid biosynthesis in Arabidopsis. Branch pathways, enzymes, and end products present in other plants but not Arabidopsis are shown in light gray. Abbreviations cinnamate-4-hydroxylase (C4H), chalcone isomerase (CHI), chalcone synthase (CHS), 4-coumarate CoA-ligase (4CL), dihydroflavonol 4-reductase (DFR), flavanone 3-hydroxylase (F3H), flavonoid 3 or 3 5 hydroxylase (F3 H, F3 5 H), leucoanthocyanidin dioxygenase (LDOX), leucoanthocyanidin reductase (LCR), O-methyltransferase (OMT), phenylalanine ammonia-lyase (PAL), rhamnosyl transferase (RT), and UDP flavonoid glucosyl transferase (UFGT).

Researchers studying the metalloenzyme hydrogenase would like to design small compounds that mimic this enzyme s ability to reversibly reduce protons to H2 and H2 to 2H+, using an active center that contains iron and nickel. Cobalamins (vitamin and its derivatives) contain an easily activated Co-C bond that has a number of biological functions, one of which is as a methyl transferase, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR). This enzyme converts homocysteine (an amino acid that has one more CH2 group in its alkyl side chain than cysteine see Figure 2.2) to methionine as methylcobalamin is converted to cobalamin. 

Fig. 14.2 Scheme of thiopurine drug metabolism. HPRT, hypoxanthine phosphoribosyl transferase 6-MMP, 6-methylmercaptopurine 6-TGN, 6-thioguanine nucleotides 6-TIMP, 6-thiosine monophosphate TPMT, thiopurine methyltransferase XO, xanthine oxidase... 

How are the differential patterns of methylation in the imprinted alleles established and maintained Some evidence exists that differential methylation at the paternal and maternal imprinted alleles are established independently. Knockout of Dnmt3L, a protein that shares homology with de novo DNA methyl-transferase enzymes but does not possess methyltransferase activity, results in a complete loss of the maternally imprinted DNA methylation marks [57,58]. The precise mechanisms by which the paternal methylation imprints are established are yet to be determined. 

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). 

Lysine methyltransferases catalyze the transfer of methyl groups from the cosubstrate SAM to certain lysine residues in histone proteins. To characterize modulators of these transferases, the above-mentioned antibody-based assay protocols are also applicable. 

With DNA-(cytosine-C ) methyltransferases, the binding mechanism also appears to be ordered with an active-site cysteinyl residue required in the enzyme reaction path. However, in this case SAM is the second substrate. The binding mechanism appears to be random with DNA-(adenine-A ) methyl transferase. 

Methyltransferase enzymes are widespread in the plant kingdom, but they are frequently very substrate specific. The substrate specificity of the methyl transferase from the onion enzyme system was tested with 18 different substrates (Table I). Pentachlorothiophenol was the best substrate tested however, four other substrates showed high levels of activity and only nine substrates showed less than 5% of the activity of pentachlorothiophenol. The three most active substrates were ortho substituted thlophenols. 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

---