Methionine adenosyl transferase reaction

In addition to the above reactions of amino acids, two others involving specific amino acids must be mentioned. The first reaction involves the conversion of methionine to 5-adenosyl methionine by methionine adenosyl transferase. In this form, methionine is an important methyl donor in animal tissue. The second reaction is that involving aspartate carbamoyl transferase which converts L-aspar-tate to Ai -carbamoyl-I aspartate. This is the begiiming of a series of reactions culminating in the synthesis of pyrimidines. An intermediate step which forms orotidine-S-phosphate is catalysed by orotidine-5-phosphate pyrophosphoxylase. The orotidine-5-phosphate formed is the immediate precursor of uridine-S-phos-phate (UMP) which occupies a central position in pyrimidine synthesis. After some degree of transformation, UMP can be converted to cytosine-5-phosphate (CMP) or thymine-5-phosphate (TMP). It might be noted here that the purine... 

Glutamine transaminase from bovine liver, one of the enzymes involved in methionine catabolism, utilizes SeMet as well as methionine (Blazon et al., 1994). However, with some enzymes, differences in the reaction rates for SeMet and Met have been observed. For example, SeMet is a better substrate than Met for the a,7-elimination by L-methionine 7-lyase of Pseudomonas putida (Esaki et al., 1979). The adenosyltransferase from rat liver reacts with L(+)-SeMet at 51% of the rate with L(+)-Met, and with the corresponding d(—) isomers at only 13 and 10% of the rate of L-Met (Pan and Tarver, 1967). The adenosyl transferase from yeast, on the other hand, is more active with SeMet than with Met (Mudd and Cantoni, 1957). This enzyme produces the... 

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... 

A large number of both endogenous and exogenous compounds can be methylated by several N-, 0-, and S-methyl transferases. The most common methyl donor is S-adenosyl methionine (SAM), which is formed from methionine and ATP. Even though these reactions may involve a decrease in water solubility, they are generally detoxication reactions. Examples of biologic methylation reactions are seen in Figure 7.18. 

S-adenosyl-L methionine (ADO-Met) dependent DNA methyl transferase catalyzed the transfer of a methyl group from AdoMet to a specific nucleotide within the DNA helix (Cheng et al., 1993). In a concerted reaction in the enzyme active site (Fig X) with a simultaneous addition of methyl residue of AdoMet to the cytosine ring and with an elimination of the ring proton by a water molecule requires involving seven heavy nuclei (two ofCys 81, four of AdoMet and one of water. An estimation with aid of Eq. 2.44 leads to value of the reaction synchronization factor asyn 10 4, that does not rule out the concerted mechanism, if the activation energy is less than 10 kcal/mole Nevertheless, a... 

Subsequently, JA can be metabolized in the cytoplasm by at least seven different reactions. Well-characterized reactions include methyla-tion to methyl-jasmonate (MeJA) by S-adenosyl-L-methionine jasmonic acid carboxyl methyl transferase (JMT), conjugation to amino acids by JA amino acid synthase (JARl) or hydroxylation to 12-hydroxyjasmonic acid(12-OH-JA). OPDA, JA, MeJA and JA-Ile are active signalling molecules, whereas 12-OH-JA is thought to be a biologically inactive derivative (Wastemack, 2007). 

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