SAM S-adenosyl-L-methionine

Scheme 1 The ethylene biosynthetic pathway. The enzymes catalyzing each step are shown above the arrows. SAM S-adenosyl-L-methionine SAMS S-adenosyl-i-methionine synthetase ACC 1-aminocyclopropane-1-carboxylic acid ACS 1-aminocyclopropane-1-carboxylate synthase ACO 1-aminocyclopropane-1-carboxylate oxidase Ade adenine MTA methylthioadenosine. The atoms of SAM recycled to methionine through methionine cycle are marked in red and the atoms of methionine converted to ethylene are marked in bold. For details see text.

NADP Oxidized nicotinamide adenine SAM S-adenosyl L-methionine... 

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

Figure 2.16 Biosynthesis of rutacridone in Ruta graveolens. SAM, S-adenosyl-L-methionine SAH, S-adenosyl-L-homocysteine.

MT methyltransferase NMT N-methyltransferase OMT 0-methyltransferase SAH S-adenosyl-L-homocysteine SAM S-adenosyl-L-methionine Met methionine THF tetrahydrofolate... 

S-adenosyl-L-methionine (AdoMet, SAM) is a cofactor and the most important donor of the methyl (CH3-) group for methyltransferases, including COMT. When the methyl-group has been transferred, the remaining demethylated compound is called S-adenosyl-L-homo-cysteine. 

The possibility that many organic compounds could potentially be precursors of ethylene was raised, but direct evidence that in apple fruit tissue ethylene derives only from carbons of methionine was provided by Lieberman and was confirmed for other plant species. The pathway of ethylene biosynthesis has been well characterized during the last three decades. The major breakthrough came from the work of Yang and Hoffman, who established 5-adenosyl-L-methionine (SAM) as the precursor of ethylene in higher plants. The key enzyme in ethylene biosynthesis 1-aminocyclopropane-l-carboxylate synthase (S-adenosyl-L-methionine methylthioadenosine lyase, EC 4.4.1.14 ACS) catalyzes the conversion of SAM to 1-aminocyclopropane-l-carboxylic acid (ACC) and then ACC is converted to ethylene by 1-aminocyclopropane-l-carboxylate oxidase (ACO) (Scheme 1). 

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... 

So far only a few dozen organofluorine compounds have been isolated from living organisms, for example fluoroacetic acid, 4-fluorothreonine and rw-fluoro-oleic acid [244-246], The reason that nature has not invested in fluorine chemistry could be a combination of low availability of water-dissolved fluoride in the environment due to its tendency to form insoluble fluoride salts, and the low reactivity of water-solvated fluoride ion. However, in 2002, O Hagan and collaborators [247] published the discovery of a biochemical fluorination reaction in a bacterial protein extract from Streptomyces cattleya converting S-adenosyl-L-methionine (SAM) to 5 -fluoro-5 deoxyadenosine (5 -FDA). The same protein extract contained also the necessary enzymatic activity to convert 5 -FDA into fluoroacetic acid. In 2004, the same authors published the crystal structure of the enzyme and demonstrated a nucleophilic mechanism of fluorination [248,249]. 

Recent developments on research into a bacterial C-F bond forming enzyme are reviewed. The fluorinase enzyme was isolated from Streptomyces cattleya in 2002 and shown to catalyse the conversion of fluoride ion and S-adenosyl-L-methionine (SAM) to 5 -fluoro-5 -deoxyadenosine (5 -FDA) and L-methionine. Subsequently, the enzyme has been the subject of cloning, crystallisation, mechanism and substrate specificity studies. This review summarises the current status of this research. 

Fig. 2. A representation of the X-ray-derived structure of the fluorinase. Inset (a) show/s the full structure as a hexamer (dimer of trimers) and inset (b) show/s the protein trimer with three S-adenosyl-L-methionine (SAM) 8 substrate molecules bound at the subunit interfaces [10]. (See Colour Plate Section at the end of this book.)...

Fig. 3. S-adenosyl-L-methionine (SAM) 8 bound to the fluorinase showing hydrogen bonding contacts [10],...

Protein lysine methyltransferases (PKMTs) are a family of enzymes that transfer the activated methyl group from S-adenosyl-L-methionine (SAM) to specific lysine residues on various substrates. The PKMTs have been causally linked to various human diseases including cancer [140], Huntington s disease [141], and growth defects [142, 143]. The substrates of the PKMTs are typically histones [144-146], but there are several methyltransferases methylate non-histone substrates, such as the tumor suppressor p53 [147, 148], the estrogen receptor ERa [149], and the ATPase Reptin [150]. Given the importance of these enzymes in normal and... 

The most widely used methyl donor for enzymatic methyl transfer is the cofactor S-adenosyl-L-methionine (SAM). The methyl moiety on the L-methionine is supplied by another known methyl donor, N5-methyl tetrahydrofolate.30 To date, numerous enzymes that perform SAM- dependent methylation reactions have been described in plants, and several reports attempting to sort out their evolutionary relationships have been published.31- 3... 

Biosynthesis of homoanatoxin-a was examined using Oscillatoria formosa and a mechanism similar to that for anatoxin-a was proposed [13,14]. The origin of the C-12 methyl group that distinguishes homoanatoxin-a from anatoxin-a, was shown through feeding experiments performed with L-[methyl- C]-methionine in the culture of Raphidiopsis mediterranea Skuja. It was proposed that the S-methyl of methionine is transferred to the toxin via S-adenosyl-L-methionine (SAM)-mediated methylation [55]. 

Figure 9.14 Typical elution pattern of phenylethanolamine N-methyltransferase incubation mixtures with the homogenate of rat pons plus medulla oblongata as enzyme. The incubation mixture contained 10 mg of rat pons plus medulla oblongata as enzyme and 16 fxM noradrenaline (NA) and 18 fxM S-adenosyl-L-methionine (SAM) as substrates. (A) Experimental incubation with homogenate of 10 mg of rat pons plus medulla oblongata. (B) Blank incubation without enzyme. (C) Another blank incubation, to which was added IS pmol of adrenaline (AD) the reaction had been stopped. Formation of 16. 6 pmol of AD from NA during 60 minutes of incubation at 37°C was calculated from a calibration curve. DHBA, dihydroxybenzylamine (internal standard) UN, unknown peak. (From Trocewicz et al., 1982.)...

The enzyme catechol-O-methyltransferase (COMT) is involved in pain regulation and regulation of neurotransmitters. COMT acts to catalyze the transfer of a methyl group from the cofactor S-adenosyl-L-methionine (SAM, 5) to the 0 of catecholate 4 (Reaction 9.5). The catechol structure is found in neurotransmitters such as dopamine, noradrenahne, and adrenaline. A related set of enzymes are the DNA methyltransferases that appear to have similar structure, especially in their active sites, to the structure of COMT. ... 

According to the above-mentioned hypothesis, the caffeic acid moiety is retransferred to coenzyme A for further modification reactions. Methylation of the caffeoyl moiety in position 3 is achieved by S-adenosyl-L-methionine (SAM)-dependent 0-methyltransferases (OMTs) either acting on the level of the free acid or the coenzyme A thioester. Hydroxylation in position 5 is catalysed by a cytochrome P450 of the CYP84 family which will be described in more detail. Establishment of the sinapoyl substitution pattern by adding another methyl group will be depicted below. 

S-Adenosyl-L-homocysteine (10) (Fig. 17.12), the product of the reaction, and 2-(2,5-dichlorophenyDcyclopropylamine (1 l)are analogs of S-adenosyl-L-methionine and norepinephrine, respectively. Using these inhibitors it was possible to ascertain the binding order of the two substrates (75). Kinetic analyses showed that SAH was a competitive inhibitor of SAM and a noncompetitive inhibitor of norepinephrine, whereas (1 l)was a competitive inhibitor of norepinephrine and an uncompetitive inhibitor of SAM. This indicates that the binding of substrates is ordered, with SAM binding first. If norepinephrine bound first, it would be expected that SAH would be an uncompetitive inhibitor and (1 l)would be noncompetitive with respect to SAM. If a random... 

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