5 -Methylthioadenosine

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). 

In addition to ACC, ACS produces 5 -methylthioadenosine (MTA), which is recycled through methionine cycle to methionine (see Scheme 1). Recycling of MTA back to methionine requires only the available ATP. A constant concentration of cellular methionine is maintained even when ethylene is rapidly synthesized or when the pool of free methionine is small. The methionine cycle involves the following subsequent intermediates MTA, 5-methylthioribose (MTR), 5-methylthioribose-1-phosphate (MTR-l-P), 2-keto-4-methylthiobutyrate (KMB), and then the recycled 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.

The synthesis of 964 was achieved by coupling 6-chloropurine and 4-methylthiobutan-l-ol (962) under the influence of DEAD/PhaP to give 963, followed by ammonolysis to 964 [93JCS(P1)1109], It was a potent inhibitor of the enzyme methylthioadenosine nucleosidase. 

ACC 71 synthase, i. e. (S)-adenosylmethionine methylthioadenosine lyase (EC 4.4.1.14), has been purified from several plant tissues [116]. Recently, ACC synthase cDNA clones have been isolated and sequenced from wounded fruit tissues of tomato, winter squash, zucchini, ripening apple and tomato fruit. Using the polymerase chain reaction (PCR), four different ACC synthase gene fragments were obtained by amplification of cDNA derived from mRNA of tomato... 

This enzyme [EC 3.2.2.9] catalyzes the hydrolysis of 5-adenosylhomocysteine to generate adenine and S-ribosylhomocysteine. The enzyme will also act on 5 -methylthioadenosine to give adenine and 5-methyl-thioribose. See also Methylthioadenosine Nucleoside Hydrolase... 

This enzyme [EC 3.3.1.2], also referred to as -adenosyl-methionine cleaving enzyme and methylmethionine-sulfonium-salt hydrolase, catalyzes the hydrolysis of -adenosylmethionine to produce methylthioadenosine and homoserine. The enzyme will also convert methyl-methionine sulfonium salt to dimethyl sulfide and homoserine. 

V. Singh, W. Shi, G. B. Evans, P. C. Tyler, R. H. Fumeaux, S. C. Almo, and V. L. Schramm, Picomolar transition state analogue inhibitors of human 5 -methylthioadenosine phosphorylase and X-ray stmcture with MT-immucillin-A, Biochemistry, 43 (2004) 9-18. 

J. A. Gutierrez, T. Crowder, A. Rinaldo-Matthis, M.-C. Ho, S. C. Almo, and V. L. Schramm, Transition state analogs of 5 -methylthioadenosine nucleosidase disrupt quorum sensing, Nat. Chem. Biol., 5 (2009) 251-257. 

Cyclization. A second kind of reaction is represented by the conversion of S-adenosylmethionine to aminocyclopropanecarboxylic acid, a precursor to the plant hormone ethylene (see Chapter 24).159 The quinonoid intermediate cyclizes with elimination of methylthioadenosine to give a Schiff base of the product (Eq. 14-27).160-161a The cyclization step appears to be a simple SN2-like reaction.162... 

A salvage pathway. Another product of S-adenosylmethionine is S -methylthioadenosine,... 

Adenine phosphoribosyltransferase catalyzes the conversion of adenine to AMP in many tissues, by a reaction similar to that of hypoxanthine-guanine phosphoribosyltransferase, but is quite distinct from the latter. It plays a minor role in purine salvage since adenine is not a significant product of purine nucleotide catabolism (see below). The function of this enzyme seems to be to scavenge small amounts of adenine that are produced during intestinal digestion of nucleic acids or in the metabolism of 5 -deoxy-5 -methylthioadenosine, a product of polyamine synthesis. 

Adams and Yang (10) have suggested that the S atom of methionine is recycled in the ethylene reaction pathway, as shown in Fig. 2. In this scheme, 5 -methylthioadenosine, the residual molecule which derives from the reaction converting SAM to ACC, is further metabolized to 5 -methylthioribose, which then transfers the S-methyl group to homoserine to form methionine. This scheme is hypothetical, and the enzymes necessary for all these reactions have not as yet been demonstrated. 

Porcelli, M., Cacciapuoti, G., Cimino, G., Gavagnin, M., Sodano, G., and Zappia, V., Biosynthesis and metabolism of 9-[5 deoxy-5 -(methylthio)-P-D-xylofuranosyl]adenine, a novel natural analogue of methylthioadenosine, Biochem. J., 263, 635, 1989. 

Figure 7-11. 5 -p-Cl-phenylthio-DADMe-Immucillin-A is a femtomolar transition state analogue for E.coli 5 -methylthioadenosine/S-adenosylhomocysteine nucleosidase (MTAN). Initial (Kj) and equilibrium (Kf) dissociation constant are shown... 

KIEs analysis and its practical application to the design of TS analogues were recently applied to other enzymes belonging to the N-ribosyltransferase family. Besides human PNP78 and M. tuberculosis PNP64 inhibitors in the picomolar dissociation constant range have been revealed for human 5 -methylthioadenosine phosphorylase (MTAP), an enzyme from the polyamine pathway considered as a target in proliferative diseases.79,80... 

Evans GB, Furneaux RH, Schramm VL, Singh V, Tyler PC (2004) Targeting the polyamine pathway with ttansition-state analogue inhibitors of 5 -methylthioadenosine phosphorylase. J. Med. Chem. 47 3275-3281... 

Singh V, Evans GB, Lenz DH, Mason JM, Clinch K, Mee S, Painter GF, Tyler PC, Furneaux RH, Lee IE, Howell PL, Schramm VL (2005) Femtomolar transition state analogue inhibitors of 5 -methylthioadenosine/S-adenosylhomocysteine nucleosidase from Escherichia coli. J. Biol. Chem. 280 18265-18273... 

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