Methionine ethylene biosynthesis from

The nonprotein amino acid, 1-aminocyclopropane-l-carboxylic acid, is an intermediate of ethylene biosynthesis in plants. This amino acid is synthesized from the L-a-amino acid methionine through the intermediate 5 -adenosyl-L-methionine (SAM) (Scheme 8). ... 

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 pathway of ethylene biosynthesis in higher plants is from l-methionine4 (Figure 5.9). Methionine is an intermediate in other metabolic processes and the control of ethylene biosynthesis via the interference of methionine production is not realistic. The ACC synthase step from S-adenosyl methionine to ACC appears more susceptible to chemical modification auxin promotes ethylene production by increasing the activity of ACC synthase. Subsequent steps from ACC are less controlled and ethylene is readily produced from the conversion of ACC in most tissues. 

Aminocyclopropane carboxylic acid (6) has been detected in several plant tissues a procedure for preparing 6 from agricultural wastes, by extraction with a diluted solution of sulfosalicylic acid, has been described . 6 was established to be an intermediate product in ethylene biosynthesis " . Ethylene acts as a phytohormone which is involved in many metabolic processes in plants, e.g. in ripening, in stress situations or after wounding (see review and references cited therein). Natural 6 is formed from methionine via sulfonium salt (640) only S,S-(640) acted as a substrate for aminocyclopropanecarbo-xylate synthase, the S,R and R,R isomers of 640 were inactive as substrates . 6 can be... 

The subjects covered in this volume are not unique, with the exception of Chapter 6 on ethylene biosynthesis. This subject was added to this volume because it describes the synthesis of the simplest two-carbon double bond system. Its mechanism of synthesis from methionine is unusual and may indeed serve as one of several models for the equally complex desaturation systems found in plants. 

The biosynthesis begins from methionine (63 see Chapter 27) with 1-amino-cyclopropane-l-carboxylic acid (64, ACC see Chapter 28) as a key intermediate. ACC oxidase aminocyclopropanecarboxylate oxidase) is involved in the last step of ethylene biosynthesis. ACC oxidase is an enzyme that catalyzes ethyleneforming reactions. 

Fig, 7. Pathways for the metabolism of methionine to 5 -methylthioadenosine (MTA) and recycling of MTA to methionine. Methionine can serve as a carbon source for the synthesis of polyamines and, in some tissues, ethylene. 5 -Methylthioadenosine is a product of both processes. Only the methylthio group of methionine is recycled, the C4 moiety for the resynthesis of methionine being derived from the ribosyl moiety of ATP. The enzymes involved are (1) SAM synthetase, (2) SAM decarboxylase, (3) various C3 transfer enzymes of polyamine biosynthesis, (4) MTA nucleosidase, (5) methylthioribose kinase, (6) three( ) uncharacterized enzymes, (7) aminotransferase, and (8) aminocyciopropane carboxylate synthase. 

Fig. 1. A postulated mechanism for the biosynthesis of ethylene from methionine. The substituted pyridine c arboxaldehyde stands for pyridoxal phosphate. The scheme is modified from that of Adams and Yang (1979).

The aminopropyl transfer from dcSAM results in the release of 5 -methylthio-adenosine (MTA), which is rapidly metabolized and recycled to the SAM precursor methionine in a cyclic pathway known as the methionine salvage cycle (Sauter et al. 2013). MTA is also released from SAM in the biosynthesis of ethylene and nicoti-anamine and considered a toxic metabolite because of product inhibition. A study has shown that MTA affects the synthesis of polyamines (Waduwara-Jayabahu et al. 2012). 

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