Aminocyclopropane-1-carboxylic acid synthase

Fig. 1. Ethylene biosynthesis. The numbered enzymes are (1) methionine adenosyltransferase, (2) ACC (l-aminocyclopropane-l-carboxylic acid) synthase, (3) ethylene forming enzyme (EFE), (4) 5 -methylthio-adenosine nucleosidase, (5) 5 -methylthioribose kinase. Regulation of the synthesis of ACC synthase and EFE are important steps in the control of ethylene production. ACC synthase requires pyridoxal phosphate and is inhibited by aminoethoxy vinyl glycine EFE requires 02 and is inhibited under anaerobic conditions. Synthesis of both ACC synthase and EFE is stimulated during ripening, senescence, abscission, following mechanical wounding, and treatment with auxins.

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... 

ZHOU, Z., ALMEIDA, D.E., ENGLER, J., ROUAN, D., MICHIELS, F., VAN MONTAGU, M., VAN DER STRAETEN, D., Tissue localization of a submergence-induced 1-aminocyclopropane-l-carboxylic acid synthase in rice. Plant Physiol., 2002,129,72-84. 

Matarasso, N. et al. (2005) A novel plant cysteine protease has a dual function as a regulator of 1-aminocyclopropane-l-carboxylic acid synthase gene expression. Plant Cell 17,1205-1216... 

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). 

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.

Wounding is assumed to exert its effect at the step where SAM (=S-adenosylmethionine) is converted into ACC (1-aminocyclopropane-l-carboxylic acid), the direct precursor of ethylene. Normally this step, regulated by the enzyme ACC synthase, is rate-limiting in the cascade of events leading to ethylene production (Yang and Hoffman 1984). By means of stress the rate can be increased... 

Ethylene production in plants is driven by two enzyme activities, 1-aminocyelopropane-l-carboxylate synthase (ACC synthase EC 4.4.1.14) that produces 1-aminocyclopropane acid from 5 -adenosylmethionine followed by aminocyclopropanecarboxylate oxidase (ACC oxidase EC 1.14.17.4) decomposing 5 -adenosylmethionine to ethylene, cyanide and... 

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