Ethylene biosynthetic pathway

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.

Ethylene is now considered to be one of the main plant-hormones involved in fruit development. Many responses formerly believed to result from the presence of auxins are now ascribed to induced ethylene production.425 The biosynthetic pathway for formation of ethylene from methionine, in a wide variety of plant tissues, including shoots of mung bean,426 tomato,427 and pea427 carrot427 and tomato428 roots and the fruits of apple,429,430 tomato,427 and avocado,427 has been elucidated, and is as follows. 

Advances in ethylene biochemistry and physiology have preceded along a number of fronts. Firstly the biosynthetic pathway from methionine to ethylene has been further clarified and intermediates identified. Secondly some progress has been made in recognising two possible receptor sites which are inhibited by Ag ions and C0 , respectively. Thirdly the localization of ethylene production has been shown to be associated with membranes in studies with protoplasts. 

Finally, there are a mixed bag of oxidases, catalysing ethylene formation in plants and many other diverse reactions, illustrated in Figure 13.20, by isopenicillin N-synthase, IPNS, which catalyses the cyclisation of the heterocyclic P-lactam ring. The importance of penicillin- and cephalosporin-related antibiotics in clinical medicine cannot be underestimated and has stimulated the study of their biosynthetic pathways. A key step in the biosynthesis of these antibiotics involves oxidative ring closure reactions of S-(L-a-aminoadipoyl)-L-cysteinyl-D-valine (ACV) to form isopenicillin N, the precursor of penicillins and cephalosporins, catalysed by IPNS (Figure 13.20). The overall reaction utilizes the full oxidative potential of O2, reducing it to two molecules of H2O. As discussed earlier, these enzymes are technically oxidases and the four electrons required for dioxygen reduction come from the substrate. 

Ethylene is amongst the best known plant hormones because of the extensive biochemical and molecular studies on its biosynthesis and its regulation. Ethylene is synthesized in both higher plants and microorganisms, but via different biosynthetic pathways. 

The major biosynthetic pathway of ethylene in higher plants includes L-methionine, S-adenosylmethionine (AdoMet) and 1-anunocyclopropane-l-carboxylic acid (ACC) as the intermediates, and this pathway is commonly called the ACC pathway. It took a decade of extensive studies since methionine was confirmed as the precursor of ethylene in high plant tissues [24,25]. Before ACC was recognised as a direct precursor of ethylene [26,27]. However, some lower plants such as the semiaquatic fern Regnellidium diphyllum and the liverwort Riella helicophylla do not use ACC as a precursor, and there is convincing evidence for the presence of a non-ACC pathway [5]. However, the biochemical characterisation of this non-ACC pathway is yet to be performed. 

Schaffer, R.J., et al. (2007) A Genomics approach reveals that aroma production in apple is controlled by ethylene predominantly at the final step in each biosynthetic pathway([w]). Plant Physiol. 144, 1899-1912... 

The dramatic increase in 6-MM content--90 to 270 fold in two samples--was unexpected. 6-MM is a potent antifungal agent (4) and one of the most important carrot phytoalexins. Usually 6-MM, one of the components that contributes to the bitterness of stored carrots ( O), is not detected in fresh carrots, but develops during storage. Biosynthetic studies indicate that 6-MM is synthesized via the acetate pathway and its production is stimulated by ethylene (. Thus, UV light may trigger ethylene production in carrots which in turn leads to 6-MM accumulation. 

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