Ethylene biosynthesis pathway

The ACS function is known only in higher plants. The activity of ACS isozymes is a key regulatory factor of ethylene biosynthesis pathway. In general, microorganisms liberate ethylene but their ethylene synthesis pathways do not involve ACC as an intermediate. Penicillium citrinum is the first reported microorganism that is able to synthesize ACC from SAM and to degrade it into ammonia and a-ketobutyrate, not to ethylene. ACS from P. citrinum shows a 100-fold higher for SAM than its plant counterparts. ... 

Yu and Wang431 considered that indole-3-acetic acid exerts its stimulating effect on expansion growth by inducing the synthesis of the enzyme catalyzing the conversion of S-adenosylmethionine into ACC, a conclusion at variance with the suggestion of Vioque and coworkers432 that indoleacetic acid oxidase and its substrate (IAA) participate in the last reaction in the ethylene biosynthesis pathway, namely, the formation of ethylene from ACC. 

Maillard, P., Thepenier, C Gudin, C., 1993. Determination of an ethylene biosynthesis pathway in the unicellular green alga. Haematococcus pluvialis. Relationship between growth and ethylene production. J Appl. Phycol. 5, 93-98. 

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. 

Fig. 2. Pathway and regulation of ethylene biosynthesis. Dotted arrow represents the rate-limiting reaction solid heavy arrow indicates induction by auxin of synthesis of the enzyme hollow arrow indicates inhibition by AVG of the conversion (from Yu et al., 1979).

The mechanistic dichotomy for conversion of ACC to ethylene seems clear from the large body of work presented above. Formation of N-heteroatom derivatives leads to the nitrene or nitrenium ion and results in a concerted mechanism, while electron transfer/free radical oxidants lead to a radical cation and result in a non concerted mechanism. Despite the significant evidence in favor of the radical pathway, reference to N-hydroxylation and nitrenium ion formation as a key step in ethylene biosynthesis has persisted, particularly in the plant physiology literature (2, 43-46). The sequence similarity of the EFE and several hydroxylase enzymes (vide supra) has only added fuel to this fire. However, consideration of the mechanisms for known hydroxylation processes makes the intermediacy of N-hydroxy-ACC very unlikely. 

The most commercially successful PGRs are those that operate either through the inhibition of gibberellin biosynthesis, from mevalonic acid (Knee, 1982) or through the production of ethylene. These represent the two most valuable pathways for growth modification since they have key roles in extension growth, ripening, fruit set and dominance. 

Most of the compounds cited in this introductory section are produced in metabolic processes where the cyclopropane-containing metabolite appears to be the stable end product or secondary product with as yet unobvious metabolic function. However, this is not the case in at least two types of systems, in which cyclopropyl species are key and necessary intermediate structures in high flux metabolic pathways. The first example is the squalene (76) and phytoene (88) biosynthesis where presqualene pyrophosphate (77) and prephytoene pyrophosphate (89) are obligate cyclopropanoid intermediates in the net head-to-head condensations of two farnesyl pyrophosphate (73) or two geranylgeranyl pyrophosphate (66) molecules respectively. The second example is in plant hormone metabolism where C(3) and C(4) of the amino acid methionine are excised as the simple hormone ethylene via intermediacy of 1-aminocyclopropane-l-carboxylic acid (9). Both examples will be discussed in detail in the Section II. 

Degradation products of LOOHs are able to initiate the production of the ethylene, kinases and G-proteins required to induce an oxidative burst. These events are apparently followed by activation of the genes which encode the generation of jasmonic acid and of salicylic acid. These are in turn able to induce the production of enzymes of the phenylpropanoid pathway [149,150], and enzymes which initiate the biosynthesis of terpenes (e.g. 3-hydroxy-3-methylglutaryl coenzyme A reductase [145,151-153]) and lignins. 

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. 

Another catabolic pathway is transamination (Fig. 24-25, rection h) to 3-mercaptopyravate. The latter compound can be reductively cleaved to pyruvate and sulfide. Cysfeine can also be oxidized by NAD and lacfafe dehydrogenase to 3-mercaptopyruvate. An interesting PLP-dependent p-replacement reaction of cysfeine leads to P-cyanoalanine, the lathyritic factor (Box 8-E) present in some plants. This reaction also detoxifies the HCN produced during the biosynthesis of ethylene from ACC. 

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