Ethylene from methionine

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. 

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).

A reduction and activation of HjOj by other one-electron donors, like semiquinones, has also to be considered. This follows from a study of the ethylene production from methionine in the presence of pyridoxal phosphate, a reaction characteristic for OH radicals or for Fenton-type oxidants. The ethylene production in the presence of dioxygen, anthraquinone-2-sulfonate, and an NADPH-generating system in phosphate buffer pH 7.6 was inhibited by SOD and by catalase, but stimulated by scavengers of OH radicals, like 0.1 mM mannitol, a-tocopherol, and formiate... 

However, it should be pointed out here that not all plants follow the same synthetic pathway for ethylene. Lower plants (liverworts, mosses, ferns, lycopods) do not produce it from methionine, nor from ACC—an alternative ethylene pathway therefore exists (Osborne et al., 1996). In evolutionary terms this is very significant and it remains to be established whether any cells in higher plants still retain this earlier primitive route for ethylene synthesis as part of their metabolic repertoire. 

Ethylene plays an important role in a number of plant developmental processes, including senescence and abscission of leaves and flowers, responses to wounding, and the ripening of climacteric fruits (Abeles, 1973). In each case ethylene is produced from methionine (Fig. 1). The two enzymes specific to the pathway, ACC synthase and ethylene forming enzyme, increase in activity in response to wounding and during ripening,... 

Methionine is the major precursor in the biochemical pathway to ethylene (9). Ethylene is formed from carbons 3 and 4 of methionine which is degraded in reactions possibly involving free radicals and oxygen (9). Recently Adams and Yang (10,11) identified S-adenosylmethionine (SAM) and 1-aminocyclopropane-l-carboxylic acid (ACC) as intermediates in the pathway from methionine to ethylene. The sequence of reactions in the pathway... 

Figure 1. Reactions from methionine to ethylene showing intermediates and inhibitors of each step in the pathway and the possible direct conversion of...

Figure 2. Proposed pathway from methionine to ethylene indicating recycling of the S atom according to Adams and Yang (10J...

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. 

Rhizobitoxine. Various Rhizoblum. laponlcum strains produce rhizobitoxine [75], whose structure was reported In 1972 (2691. Although the host of this bacterium Is soybean, this compound Is also phytotoxic to many other plant species. This phytotoxin Is an analog of cystathionine and acts as an Irreversible Inhibitor of -cystathlonase which catalyzes production of homoserlne from cystathionine (2701. Rhizobitoxine also Inhibits ethylene production from methionine (2711. as does a similar phytotoxin, 2-am1no-4-methoxy-3-eno1c acid (28). 

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... 

Elstner EE, Saran M, Bors W and LengfelderE (1978) Oxygen activation in isolated chloroplasts. Mechanism offerredoxin-dependent ethylene formation from methionine. Eur J Biochem 89 61-66... 

Some microorganisms in culture show methionine-dependent ethylene formation. In studies with Escherichia coli, 2-oxo-4-methylthiobutyrate (KMB) produced from methionine by transamination was suggested as the precursor of ethylene [19], and subsequently a cell-free system which produced ethylene from KMB in the presence of NAD(P)H, EDTA-Fe and oxygen was established [20]. An enzyme which catalysed a similar ethylene-forming activity was purified from Cryptococcus albidus [15]. The purified enzyme of molecular mass 62 kDa turned out to be NADH EDTA-Fe oxidoreductase. The proposed mechanism involves reduction of EDTA-Fe to EDTA-Fe by the enzyme, reduction of oxygen to superoxide by EDTA-Fe, of hydrogen peroxide to hydroxyl radical, and oxidation of KMB by hydroxyl radical to ethylene. However, an extensive physiological evaluation of this enzyme must be done before it can... 

C4H,N02,Mr 101.1 l,mp. 229-231 °C. A non-proteinogenic amino acid first known as a synthetic product and later isolated from pears and apples. ACC is formed from methionine via 5-adenosylmethionine with the help of ACC synthase (EC 4.4.1.14) and cleaved by ACC oxidase to the multifunctional plant growth substance ethylene which plays key roles in various plant physiological processes such as ripening of fruit, aging, germination, and response to stress. 

The degradation of methionine in its role of ethylene production leads to CO2, formate, and ethylene from the Ci, C2, and Ca,4, respectively, of the a-aminobutyryl moiety. The fate of the methylthio fragment is unclear but it is suggested that it is utilized to resynthesize methionine by transfer to some four-carbon acceptor (Adams and Yang, 1977). 

These authors have shown the release of ethylene from methional in the presence of phagocytizing neutrophils. The oxidizing agent thought responsible for this reaction is OH and not 0 or H202- Further support for the production of OH in the ASC system may be provided by the fact that ethylene is endogenously produced in plants from methionine by a copper-ascorbate-H202 non-enzymatic system (Bors, 1974). 

A similar peptide toxin, rhizobitoxin (16), is produced by the root-nodulating organism Rhizobium japonicum. Rhizobitoxin causes chlorosis in the developing leaflets of plants, such as soybean Glycine max), which have nodules colonized with these strains of Rhizobium japonicum (Mitchell, 1981). This compound is a irreversible inhibitor of ethylene production from methionine, as it blocks the conversion... 

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. 

Free arabinose and galactose, which are often associated with hydroxyproline-rich proteins, are found in the free space of cell walls (132). Levels of proteins and free hydroxyproline are increased by ethylene treatment of pea (Fisum sativa) stem segments. However, the total amount of hydroxyproline in ethanol-insoluble polymers after extraction of the free space with water and Ca was not influenced by ethylene. Terry et al. (132) propose that the response to ethylene, which is now known to be derived from methionine, can be divided into two components. One requires changes in cellulose microfibrils of the cell wall, which result in reorientation of the plane of cell expansion. The other involves a change in the hemicellulosic xyloglucan, which inhibits extension growth of these cells. 

A common neutral amino acid derived from cyclopropane is 1-aminocyclopropane-l-carboxylic acid. The precursor of this carboxylic acid is methionine, or strictly S-adenosyl-L-methionine which is derived from methionine (Figure 2.1). 1-Aminocyclopropane-l-carboxylic acid is also present in apples, pears and other fruits. 1-Aminocyclopropane-l-carboxylic acid serves as a precursor of ethylene (ethene) in essentially all tissues of... 

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. 

The formation of ethylene is often induced by the hormone auxin (Chapter 30), which stimulates activity of the synthase that forms 1-aminocyclopropane-1-carboxylate (ACC) from S-adenosyl methionine (Eq. 14-27, step j Fig. 24-16).320a/b Although ACC has... 

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