Ethylene methionine cycle

In addition to ACC, ACS produces 5 -methylthioadenosine (MTA), which is recycled through methionine cycle to methionine (see Scheme 1). Recycling of MTA back to methionine requires only the available ATP. A constant concentration of cellular methionine is maintained even when ethylene is rapidly synthesized or when the pool of free methionine is small. The methionine cycle involves the following subsequent intermediates MTA, 5-methylthioribose (MTR), 5-methylthioribose-1-phosphate (MTR-l-P), 2-keto-4-methylthiobutyrate (KMB), and then the recycled methionine. ... 

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.

AdoMet is an important metabolic intermediate in all organisms, from bacteria to higher animals and plants. It supplies the methyl group to nucleic acids, phenolic substances and alkaloids, or the propylamine moiety to polyamines after decarboxylation. The methionine cycle operates in animals and microorganisms in relation to polyamine synthesis. Thus, enzymes which catalyse all of these reactions are present in all organsims. However, two enzymes in the ACC pathway, ACC synthase (AdoMet methylthioadenosine-lyase) and ACC oxidase, are unique to higher plants. ACC is also malonylated to form N-malonyl ACC, which does not serve as a precursor of ethylene [48,49]. 

The biological significance of the methionine cycle is of great importance for the synthesis of ethylene and polyamines, both of which are biologically active compounds. A small amount of methionine serves as a catalyst of the cyclic process, while ethylene or polyamines are continuously produced. Thus, even in tissues in which supply free methionine through de novo synthesis or degradation of proteins is limited, ethylene and polyamines are produced in large amounts if a sufficient supply of ATP is available. One could say that the real precursor of ethylene and polyamines is ATP. 

The biosynthesis of ethylene has been reviewed recently [9, 33]. In this paper we describe research progress that has occurred since then with regard to (a) ACC synthase, (b) ACC oxidase, (c) ACC N-malonyltransferase, and (d) the methionine cycle, as they related to ethylene biosynthesis. 

The overall result of the methionine cycle is that the 4-carbon moiety of methionine, from which ACC is derived, is ultimately furnished from the ribose moiety of ATP via SAM, while the CH3S group of methionine is conserved for continued regeneration of methionine. The fate of ATP in relation to ethylene production is shown below, where the numbers refer to the carbon position of the ribose moiety of ATP from which each product is derived. 

Ethylene is produced in plants in a complex series of reactions known as the Yang cycle (after its discoverer, S. F. Yang) that starts with the amino acid methionine. 

Plants, in common with microorganisms and animals, require methionine chiefly for three roles, (a) As a component of protein, a role which accounts for most of the methionine in the cell, (b) As a component of methionyl tRNA (in eukaryotes) and formylmethionyl tRNA (in chloroplasts, mitochondria, and prokaryotes), factors required for initiation of protein synthesis. (c) As a component of AdoMet, the chief biological methyl donor, the obligatory precursor of spermidine and spermine, and an effector of certain enzymes. In addition to these chief roles, a major pathway for the metabolism of methionine in certain plant tissues is its conversion to ethylene (see Yang and Adams, this series, Vol. 4, Chapter 6). Only plants and microorganisms can synthesize the homocysteine moiety of methionine novo, and the importance of this synthesis in the sulfur cycle has been noted in the introduction. 

Biosynthesis ofethyiene in plant tissue. The upper cycle is known as the Yang cycle. The two methylene groups of S-adenosyl-L-methionine finally give rise, via the two methylene groups of ACC, to the two carbon atoms of ethylene (indicated by the solid circles). A = Adenine. 

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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