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MetNet - plant pathway - choline biosynthesis I
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Pathway details: choline biosynthesis I


General info Interaction details Linked pathways Protein-protein interactions
Notes
  Pathway was created on Mon Jul 29, 2013.
 Contributed by aracyc:
General information: Choline is a fundamental metabolite in plants because of its contribution to the synthesis of the membrane phospholipid phosphatidylcholine, which accounts for 40 to 60% of lipids in non-plastid plant membranes . Choline is also a precursor for the formation of glycine betaine (glycine betaine biosynthesis III (plants)) in certain plants such as spinach, where this osmoprotectant is accumulated and confers also tolerance to salinity, drought, and other environmental stresses. In addition choline has been recognized as an essential nutrient for humans . The choline biosynthetic pathway enables plants to decouple choline synthesis from lipid metabolism (Kennedy pathway - triacylglycerol biosynthesis) and provides them with the metabolic flexibility to adapt to environmental conditions where large and variable amounts of choline are beneficial for survival . Pathway information: The first step in choline biosynthesis is the direct decarboxylation of serine to ethanolamine , which is catalyzed by a serine decarboxylase unique to plants . Ethanolamine is widely recognized as the entrance compound to choline biosynthesis. The synthesis of choline from ethanolamine may take place at three parallel pathways, where three consecutive N-methylation steps are carried out either on free bases, phospho-bases, or phosphatidyl-bases . Although choline biosynthesis accommodates different pathways in plants, most of them share the enzymatic step forming phosphoethanolamine |CITS: [DATKO88A]| |CITS: [DATKO88B]|. The N-methylation, catalyzed by the phosphoethanolamine N-methyltransferase (PEAMT), is considered to be the committing step exerting a major control over flux throughout the pathway. It has been demonstrated that the PEAMT of spinach catalyzes all three N-methylations necessary to yield phosphocholine. The confirmed intermediates of this pathway have been determined as monomethyl- and dimethylphosphoethanolamine . However, Weretilnyk and coworkers argued that the N-methylation of those three steps may be exercised by two different enzymes as they found a different behavior of the enzyme(s) in response to different light settings. The pathway displayed (methylation pathway) represents the main biosynthetic route as found in the chenopods family. The N-methylation may also occur species-specific on the free-base level |CITS: [PRUDHOMME92A]|or the phosphatidyl-base level as shown for soybean |CITS: [DATKO88A]|, castor bean |CITS: [TANG97]| and barley |CITS: [HITZ81]|. Consequently, it has been assumed that the reactions embedded in the nucleotide and methylation pathways may be two characteristics of one overall phosphoaminoalcohol pathway for the formation of phosphatidylcholine |CITS: [KINNEY93]| in plants. The further metabolic fate of phosphocholine is also species-specific. Phosphocholine can either be directly dephosphorylated to release choline as observed in spinach or incorporated into phosphatidylcholine with the subsequent release of choline, as in tobacco . It has also been suggested that the hydrolization of sinapine yielding sinapic acid and cholin (sinapate ester biosynthesis) may contribute to the content of free choline in plants |CITS: [Strack81]|. The biosynthesis of choline appears to be regulated by a feedback response of phosphocholine inhibiting its own synthesis by decreasing N-methyltransferase (PEAMT) activities involved in the pathway . Pre-growth of carrot and soybean suspension cells as well as Lemna paucicostata in choline enriched culture medium caused a significant decrease of methylated derivatives of phosphoethanolamine |CITS: [MUDD89A]| |CITS: [MUDD89B]|. In contrast, the activity of PEAMT was considerably increased when plants were exposed to salt stress , which is consistent with the high demand for choline as osmoprotectant precursor in such plants.
  Parts of this pathway occur in:   cytosol     nucleus  


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metabolite [19]
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