This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Biosynthesis → Fatty Acid and Lipid Biosynthesis → Choline Biosynthesis|
Some taxa known to possess this pathway include : Ricinus communis
Expected Taxonomic Range: Magnoliophyta
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 [Mou02]. 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 [McNeil01].
The choline biosynthetic pathway enables plants to decouple choline synthesis from lipid metabolism (Kennedy pathway - diacylglycerol and 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 [Rontein01].
About This Pathway
The first step in choline biosynthesis is the direct decarboxylation of serine to ethanolamine [Rontein01], which is catalyzed by a serine decarboxylase unique to plants [Rontein03]. 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 [McNeil01].
The N-methylation of free base intermediates has been demonstrated in crude extracts of castor bean (Ricinus communis) endosperm tissue. In this nutritive tissue the first methylation step appears to favor ethanolamine [Prudhomme92] over phosphoethanolamine, which was thought to be the key substrate for further metabolic steps in the plant's choline/phosphocholine biosynthesis [Datko88] [Datko88a]. The resulting monomethylethanolamine (MMEta) is than converted partially to dimethylethanolamine (DMEta) or phosphorylated to phosphomonomethylethanolamine (PMMEta). The DMEta is again partitioned between methylation to choline (Cho) and phosphorylation to phosphodimethylethanolamine (PDMEta). Taken together, it has been demonstrated that N-methylation in castor bean takes part on two different levels of intermediates, the free base laevel to produce choline (Cho) and the phospho-base level to generate phosphocholine (P-Cho) [Prudhomme92a].
The metabolic fate of the end products of those two pathways is different. Free choline accumulates in a major sequestered pool, and is more or less inaccessible for phosphatidylcholin biosynthesis. In contrast, phosphocholin is decidedly involved in this pathway supported by a high in vitro activity of the nucleotide (CDP) pathway [Kinney87] [Tang97] as found in castor bean endosperm. Although the rate and direction of choline recycling still remains to be investigated, the extractable enzyme activities acting on different intermediate levels indicated a well developed capability of this plant to work efficiently with small, highly regulated pools.
Superpathways: superpathway of choline biosynthesis
McNeil01: McNeil SD, Nuccio ML, Ziemak MJ, Hanson AD (2001). "Enhanced synthesis of choline and glycine betaine in transgenic tobacco plants that overexpress phosphoethanolamine N-methyltransferase." Proc Natl Acad Sci U S A 98(17);10001-5. PMID: 11481443
Mou02: Mou Z, Wang X, Fu Z, Dai Y, Han C, Ouyang J, Bao F, Hu Y, Li J (2002). "Silencing of phosphoethanolamine N-methyltransferase results in temperature-sensitive male sterility and salt hypersensitivity in Arabidopsis." Plant Cell 14(9);2031-43. PMID: 12215503
Prudhomme92: Prud'homme M-P, Moore TS Jr. (1992). "Phosphatidylcholine synthesis in castor bean endosperm. Occurrence of an S-adenosyl-L-methionine:ethanolamine N-methyltransferase." Plant Physiol. 100; 1536-1540.
Rontein01: Rontein D, Nishida I, Tashiro G, Yoshioka K, Wu WI, Voelker DR, Basset G, Hanson AD (2001). "Plants synthesize ethanolamine by direct decarboxylation of serine using a pyridoxal phosphate enzyme." J Biol Chem 276(38);35523-9. PMID: 11461929
Rontein03: Rontein D, Rhodes D, Hanson AD (2003). "Evidence from engineering that decarboxylation of free serine is the major source of ethanolamine moieties in plants." Plant Cell Physiol 44(11);1185-91. PMID: 14634155
Tang97: Tang F, Moore ST Jr. (1997). "Enzymes of the Primary Phosphatidylethanolamine Biosynthetic Pathway in Postgermination Castor Bean- Endosperm. Developmental Profiles and Partia1 Purification of the Mitochondrial C1P:Ethanolaminephosphate Cytidylyltransferase." Plant Physiol. 115; 1589-1597.
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