Choline is a nitrogen-containing, water-soluble nutrient that is incorporated into the headgroups of membrane phospholipids such as phosphatidylcholine. Two pathways exist for choline biosynthesis whereby serine becomes choline. Both of these pathways take place in the cytosol. This is the second pathway of choline biosynthesis. First, serine decarboxylase (SDC) uses a proton and a pyridoxal 5'-phosphate cofactor to catalyze the conversion of L-serine to ethanolamine, producing carbon dioxide as a byproduct. Second, ethanolamine kinase, localized to the cell membrane (coloured dark green in the image), uses ATP to catalyze the conversion of ethanolamine to O-phosphoethanolamine. Note that this is only the probable ethanolamine kinase in Arabidopsis thaliana and requires further research to confirm its function. Steps 3, 4, and 5 are catalyzed by phosphoethanolamine N-methyltransferase (PEAMT). These three sequential N-methylation steps convert phosphoethanolamine to phosphocholine and utilize S-adenosyl-L-methionine as a methyl donor. The intermediates are as follows: O-Phosphoethanolamine, N-methylethanolamine phosphate, and N-dimethylethanolamine phosphate. Sixth, choline-phosphate cytidylyltransferase (CCT) uses CTP to convert phosphocholine to CDP-choline. Seventh, choline/ethanolaminephosphotransferase (AAPT) uses a 1,2-diacyl-sn-glycerol and either magnesium or manganese ions as cofactors to convert CDP-choline into a phosphatidyl choline, producing CMP and a proton as byproducts. Eighth, phospholipase D uses a calcium cofactor and water to convert a phosphatidylcholine to choline, producing a 1,2-diacyl-sn-glycerol 3-phosphate and a proton as byproducts.

Choline Biosynthesis II References