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 first 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, phosphoethanolamine/phosphocholine phosphatase catalyzes the synthesis of choline from phosphocholine. It requires magnesium as a cofactor.

Choline Biosynthesis I References