Glycine betaine biosynthesis I, beginning in the cytosol and ending in the peroxisome results in the synthesis of glycine betaine from choline produced from the choline biosynthesis I pathway. Glycine betaine is an amino acid derivative that occurs in plants which serves as a protectant against osmotic stress, mutagenesis, and radiation-induced damage. 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 is then theorized to be transported into the peroxisome. Seventh, choline monooxygenase is a predicted enzyme that is theorized to catalyze the conversion of choline into betaine aldehyde hydrate. Eighth, betaine aldehyde hydrate spontaneously becomes betaine aldehyde, producing water as a byproduct. Ninth, betaine aldehyde uses NAD and water to convet betaine aldehyde into glycine betaine.

Glycine Betaine Biosynthesis I References