Vitamin B9 (folate) biosynthesis pathway (Zea mays)

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Tetrahydrofolate (THF) acts as a carrier for one-carbon units and, less often, as an electron donor. THF contains pterin, p-aminobenzoate, and glutamate moieties. The pterin is made in the cytosol from GTP, p-aminobenzoate is made from chorismate in plastids, and the two are coupled together in mitochondria to give dihydropteroate. Glutamylation and reduction steps in mitochondria then yield THF (Hanson and Gregory, 2011; Ravanel et al., 2011). THF and its one-carbon derivatives (collectively termed folates) occur mainly as polyglutamates; these have a short polyglutamyl tail that can be added in mitochondria, plastids, or cytosol. Folate polyglutamates can be imported into vacuoles, where the polyglutamyl tail can be removed by γ-glutamylhydrolase. p-Aminobenzoate can be converted to its glucose ester in the cytosol, and imported into vacuoles.