Fig. 1.

Structure and biosynthesis of folates. (A) Chemical structure of THF, monoglutamyl form. Plant folates have γ-linked polyglutamyl tails of up to approximately six residues attached to the first glutamate. One-carbon units at various levels of oxidation are attached to N-5 and/or N-10. The pteridine ring of folates and free pteridines can exist in tetrahydro-, dihydro-, and fully oxidized forms. (B) The plant folate biosynthesis pathway. Pteridines are in blue. PABA and its precursor aminodeoxychorismate (ADC) are in green. Red arrows are the engineered GTP cyclohydrolase I (GCHI) and ADC synthase (ADCS) reactions. Asterisks show two possible constraints in the pathway in engineered fruit (pteridine transport and phosphorylation). DHN, dihydroneopterin; -P, monophosphate; -PP, pyrophosphate -PPP, triphosphate; DHM, dihydromonapterin; HMDHP, hydroxymethyldihydropterin.