The Biosynthesis of Folic Acid

A procedure is described for the purification from extracts of Escherichia coli of the enzyme that catalyzes the release of carbon 8 of guanosine triphosphate as formic acid. This reaction is the first in a series of steps involved in the conversion of GTP to the pteridine component of folic acid. The enzyme, which has been purified some 700-fold, has been named GTP cyclohydrolase. No purine, nucleoside, or nucleotide other than GTP can be used as substrate. No coenzyme or metal activator is required in the reaction. The Km for GTP has been determined to be 2.2 × 10−5M. The molecular weight of the enzyme has been estimated to be larger than 300,000. The purified enzyme has been shown to catalyze the formation from GTP of formic acid and the triphospho ester of 2-amino-4-hydroxy-6-(erythro-1′,2′,3′-trihydroxypropyl)dihydropteridine (dihydroneopterin triphosphate) as products. No evidence was obtained to indicate that more than a single protein is involved in this transformation, although theoretical considerations suggest that this phenomenon is the sum of several individual reactions. Evidence is presented which shows that an arsenate-sensitive phosphatase is present in extracts of E. coli and that this enzyme is involved in the dephosphorylation of dihydroneopterin triphosphate, an enzymatic step which is necessary in order for this compound to be converted to dihydropteroic acid.