The Biosynthesis of Folic Acid

The enzyme that catalyzes the removal of a two-carbon compound from 2-amino-4-hydroxy-6-(d-erythro-1',2',3'-trihydroxypropyl)-7,8-dihydropteridine (known by the trivial name of dihydroneopterin) to yield 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine, has been purified by approximately 1100-fold from extracts of Escherichia coli B. The two-carbon compound formed as a product has been identified as glycolaldehyde. Since the reaction is similar to those reactions catalyzed by aldolases, the enzyme has been named dihydroneopterin aldolase. The enzyme is relatively heat-stable in that it can withstand a temperature of 100° for 5 min with virtually no loss of activity. The enzyme functions optimally at pH 9.6. The addition of chelating agents such as EDTA markedly increases the activity of the enzyme. The Km for dihydroneopterin is approximately 9 µm. The enzyme cannot use the following compounds as substrates in place of dihydroneopterin: neopterin, tetrahydroneopterin, dihydroneopterin monophosphate, and dihydroneopterin triphosphate. The product of the reaction, 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine, is a potent inhibitor of the reaction. Other dihydropteridines, including dihydropteroic acid and dihydrofolic acid, are less potent inhibitors. It was found that purified dihydroneopterin aldolase could use as substrate an epimer of dihydroneopterin, i.e. 2-amino-4-hydroxy-(6-l-threo-trihydroxypropyl)-7,8-dihydropteridine, but the corresponding d-threo and l-erythro compounds could not be utilized, although the l-erythro compound could be utilized for the synthesis of 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine in the presence of purified dihydroneopterin aldolase and another enzyme fraction obtained from extracts of E. coli. This suggests that E. coli contains an enzyme that catalyzes the epimerization of the l-erythro compound to a compound that can be used directly as substrate by the purified aldolase (presumably the corresponding l-threo compound).