BIOSYNTHESIS OF PTERIDINES IN ESCHERICHIA-COLI - STRUCTURAL AND MECHANISTIC SIMILARITY OF DIHYDRONEOPTERIN-TRIPHOSPHATE EPIMERASE AND DIHYDRONEOPTERIN ALDOLASE

An open reading frame located at 69.0 kilobases on the Escherichia col i chromosome was shown to code for dihydroneopterin aldolase, catalyzi ng the conversion of 7,8 dihydroneopterin to 6-hydroxymethyl-7,8-dihyd ropterin in the biosynthetic pathway of tetrahydrofolate, The gene was subsequently designated folB. The FolB protein shows 30% identity to the paralogous dihydroneopterin-triphosphate epimerase, which is speci fied by the folX gene located at 2427 kilobases on the E. coli chromos ome, The folX and folB gene products were both expressed to high yield in recombinant E. coli strains, and the recombinant proteins were pur ified to homogeneity. Both enzymes form homo octamers. Aldolase can us e L-threo-dihydroneopterin and D-erythro-dihydroneopterin as substrate s for the formation of 6-hydroxymethyldihydropterin, but it can also c atalyze the epimerization of carbon 2' of dihydroneopterin and dihydro monapterin at appreciable velocity. Epimerase catalyzes the epimerizat ion of carbon 2' in the triphosphates of dihydroneopterin and dihydrom onapterin, However, the enzyme can also catalyze the cleavage of the p osition 6 side chain of several pteridine derivatives at a slow rate. Steady-state kinetic parameters are reported for the various enzyme ca talyzed reactions. We propose that the polarization of the 3'-hydroxy group of the substrate could serve as the initial reaction step for th e aldolase as web as for the epimerase activity. A deletion mutant obt ained by targeting the folX gene of E, coli has normal growth properti es on complete medium as well as on minimal medium. Thus, the physiolo gical role of the E. coli epimerase remains unknown. The open reading frame ygiG of Hemophilus influenzae specifies a protein with the catal ytic properties of an aldolase. However, the genome of H. influenzae d oes not specify a dihydroneopterin-triphosphate epimerase.