ENZYMATIC CHARACTERIZATION OF BACILLUS-SUBTILIS GTP CYCLOHYDROLASE-I - EVIDENCE FOR A CHEMICAL DEPHOSPHORYLATION OF DIHYDRONEOPTERIN TRIPHOSPHATE

GTP cyclohydrolase I catalyses the first committing step in the biosynthesis of the pterin moiety of folic acid: conversion of GTP to dihydroneopterin triphosphate. GTP cyclohydrolase I of Bacillus subtilis was purified to homogeneity and shown to have a homo-octameric structure. The enzyme had an apparent K-m for GTP of 4 mu M and, in the absence of cations, a V-max of 80 nmol/min per mg of protein. K+ ions moderately increased its V-max, whereas UTP and Ca2+ and Mg2+ ions drastically increased its K-m for GTP. Dihydrofolate and other products of the folate and tetrahydrobiopterin pathways did not inhibit GTP cyclohydrolase I. In addition to their effect on the enzyme activity, Ca2+ and Mg2+ ions catalysed the chemical dephosphorylation of dihydroneopterin triphosphate to non-cyclic dihydroneopterin monophosphate, the substrate for the phosphomonoesterase reaction in folate biosynthesis. This dephosphorylation was specific and did not require the action of a phosphatase. We suggest a physiological role for Ca2+ ions and UTP in regulation of folate biosynthesis at the levels of GTP cyclohydrolase I and dephosphorylation of dihydroneopterin triphosphate.