N-5-CARBOXYAMINOIMIDAZOLE RIBONUCLEOTIDE - EVIDENCE FOR A NEW INTERMEDIATE AND 2 NEW ENZYMATIC-ACTIVITIES IN THE DE-NOVO PURINE BIOSYNTHETIC-PATHWAY OF ESCHERICHIA-COLI

Conversion of aminoimidazole ribonucleotide (AIR) to 4-carboxyaminoimi dazole ribonucleotide (CAIR) in Escherichia coli requires two proteins , PurE and PurK, previously thought to be subunits of a single enzyme, AIR carboxylase. Past studies revealing an ATP requirement for this r eaction (Meyer et al., 1992), in conjunction with present studies, rev eal that PurE and PurK possess independent catalytic activities. PurK is shown, by NMR spectroscopy, to catalyze the conversion of AIR in th e presence of HCO3- and ATP to ADP, P-i, and the carbamate of AIR (des ignated N-5-CAIR). PurE has been shown by NMR spectroscopy and kinetic analysis, to catalyze the reversible conversion of N-5-CAIR and CAIR. N-5-CAIR has a half-life of 0.9 min at pH 7.8 and 30 degrees C. Thus, two new enzymatic activities and a new intermediate have been discove red in the de novo purine biosynthetic pathway of E. coli.