N5-carboxyaminoimidazole ribonucleotide: evidence for a new intermediate and two new enzymatic activities in the de novo purine biosynthetic pathway of Escherichia coli.

Conversion of aminoimidazole ribonucleotide (AIR) to 4-carboxyaminoimidazole 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 reaction (Meyer et al., 1992), in conjunction with present studies, reveal that PurE and PurK possess independent catalytic activities. PurK is shown, by NMR spectroscopy, to catalyze the conversion of AIR in the presence of HCO3- and ATP to ADP, P(i), and the carbamate of AIR (designated N5-CAIR). PurE has been shown by NMR spectroscopy and kinetic analysis, to catalyze the reversible conversion of N5-CAIR and CAIR. N5-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 discovered in the de novo purine biosynthetic pathway of E. coli.