A Fluorescence-Based Assay for N⁵ -Carboxyaminoimidazole Ribonucleotide Mutase

The enzyme N⁵ -carboxylaminoinidazole ribonucleotide (N⁵ -CAIR) mutase is found in microbial de novo purine biosynthesis but is absent in humans making it an attractive antimicrobial target. N⁵ -CAIR mutase catalyzes the synthesis of carboxyaminoimidazole ribonucleotide (CAIR) from N⁵ -CAIR which is itself prepared from aminoimidazole ribonucleotide (AIR) by the enzyme N⁵ -CAIR synthetase. During our research on identifying inhibitors of N⁵ -CAIR mutase, we developed an innovative, fluorescence-based assay to measure the activity of this enzyme. This assay relies upon our recent serendipitous observation that AIR reversibly reacts with the compound isatin. Reaction of a fluorescently-tagged isatin with AIR resulted in a large increase in fluorescence intensity allowing a measurement of the concentration of AIR in solution. From this observation, we developed a reproducible, non-continuous assay that can replicate the known kinetic parameters of the enzyme and can readily detect a recognized inhibitor of the enzyme. This assay should find utility in screening for inhibitors targeting N⁵ -CAIR mutase.