Aryl Acid Adenylating Enzymes Involved in Siderophore Biosynthesis: Fluorescence Polarization Assay Ligand Specificity and Discovery of Non-nucleoside Inhibitors via High-Throughput Screening

摘要

The design and synthesis of a fluorescent probe Fl-Sal-AMS >6 based on the tight-binding inhibitor 5′-O-[N-(salicyl)sulfamoyl]adenosine (Sal-AMS) is described for the aryl acid adenylating enzymes (AAAEs) known as MbtA, YbtE, EntE, VibE, DhbE and BasE involved in siderophore biosynthesis from Mycobacterium tuberculosis, Yersinia pestis, Escherichia coli, Vibrio cholerae, Bacillus subtilis, and Acinetobacter baumannii respectively. The probe was successfully used to develop a fluorescence polarization assay for these six AAAEs and equilibrium dissociation constants were determined in direct binding experiments. Fl-Sal-AMS was effective for AAAEs, which utilize salicylic acid or 2,3-dihydroxybenzoic acid as native substrates, with dissociation constants ranging from 9–369 nM, but was ineffective for AsbC, the AAAE from Bacillus anthracis that activates 3,4-dihydroxybenzoic acid. Competitive binding experiments using a series of ligands including substrates, reaction products, and inhibitors provided the first comparative structure-activity-relationships for AAAEs. The fluorescence polarization assay was then miniaturized to a 384-well plate format and high-throughput screening was performed at the National Screening Laboratory for the Regional Centers of Excellence in Biodefense and Emerging Infectious Diseases (NSRB) against BasE, an AAAE from Acinetobacter baumannii involved in production of the siderophore acinetobactin. Several small molecule inhibitors with new chemotypes were identified and compound >23 containing a pyrazolo[5,4-a]pyridine scaffold emerged as the most promising ligand with a KD of 78 nM, which was independently confirmed by isothermal calorimetry and inhibition was also verified in an ATP-[³²P]–pyrophosphate exchange steady-state kinetic assay.