Inhibition of the Escherichia coli 6-Oxopurine Phosphoribosyltransferases by Nucleoside Phosphonates: Potential for New Antibacterial Agents

Escherichia coli (Ec) cells possess two purine salvage enzymes: xanthine-guanine phosphoribosyltransferase (XGPRT) and hypoxanthine phosphoribosyltransferase (HPRT). EcXGPRT shares a common structural feature with other members of this family, a flexible loop that closes over the active site during catalysis. The replacement of six of these amino acids by alanine has no effect on the K-m for the two substrates. However; the K-i for the nucleoside monophosphate increases by 27-fold, and the k(cat) is reduced by similar to 200-fold. Nucleoside. phosphonates (NP) are good inhibitors of EcXGPRT and EcHPRT, with K-i values as low as 10 nM. In the absence of the flexible loop, these values increase by 5- to 30-fold, indicating the importance of the loop for high-affinity inhibition. Crystal structures of two NPs in complex with EcXGPRT explain the tight binding. Prodrugs of NPs with low K-i values for EcXGPRT or EcHPRT exhibit IC50 values between 5 and 23 mu M against Mycobacterium tuberculosis in cell-based assays, suggesting that these compounds are therapeutic leads against pathogenic bacteria.