Drug selectivity is determined by coupling across the NAD+ site of IMP dehydrogenase.

Drug resistance often results from mutations that are located far from the drug-binding site. The effects of these mutations are perplexing. The inhibition of IMPDH by MPA is an example of this phenomenon. Mycophenolic acid (MPA) is a species-specific inhibitor of IMPDH; mammalian IMPDHs are very sensitive to MPA, while the microbial enzymes are resistant to the inhibitor. MPA traps the covalent intermediate E-XMP and binds in the nicotinamide half of the dinucleotide site. Previous results indicated that about half of the difference in sensitivity derives from residues in the MPA-binding site [Digits, J. A., and Hedstrom, L. (1999) Biochemistry 38, 15388-15397]. The remainder must be attributed to regions outside the MPA-binding site. The adenosine subsite of the NAD+ site is not conserved among IMPDHs and is, therefore, a likely candidate. Our goal is to examine the coupling between the nicotinamide and adenosine sites in order to test this hypothesis. We performed multiple inhibitor experiments with the Tritrichomonas foetus and human type 2 IMPDHs using tiazofurin and ADP, which bind in the nicotinamide and adenosine subsites, respectively. For T. foetus IMPDH, tiazofurin and ADP are extraordinarily synergistic. In contrast, these inhibitors are virtually independent for the human type 2 enzyme. We suggest that the difference in coupling of the nicotinamide and adenosine subsites accounts for the remaining difference in MPA affinity between T. foetus and human IMPDH.