| Abstract
| - 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 thisphenomenon. Mycophenolic acid (MPA) is a species-specific inhibitor of IMPDH; mammalian IMPDHsare very sensitive to MPA, while the microbial enzymes are resistant to the inhibitor. MPA traps thecovalent intermediate E-XMP* and binds in the nicotinamide half of the dinucleotide site. Previous resultsindicated that about half of the difference in sensitivity derives from residues in the MPA-binding site[Digits, J. A., and Hedstrom, L. (1999) Biochemistry38, 15388−15397]. The remainder must be attributedto regions outside the MPA-binding site. The adenosine subsite of the NAD+ site is not conserved amongIMPDHs and is, therefore, a likely candidate. Our goal is to examine the coupling between the nicotinamideand adenosine sites in order to test this hypothesis. We performed multiple inhibitor experiments with theTritrichomonasfoetus and human type 2 IMPDHs using tiazofurin and ADP, which bind in the nicotinamideand adenosine subsites, respectively. For T. foetus IMPDH, tiazofurin and ADP are extraordinarilysynergistic. In contrast, these inhibitors are virtually independent for the human type 2 enzyme. We suggestthat the difference in coupling of the nicotinamide and adenosine subsites accounts for the remainingdifference in MPA affinity between T. foetus and human IMPDH.
|
