Differential signatures of bacterial and mammalian IMP dehydrogenase enzymes

IMP dehydrogenase (IMPDH) is an essential enzyme of de novo guanine nucleot ide synthesis. IMPDH inhibitors have clinical utility as antiviral, antican cer or immunosuppressive agents. The essential nature of this enzyme sugges ts its therapeutic applications may be extended to the development of antim icrobial agents. Bacterial IMPDH enzymes show bio- chemical and kinetic cha racteristics that are different than the mammalian IMPDH enzymes, suggestin g IMPDH may be an attractive target for the development of antimicrobial ag ents. We suggest that the biochemical and kinetic differences between bacte rial and mammalian enzymes are a consequence of the variance of specific, i dentifiable amino acid residues. Identification of these residues or combin ation of residues that impart this mammalian or bacterial enzyme signature is a prerequisite for the rational identification of agents that specifical ly target the bacterial enzyme. We used sequence alignments of IMPDH protei ns to identify sequence signatures associated with bacterial or eukaryotic IMPDH enzymes. These selections were further refined to discern those likel y to have a role in catalysis using information derived from the bacterial and mammalian IMPDH crystal structures and site-specific mutagenesis. Candi date bacterial sequence signatures identified by this process include regio ns involved in subunit interactions, the active site flap and the NAD bindi ng region. Analysis of sequence alignments in these regions indicates a pat tern of catalytic residues conserved in all enzymes and a secondary pattern of amino acid conservation associated with the major phylogenetic groups. Elucidation of the basis for this mammalian/bacterial IMPDH signature will provide insight into the catalytic mechanism of this enzyme and the foundat ion for the development of highly specific inhibitors.