CRYSTAL-STRUCTURE OF TRITRICHOMONAS-FETUS INOSINE-5'-MONOPHOSPHATE DEHYDROGENASE AND THE ENZYME-PRODUCT COMPLEX

Inosine-5'-monophosphate dehydrogenase (IMPDH) is an attractive drug t arget for the control of parasitic infections. The enzyme catalyzes th e oxidation of inosine monophosphate (IMP) to xanthosine monophosphate (XMP), the committed step in de novo: guanosine monophosphate (GMP) b iosynthesis. We have determined the crystal structures of IMPDH from t he protozoan parasite Tritrichomonas foetus in the apo form al 2.3 Ang strom resolution and the enzyme-XMP complex at 2.6 Angstrom resolution . Each monomer of this tetrameric enzyme is comprised of two domains, the largest of which includes an eight-stranded parallel beta/alpha-ba rrel that; contains the: enzyme active site at the C termini of the ba rrel beta-strands. A second domain, comprised of residues 102-220, is disordered in the crystal. IMPDH is expected to be active as a tetrame r, since the active site cavity is formed by strands from adjacent sub units. An intrasubunit disulfide bond, seen in the crystal structure, may stabilize the protein in a less active form, as high concentration s of reducing agent: have been shown to increase enzyme activity, Diso rder at the active site suggests that a high degree of flexibility may be inherent in the catalytic function of IMPDH. Unlike IMPDH from oth er species, the T. foetus enzyme has a single arginine that is largely responsible for coordinating the substrate phosphate in the active si te, This structural uniqueness may facilitate structure-based identifi cation and design of compounds that specifically inhibit the parasite enzyme.