Crystal structures of Tritrichomonas foetus inosine-5'-monophosphate dehydrogenase: Structural basis for the random-in ordered-out kinetic mechanism

摘要

Inosine-5'-monophosphate dehydrogenase (IMPDH) (E.C. 1.1.1.205) is the enzyme that catalyzes the NAD+-dependent oxidation reaction that converts inosine monophosphate to xanthosine monophosphate. This is the rate-limiting step in guanine nucleotide biosynthesis. Because of this, IMPDH is a well-established drug target. Inhibitors that target IMPDH have been approved or are currently evaluated for antiproliferative, antiviral, and anticancer chemotherapies as well as immunosuppressive agents. Most IMPDH inhibitors are far more effective against mammalian IMPDH than the microbial form of the enzyme. Greater knowledge of the enzymes structure and mechanism, may lead to the development of an effective and selective inhibitor of microbial IMPDH for use as a drug against multi-drug resistant bacteria and protists.;The high resolution crystal structures of IMPDH from the protozoan parasite Tritrichomonas foetus in six complexes containing substrate, product, cofactor, and inhibitors have been solved. Examination of these complexes as well as other previously published complexes has led to the development of a structural model for the random-in ordered-out kinetic mechanism. In the proposed model, the active site loop remains in an open conformation until substrate binding. Upon binding, the loop closes, preventing release of the nucleotide until the cofactor leaves. Differences in this mechanism were observed between the mammalian and microbial enzymes, which may lead to a new class of antibiotic.;The active site cation has been observed for the first time in the T. foetus enzyme. The cation binding site appears to be formed only during catalysis. In the mammalian and T. foetus enzymes, the site is formed only when the covalent intermediate is formed. However, due to amino acid substitutions in the active site loop of the bacterial enzyme, only substrate binding is required for cation binding. The cation appears to stabilize the loop for catalysis.;The gene for Plasmodium falciparum IMPDH has been cloned and sequenced. A homology model of the protein structure has revealed several key differences when aligned with mammalian and other protist forms of IMPDH, which may lead to new chemotherapies for malaria.