BIOL 278 |
| Transition state theory suggests that enzymatic rate acceleration (kcat/knon) is related to the stabilization of the transition state. Chemically stable analogues of a transition state complex are predicted to convert this stabilization energy into binding energy. Transition state structures have been reported for the bovine (Bos taurus), human (Homo sapiens), and malarial (Plasmodium falciparum) orthologues of purine nucleoside phosphorylase (PNP). All three enzymes have transition states with substantial ribooxocarbenium orthologues ion character. Whereas the bovine PNP proceeds through a concerted mechanism, the human and malarial PNPs proceed through a more dissociative (SN1) stepwise mechanism. Further, the PNP from P. falciparum differs from human PNP by its ability to use for 5'-methylthioinosine as a substrate. Transition state analogues developed for specific PNPs exhibit differential inhibition specificity for the enzymatic transition state of these three enzymes based upon their differing reaction rates (kcat), mechanisms, and substrate specificity. The most powerful inhibitors of these three orthologues have picomolar slow-onset binding constants (Ki*). Two potent inhibitors are Immucillin-H and DADMe-Immucillin-H. MT-Immucillin-H has specificity for P. falciparum PNP by virtue of the 5'-methylthio group. Although the transition state for mosquito (Anopholes gambiae) PNP is yet to be determined, inhibition values support a dissociative transition state mechanism. Comparison of the transition states and substrate specificity of PNPs permits the design of species-specific inhibitors for use as therapeutic agents.
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Enzymes
4:30 PM-6:30 PM, Wednesday, 13 September 2006 Moscone Center -- Hall D, Poster
Division of Biological Chemistry |