Trypanosomes, transition states and design of transition state inhibitors

Transition state inhibitors against enzymes of purine nucleoside salvage in protozoan parasites are being designed from the experimentally determined structure of the enzymatic transition states. Protozoa lack de novo purine biosynthesis and encode salvage enzymes not found in mammals. Targeting these enzymes with specific transition state inhibitors may prevent nucleoside salvage. The interpretation of kinetic isotope effects is well established in chemical systems, and applied to enzymes, can provide reliable transition state information. Enzymes bind transition states, and therefore transition state analogues, with extreme affinity, providing the best known noncovalent inhibitors. Enzymatic transition state structures can be characterized by the systematic determination of intrinsic kinetic isotope effects followed by a combination of classical and quantum chemical descriptions of the transition state. The method requires: 1) synthesis of labeled substrates 2) measurement of intrinsic kinetic isotope effects 3) normal mode analysis to establish the geometry of the transition state 4) quantum electron distribution of the transition state 5) matching transition state features with stable inhibitor features a) synthesis and testing of inhibitors. Using these methods, a family of powerful and specific transition state inhibitors have been synthesized. The method will be described in the context of nucleoside hydrolases from trypanosomes.