Specificity transition state analogs

The requirements of protease inhibitors as drugs in terms of potency, pharmacokinetics, and toxicity will vary depending on the nature of the infection and the goals of therapy. At one extreme is treatment of HlV-1, a chroific infection that requires life-long therapy and full suppression of viral replication. At the other extreme is the treatment of human rhinovirus (i.e., the cold virus), where short-term treatment to blunt viremia will likely be sufficient to reduce the unwanted symptoms of a cold. In all cases, viral proteases represent very attractive targets with familiar mechanisms of catalysis that frequently allow for the design of transition state analogs and with distinct specificities from host proteases. 

The substrate specificities of both mammalian and yeast hexo-kinases have been extensively studied (76,77). Nevertheless, work in this area continues both in the search for isoenzyme specific inhibitors and in increasingly detailed investigations of the catalytic mechanism. Recently potential transition state analogs PI-(adenosine-5 )-P3-glucose-6 triphosphate (Ap -glucose) and P1-(adenosine-5 )-P4-glucose-6 triphosphate (Ap.-giucose) were tested as inhibitors of four hexokinase isoenzymes. However, they were found to exhibit less affinity for the enzyme than either of the natural substrates alone (78). 

Two specific enzyme-inhibitor mechanisms deserve special discnssion becanse they are the basis of action for several important drags. They are the transition-state analogs and the snicide snbstrates. 

Of special interest is the eventuality of stabilizing transition states by imprinting their features into cavities or adsorption sites using stable transition state analogs as templates. Studies towards such TSA footprint catalysis have been performed by generating TSA complementary sites as marks on the surface [7.73a] or as cavities in the bulk [7.73b] of silica gel. These imprinted catalytic sites showed pronounced substrate specificity [7.74a,b] (namely in the case of cavities [7.73 b]) and chiral selectivity [7.74c,d]. 

The idea that antibodies raised against transition-state analogs should show specific catalytic activity is beautiful and seductive. In the tenth year since the idea became an experimental reality, a preliminary assessment of their potential was made (Kirby, 1996). It was concluded that their high stereoselectivity makes abzymes excellent prospects for asymmetric synthesis, though their practical usefulness is currently limited by their catalytic efficiency. 

Harel, M., Quinn, D.M., Nair, H.K., Silman, I., Sussman, J.L. The X-ray Structure of a Transition State Analog Complex Reveals the Molecular Origins of the Catalytic Power and Substrate Specificity of Acetylcholinesterase./. Am. Chem. Soc. 1996, 118, 2340-2346. 

It, has been speculated that the catalytic specificity of an enzyme requires the active site of the enzyme and the transition state of the reaction at the substrate molecule to be structurally complementary 26). Molecules which resembled the transition state structure could thus be expected to bind the active site tightly. This concept was taken up and developed by Lienhard27) and Wolfenden 28) as transition state analogs. 

In order to provide a better estimate of the enantioselectivity of the catalyst, we prepared an authentic sample of (+)-chorismate by kinetic resolution of the racemate with 1F7 (37). Circular dichroism spectroscopy confirmed the identity and high optical purity of the recovered, HPLC-purified compound. Initial rate measurements with the individual isomers show that (-)-chorismate is favored over (+)-chorismate by the antibody by a factor of at least 90 to 1 at low substrate concentrations. The slight rate enhancements above background observed for the (+)-isomer may be due to general medium effects rather than interaction with a specific locus on the antibody surface. To test this possibility we are currently examining the ability of the transition state analog 3 to inhibit rearrangement of this optical isomer. 

Bortezomib proteosome competitive, transition state analog specific proteosome 0.62 nM chymotrypsin 320 nM (43)... 

Pepstatin A aspartic proteases competitive, transition state analog class-specific sub-nanomolar (18)... 

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