Transition-state analog substrate

Transition-State Analog Substrates for ENGase-Catalyzed T ransgly cosy lation... 

X-ray crystallographic studies of serine protease complexes with transition-state analogs have shown how chymotrypsin stabilizes the tetrahedral oxyanion transition states (structures (c) and (g) in Figure 16.24) of the protease reaction. The amide nitrogens of Ser and Gly form an oxyanion hole in which the substrate carbonyl oxygen is hydrogen-bonded to the amide N-H groups. 

TRANSITION-STATE ANALOGS are stable molecules that are designed to look more like the transition state than like the substrate or product. Transition-state analogs usually bind to the enzyme they re designed to inhibit much more tightly (by 1000-fold or more) than the substrate does. 

Concept A new approach to the rational design of enzyme inhibitors has emerged in the last ten to fifteen years that incorporates a substrate (or transition state) analog "core" molecule with additional binding determinants spanning beyond the immediate... 

Bartlett has derived a method181 for proving that a putative transition state analog exerts its inhibitory power from successfully mimicking the transition state. If a series of structurally-related inhibitors (all containing the identical core chemical structure meant to simulate the transition state) bind to the target enzyme with log (fQ) values that linearly correlate (slope = 1) with the log (KMlkcai) values of the same series of structurally-related substrates, then... 

In our context, an important class of reversible inhibitors are the transition state analogs [18], which are stable compounds designed to mimic the structure of an intermediate in the path of substrate s transformation by the enzyme. Such analogs are based in Pauling s postulate [19], which states that "an enzyme recognises and binds more tightly to the transition state than to the ground state of the substrate". 

The preceding summary and Fig. 20 present a frame-by-frame account of the pathway for ribonuclease catalysis, based predominandy on knowledge of the structures of the various intermediates and transition states involved. The ability to carry out such a study is dependent on three critical features (1) crystals of the enzyme which diffract sufficiently well to permit structural resolution to at least 2 A (2) compatibility of the enzyme, its crystals, and its catalytic kinetic parameters with cryoenzymology so as to permit the accumulation and stabilization of enzyme-substrate complexes and intermediates at subzero temperatures in fluid cryosolvents with crystalline enzyme and (3) the availability of suitable transition state analogs to mimic the actual transition states which are, of course, inaccessible due to their very short lifetimes. The results from this investigation demonstrate that this approach is feasible and can provide unparalleled information about an enzyme at work. 

Also a new family of inhibitors exemplified by SUAM 1221 65 [75] has been described in which a pyrrolidinylcarbonyl function at the PI site is the crucial entity for enzyme recognition, giving rise to the transition state analog of the enzyme-substrate interaction. 

Detailed binding with substrate and transition state analogs has also been reported on KSI [83, 84] using a wide range of techniques, highlighting the subtle interplay of the electrostatic and geometric properties of the enolate stabilizing active site. 

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). 

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