Transition-state analogues esterases

An example of esterase behaviour is provided by a catalytic antibody developed by Tramontano et al. (1988), using a phosphonate transition state analogue [53] as the hapten. The antibody cleaves the carboxylic ester [54, R = Me] with enzyme-like efficiency (kc/ku = 6.25 X 106 = 1.5 mM ... 

A similar concept was used in the development of artificial chymotrypsin mimics [54]. The esterase-site was modeled by using the phosphonate template 75 as a stable transition state analogue (Scheme 13.19). The catalytic triad of the active site of chymotrypsin - that is, serine, histidine and aspartic acid (carboxy-late anion) - was mimicked by imidazole, phenolic hydroxy and carboxyl groups, respectively. The catalytically active MIP catalyst 76 was prepared using free radical polymerization, in the presence of the phosphonate template 75, methacrylic acid, ethylene glycol dimethacrylate and AIBN. The template removal conditions had a decisive influence on the efficiency of the polymer-mediated catalysis, and best results were obtained with aqueous Na2CC>3. 

D. Lang, M. Rossi, and C. Pedone, A snapshot of a transition state analogue of a novel thermophilic esterase belonging to the subfamily of mammalian hormone-sensitive lipase, J. Mol. Biol., 2000, 303, 761-771. 

Hammock and co-workers (Hammock et ai, 1982 Abdel-Aal, 1984 Prestwich et ai, 1984) prepared several 3-alkylthio-l,l,l-trifluoro-2-propanones with juvenile hormone-like side chains. These compounds were designed as possible transition-state analogue inhibitors of JH esterases. The most active analogue (78) showed a dose-dependent delay in pupation and a selective inhibition of JH esterase of the cabbage looper, Trichopltisia ni. 

Ohkubo, K. Sawakuma, K. Sagawa, T. Shape-and stereo-selective esterase activities of cross-linked pol mers imprinted with a transition-state analogue for the hydrolysis of amino acid esters. J. Mol. Catal. A Chem. 2001,165, 1-7. 

Figure 2 Ester hydrolysis. Design of a stable analogue of the tetrahedral transition state to be used as a hapten to generate catalytic antibodies with an esterase activity.

Trifluoromethyl /1-thioalkyls and /1-amino alcohols are often good reversible inhibitors of esterases and proteases, respectively. Depending on the enzymes (serine or aspartyl enzymes), fluorinated alcohols are often less efficient inhibitors than the corresponding ketones, which act as analogues of the transition state (vide infra). Nevertheless, fluoroalcohols inhibit hydrolytic enzymes with high inhibition constants (Figure 7.25)." ... 

The most widely used strategy involves the synthesis of the network around a structural analogue of the transition state of the reaction. The imprinted sites then correspond to the conformation of the substrates in the transition state. For ester hydrolysis this state can, for instance, be simulated by a phosphonate derivative as template [156,167]. An imprinted network with an esterase-type catalytic activity can then be obtained. For the MIP represented in Fig. 19(1), the reaction rate is increased 100-fold with respect to the reaction without catalyst and kinetics of the Michaelis-Menten type, as well as inhibition by an analogue of the transition state are observed [156]. 

---