Carboxylesterases catalytic activities

Mechanism of interaction of A- and B-esterases with OPC is similar. B-esterases initially form Michaelis complex with an OPC inhibitor producing phosphorylated or inhibited enzyme that either reactivates very slowly or does not reactivate at all (Figure 3) [1], However, after forming Michaelis complex with OPC A-esterases perform intensive and permanent hydrolysis of OPC and their catalytic activity and turnover rate are very high. It was already shown that carboxylesterases, as a typical B-esterase, can hydrolyze carboxylic esters that serve as functional groups in OPC such as malathion thus performing detoxication of the compound [26, 27]. 

In an another application, the enantioselectivity of carboxylesterase A (CesA) from Bacillus subtilis towards butyrate and caprilate esters of 1,2-O-isopropylideneglycerol was improved up to 13-fold by structure-guided mutagenesis. The mutagenesis of positions 166 and 182 in CesA yielded novel variants with enhanced enantioselectivity without significant loss of catalytic activity. ... 

Crow et al. (2007) examined the catalytic activity of carboxylesterase (rCE proteins hCE-1 and hCE-2) on Type 1 pyrethroids (i.e., IRS frans-permethrin (98% pure, 93% trans and 5% cis), IRS cA-permethrin (99% pure), IR trans-resmethrin (bioresmethrin, 99% pure, 97% trans and 2% cis)). Type 11 pyrethroids (i.e., alpha-cypermethrin (99% pure, mixture of isomers), lambda-cyhalothrin (99% pure, mixture of isomers), and deltamethrin (99% pure)). 

Organophosphates phosphorylate the OH group of the catalytic serine at the active site of B-esterases (see Sect. 3.3). The rate of dephosphorylation of the enzyme is very slow, thus, the organophosphate acts as a mechanism-based inactivator. B-Esterases are classified as carboxylesterases (EC 3.1.1.1). 

Other serine hydrolases such as cholinesterases, carboxylesterases, lipases, and fl-lactamases of classes A, C, and D have a hydrolytic mechanism similar to that of serine peptidases [25-27], The catalytic mechanism also involves an acylation and a deacylation step at a serine residue in the active center (see Fig. 3.3). All serine hydrolases have in common that they are inhibited by covalent attachment of diisopropyl phosphorofluoridate (3.2) to the catalytic serine residue. The catalytic site of esterases and lipases has been less extensively investigated than that of serine peptidases, but much evidence has accumulated that they also contain a catalytic triad composed of serine, histidine, and aspartate or glutamate (Table 3.1). 

The transesterification of cocaine to cocaethylene is an enzymatic reaction catalyzed by microsomal carboxylesterases and blocked by inhibitors of serine hydrolases [124][125], In Chapt. 3, we have discussed the mechanism of serine hydrolases, showing how a H20 molecule enters the catalytic cycle to hydrolyze the acylated serine residue in the active site of the enzyme. In the case of cocaine, the acyl group is the benzoylecgoninyl moiety (Fig. 7.9,d ), which undergoes esterification with ethanol according to Steps e and/ (Fig. 7.9). 

Zhong G, Lemer RA, Barbas CF 111. Broadening the aldolase catalytic antibody repertoire by combining reactive immunization and transition state theory new enantio- and diastereoselectivi-ties. Angew. Chem. Int. Ed. Engl. 1999 38(24) 3738-3741. Schowen RL. The elicitation of carboxylesterase activity in antibodies by reactive immunization with labile organophos-phorus antigens a role for flexibility. J. Immunol. Methods 2002 269(l-2) 59-65. 

Mechanisms of Serine Hydrolases. Typical to enzymatic reactions, the enzyme (E) first binds its substrate (S) at the active site as an enzyme-substrate complex (E S). For the formation of the product P, the enzyme-catalyzed reaction then takes place through the mechanism typical of the enzyme. At the active site of serine hydrolases (lipases, carboxylesterases, and serine proteases), the catalytic machinery is called a cataljdic triad consisting of amino acid residues Ser, His, and either Asp or Glu (Fig. 5). In the E S complex, imidazole of His serves as a general acid/base catalyst, catalyzing the addition of the alcoholic hydroxyl of the serine residue to the carbonyl carbon of the acyl donor (R C02R, the first substrate S ). This leads both to the liberation of the first product P (R OH) and to the formation of the so-called acyl-enzyme intermediate. This ester intermediate then reacts with the second substrate (R OH), which leads to the... 

A carboxylesterase (EC 3.1.1.1) from T. fusca [8, 86] and a steryl esterase (EC 3.1.1.13) from Melanocarpus albomyces [59] have also shown activity with PET oligomers and fabrics. The enzyme from M. albomyces with high specificity for fatty acid esters of sterols increased the hydrophilicity of PET fabrics. The highly hydrophobic serine hydrolase from T. fusca with a catalytic triad composed of serine, glutamic acid, and histidine hydrolyzed CTR and PET nanoparticles. The esterase showed high specificity towards short and middle chain-length fatty acyl esters of p-nitrophenol. In addition, p-nitrobenzyl esterases from Bacillus subtilis and B. licheniformis that hydrolyzed short chain dialkylphthalates and PET nanoparticles have been reported [74, 87]. 

---