Catalytic triad carboxylesterases

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). 

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]. 

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... 

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