Hydrolases, enzyme catalysis

Although quite reliable empirical rules exist for the enantioselectivity of hydrolases for secondary alcohols (see Section 4.2.1.2), such rules are not as developed for primary alcohols, partly because many hydrolases often show low enantioselectivity. With some exceptions, lipases from Pseudomonas sp. and porcine pancreas lipase (PPL) often display sufficient selectivity for practical use. The model described in Figure 4.3 has been developed for Pseudomonas cepacia lipase (reclassified as Burkholderia cepacia), and, provided that no oxygen is attached to the stereogenic center, it works well for this lipase in many cases [41]. However, as soon as primary alcohols are resolved by enzyme catalysis, independent proof of configuration for a previously unknown product is recommended. 

Lodola, A., Mor, M., Zurek, J., Tarzia, G., PiomeUi, D., Harvey, J.N., MulhoUand, A.J. Conformational effects in enzyme catalysis Reaction via a high energy conformation in fatty acid amide hydrolase. Biophys. J. 2007, 92(2), L20-2. 

Sometimes the mechanism of enzyme catalysis involves more than one enzyme-substrate complex. A representative example is chymotrypsin, one of the most-studied enzymes. Chymotrypsin can act as an esterase and a protease, because the chemical mechanisms of ester and amide hydrolases are almost identical. The catalytic mechanism when chymotrypsin acts as a serine protease involves the following steps ... 

The mechanism for the lipase-catalyzed reaction of an acid derivative with a nucleophile (alcohol, amine, or thiol) is known as a serine hydrolase mechanism (Scheme 7.2). The active site of the enzyme is constituted by a catalytic triad (serine, aspartic, and histidine residues). The serine residue accepts the acyl group of the ester, leading to an acyl-enzyme activated intermediate. This acyl-enzyme intermediate reacts with the nucleophile, an amine or ammonia in this case, to yield the final amide product and leading to the free biocatalyst, which can enter again into the catalytic cycle. A histidine residue, activated by an aspartate side chain, is responsible for the proton transference necessary for the catalysis. Another important factor is that the oxyanion hole, formed by different residues, is able to stabilize the negatively charged oxygen present in both the transition state and the tetrahedral intermediate. 

This enzyme [EC 3.4.19.1], also known as acylamino-acid releasing enzyme and A-acylpeptide hydrolase, catalyzes the hydrolysis of an acylaminoacyl-peptide to generate an acylamino acid and the free peptide. Catalysis is most... 

This broad class of hydrolases constitutes a special category of enzymes which bind to and conduct their catalytic functions at the interface between the aqueous solution and the surface of membranes, vesicles, or emulsions. In order to explain the kinetics of lipolysis, one must determine the rates and affinities that govern enzyme adsorption to the interface of insoluble lipid structures -. One must also account for the special properties of the lipid surface as well as for the ability of enzymes to scooC along the lipid surface. See specific enzyme Micelle Interfacial Catalysis... 

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