Lipase mechanisms

Keywords (Co)polyesters Lipase Mechanism Polycondensation Ringopening polymerization... 

Ql. Quinn, D., Shirai, K., and Jackson, R. L., Lipoprotein lipase Mechanism of action and role in lipoprotein metabolism, frog. Lipid Res. 22, 35-78 (1983). 

Bengtsson, G., Olivecrona, T. 1980. Lipoprotein lipase. Mechanism of product inhibition. Eur. J. Biochem. 106, 557-562. 

R.L. (1986) Which class of serine is involved in the lipase mechanism Trends Biochem. Sci. (Pers. Ed.) 11, 497-498... 

Liao, T.H., Hamosh, M., and Hamosh, P. (1984) Fat Digestion by Lingual Lipase Mechanism of Lipolysis in the Stomach and Upper Small Intestine, Pediatr. Res. 18,402 09. 

Lipase catalysis is a very diverse and broad field where, still, little is known about lipase mechanisms on a molecular level. The extensive research currently going on is expected to reveal important information regarding the controlled tailoring of lipase en-antioselectivity. Important aspects other than steric effects are the involvement of water, the nature of die solvent, and the enhropic influence on substrate binding and transition state stabilization. This knowledge will provide an understanding of tiie details of lipase catalysis and facilitate the development of quantitative computer models for prediction of enantioselectivity in the very near future. 

Lipases as acylation catalysts, mechanism and preparative applications, particularly in heterocyclic chemistry 98AG(E)1608. 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

Figure 29.2 MECHANISM Mechanism of action of lipase. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine, which react cooperatively to carry out two nucleophilic acyl substitution reactions. Individual steps are explained in the text.

The plotting of Dixon plot and its slope re-plot (see 5.9.5.9) is a commonly used graphical method for verification of kinetics mechanisms in a particular enzymatic reaction.9 The proposed kinetic mechanism for the system is valid if the experimental data fit the rate equation given by (5.9.4.4). In this attempt, different sets of experimental data for kinetic resolution of racemic ibuprofen ester by immobilised lipase in EMR were fitted into the rate equation of (5.7.5.6). The Dixon plot is presented in Figure 5.22. 

However, whatever the mechanism of action is, the effect of solvents on enzyme selectivity is sometimes really dramatic. For example, Hrrose et al. [42] reported that in the Pseudomonas species lipase-catalyzed desymmetrization of prochiral... 

Figure 2.6 Mechanism of lipase-catalyzed hydrolysis of esters [34].

Esterases have a catalytic function and mechanism similar to those of lipases, but some structural aspects and the nature of substrates differ [4]. One can expect that the lessons learned from the directed evolution of lipases also apply to esterases. However, few efforts have been made in the directed evolution of enantioselective esterases, although previous work by Arnold had shown that the activity of esterases as catalysts in the hydrolysis of achiral esters can be enhanced [49]. An example regarding enantioselectivity involves the hydrolytic kinetic resolution of racemic esters catalyzed by Pseudomonasfluorescens esterase (PFE) [50]. Using a mutator strain and by screening very small libraries, low improvement in enantioselectivity was... 

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. 

Lipase from C.antarctica also catalyzes carbon-carbon bond formation through aldol condensation of hexanal. The reaction is believed to proceed according to the same mechanism as the Michael additions [113]. Lipase from Pseudomonas sp. 

Holm C et al Molecular mechanisms regulating hormone sensitive lipase and lipolysis. Annu Rev Nutr 2000 20 365. 

The catalytic alcohol racemization with diruthenium catalyst 1 is based on the reversible transfer hydrogenation mechanism. Meanwhile, the problem of ketone formation in the DKR of secondary alcohols with 1 was identified due to the liberation of molecular hydrogen. Then, we envisioned a novel asymmetric reductive acetylation of ketones to circumvent the problem of ketone formation (Scheme 6). A key factor of this process was the selection of hydrogen donors compatible with the DKR conditions. 2,6-Dimethyl-4-heptanol, which cannot be acylated by lipases, was chosen as a proper hydrogen donor. Asymmetric reductive acetylation of ketones was also possible under 1 atm hydrogen in ethyl acetate, which acted as acyl donor and solvent. Ethanol formation from ethyl acetate did not cause critical problem, and various ketones were successfully transformed into the corresponding chiral acetates (Table 17). However, reaction time (96 h) was unsatisfactory. 

Several mechanisms have been proposed for lipase-catalyzed reactions. Kinetic studies of hydrolysis [14,15] and esterification [50] catalyzed by Pseudomonas cepecia lipase, demonstrate that the enzyme has a ping-pong mechanism. 

The simplest kinetic model applied to describe lipase catalyzed reactions is based on the classic Michaelis-Menten mechanism [10] (Table 3). To test this model Belafi-Bakd et al. [58] studied kinetics of lipase-catalyzed hydrolysis of tri-, di-, and mono-olein separately. All these reactions were found to obey the Michaelis-Menten model. The apparent parameters (K and V ) were determined for global hydrolysis. 

Catalytic site of lipase is known to be a serine-residue and lipase-catalyzed reactions are considered to proceed via an acyl-enzyme intermediate. The mechanism of lipase-catalyzed polymerization of divinyl ester and glycol is proposed as follows (Fig. 3). First, the hydroxy group of the serine residue nucleophilically attacks the acyl-carbon of the divinyl ester monomer to produce an acyl-enzyme intermediate involving elimination of acetaldehyde. The reaction of the intermediate with the glycol produces 1 1 adduct of both... 

Fig. 3. Proposed mechanism of polymerization of dicarboxylic acid divinyl ester and glycol through lipase catalysis...

Fig. 9. Postulated mechanism of lactone polymerization catalyzed by lipase...

Chiesa G, Michelagnoli S, Cassinotti M, Gianfranceschi G, Werba JP, Paz-zucconi F, et al. Mechanisms of high-density lipoprotein reduction after probu-col treatment changes in plasma cholesterol esterification/transfer and lipase activities. Metabolism 1993 42 229-235. 

Zambon A, Hokanson JE, Brown BG, Brunzell JD. Evidence for a new pathophysiological mechanism for coronary artery disease regression hepatic lipase-mediated changes in LDL density. Circulation 1999 99 1959-1964. 

The regulation of fat metabolism is relatively simple. During fasting, the rising glucagon levels inactivate fatty acid synthesis at the level of acetyl-CoA carboxylase and induce the lipolysis of triglycerides in the adipose tissue by stimulation of a hormone-sensitive lipase. This hormone-sensitive lipase is activated by glucagon and epinephrine (via a cAMP mechanism). This releases fatty acids into the blood. These are transported to the various tissues, where they are used. 

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