Hydrolysis enzymic, selective

A number of examples of monoacylated diols produced by enzymatic hydrolysis of prochiral carboxylates are presented in Table 3. PLE-catalyzed conversions of acycHc diesters strongly depend on the stmcture of the substituent and are usually poor for alkyl derivatives. Lipases are much less sensitive to the stmcture of the side chain the yields and selectivity of the hydrolysis of both alkyl (26) and aryl (24) derivatives are similar. The enzyme selectivity depends not only on the stmcture of the alcohol, but also on the nature of the acyl moiety (48). 

The interest and success of the enzyme-catalyzed reactions in this kind of media is due to several advantages such as (i) solubilization of hydrophobic substrates (ii) ease of recovery of some products (iii) catalysis of reactions that are unfavorable in water (e.g. reversal of hydrolysis reactions in favor of synthesis) (iv) ease of recovery of insoluble biocatalysts (v) increased biocatalyst thermostability (vi) suppression of water-induced side reactions. Furthermore, as already said, enzyme selectivity can be markedly influenced, and even reversed, by the solvent. 

Enzyme hydrolysis of immobihzed substrates was not only used for the development of linkers in sohd-phase chemistry but also used as a key step to evaluate enzyme-selectivity in several assays. 

A detailed analysis of the effect of mixed monolayers of 15 and DMPC on the activity of phospholipase A2 was reported by Grainger et al. [53]. Monolayers composed of different ratios of DMPC and either 15 or primarily poly 5 were characterized by Langmuir isotherms and isobars. The phospholipse-A2-mediated hydrolysis of selected monolayer compositions was usefully employed to ascertain the effectiveness of the enzyme. Both 15 and polyl5 were resistant to hydrolysis. The DMPC hydrolysis was sensitive to its molecular environment in a manner that suggests the phase separation of the polyl5 from DMPC. Phospholipase A2 activity is known to be sensitive to the concentration of the hydrolytic products, i.e. the fatty acid and lysophospholipid. The effect of these reaction products of the activity of phospholipase A2 on mixed monolayers of nonpolymerizable lipids is the subject of a series of interesting studies which are beyond the scope of this review. Ahlers et al. reviewed some of this research [54],... 

Resolution of a racemic mixture can also be achieved by using an enzyme. An enzyme selectively converts one enantiomer in a racemic mixture to another compound, after which the unreacted enantiomer and the new compound are separated. For example, lipase is used in the hydrolysis of chiral esters as shown above. 

The competition between transferase and hydrolysis reactions can be described in terms of nucleophile (acceptor) selectivities of the enzymes, and selectivity constants can be defined. These constants are meant to quantify the intrinsic selectivity of the enzymes. Selectivity constants in combination with the concentrations (or thermodynamic activities) of the competing nucleophiles give the transferase/hydrolysis ratio. The selectivity constants are defined as follows [38, 39] ... 

Because only 40 to 60 amino acid residues can be determined by the Edman procedure, additional methods are needed for larger proteins. Determination of the C-terminal amino acid can be accomplished by treating the protein with carboxypeptidase. This enzyme selectively catalyzes the hydrolysis of the C-terminal amino acid. After the first amino acid has been removed, the enzyme begins to cleave the second amino acid, and so forth. By following the rates at which the amino acids appear, it is possible to determine the first few amino acids at the C-terminal end of the protein by employing this enzyme. However, because the enzyme hydrolyzes different peptide bonds at different rates, it is possible to identify only a few amino acids before the reaction mixture becomes too complex. 

In general, acid hydrolysis produces similar degradation. Since acids usually involve a much smaller molecule as the attacking agent, however, acids penetrate the amorphous area of cellulose more easily than enzymes and produce a faster drop in DP. Diffusion into the crystalline areas proceeds more slowly. Because acid hydrolysis is more easily controlled and is faster than enzymatic attack, acid hydrolysis was selected as the second aging system to be examined. 

The nature of the aqueous buffer (0.1 M NaCl, 4 mM sodium phosphate buffer pH 8.5 versus 0.1 M NaCl, 4 mM sodium borate buffer pH 8.5) had no appreciable effect on the enzyme selectivity although the hydrolysis was slightly faster with the phosphate. The buffer composition was simplified by omitting sodium chloride, while at the same time the phosphate concentration was increased to 10 mM in order to compensate for the ionic strength and increase the buffering capacity of the system. The pH value was kept at 8.5 as the optimal compromise between highest enzymatic reaction rate and lowest reaction rate of the unspecific alkaline ester hydrolysis. 

The vindoline synthesis required prior preparation of the amine coupling partner, the 2,4-dinitrobenzenesulfonamide 713, which was prepared from the pent-anal 710, as shown in Scheme 43. A notable feature of this route was the enzyme-mediated resolution of the cyanohydrin acetate 711, via enzymatic hydrolysis to selectively afford a diastereomeric mixture of only the (5)-cyanohydrins 712. 

Studies of hydrolytic stabihty of polymeric plasticizers, such as poly(propylene adipate) and poly(butylene adipate) were conducted in simulated body fluids such as saliva, gastric and intestinal fluids. It was found that no hydrolysis occurred under saliva and gastric conditions but plasticizers were hydrolyzed to a large extent in simulated intestinal fluid. Enzymes selectively catalyzed the primary alcohol ester linkage. It is expected that other polymeric ester plasticizers will behave in a similar manner. ... 

The first simple method using isotopic labelling has been developed to confirm the actual and observed selectivities in the enzymatic hydrolysis of unsymmetrical diacetates and to measure the enzyme selectivity efficiency. The simple method consists of enzymatic hydrolysis of the unsymmetrical diacetate followed by labelling of the hydroxyacetate formed with CD3CO2D-DCC and enzymatic rehydrolysis of the labelled compound under identical reaction conditions. The amount of label lost, as determined by H NMR spectroscopy, directly indicates the extent of regioselective action of the enzyme. For example, enzymic hydrolysis using pig liver acetone powder (FLAP) of the racemic glycerol diacetate (208) yielded a mixture (1 9) of the 1-hydroxy ester (209) and the 2-hydroxy ester (210). Isolation of the 2-hydroxy... 

Over the past three decades, an increasing concern was put on application of nonaqueous solvents to facilitate biocatalytic reactions where several industrially attractive advantages are presented, such as increased solubility of nonpolar substrates, reversal of hydrolysis reactions, alternation of enzyme selectivity, and suppression of water-dependent side reactions. However, there are some inherent problems and technical challenges, including inactivation of biocatalysts, potentially reduced protein stability and lowered reaction rates due to mass-transfer limitations, and/or the increased rigidity of protein structure. 

Selective hydrolysis can be accomplished by using enzymes to catalyze cleavage at specific peptide bonds... 

Chymotrypsin (Section 27 10) A digestive enzyme that cat alyzes the hydrolysis of proteins Chymotrypsin selectively catalyzes the cleavage of the peptide bond between the car boxyl group of phenylalanine tyrosine or tryptophan and some other ammo acid... 

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