Esterases selectivity

The synthesis of various phosphonate analogues of aspartic acid, glutamic acid and their homologues and serine phosphate have been reported. The kynurenine phosphinic acid analogue 217 and the corresponding phosphinate 218 have been synthesised from iV-protected 2-amino P-propiolactone (Scheme 27). Kinetic resolution was achieved by esterase-selective hydrolysis of the carboxylate group in the diester. 

Porcine liver esterase (PLE) gives excellent enantioselectivity with both dimethyl 3-methylglutarate [19013-37-7] (lb) and malonate (2b) diester. It is apparent from Table 1 that the enzyme s selectivity strongly depends on the size of the alkyl group in the 2-position. The hydrolysis of ethyl derivative (2c) gives the S-enantiomer with 75% ee whereas the hydrolysis of heptyl derivative (2d) results in the R-monoester with 90% ee. Chymotrypsin [9004-07-3] (CT) does not discriminate glutarates that have small substituents in the 3-position well. However, when hydroxyl is replaced by the much bulkier benzyl derivative (Ic), enantioselectivity improves significantly. 

Optically Active Acids and Esters. Enantioselective hydrolysis of esters of simple alcohols is a common method for the production of pure enantiomers of esters or the corresponding acids. Several representative examples are summarized ia Table 4. Lipases, esterases, and proteases accept a wide variety of esters and convert them to the corresponding acids, often ia a highly enantioselective manner. For example, the hydrolysis of (R)-methyl hydratropate [34083-55-1] (40) catalyzed by Hpase P from Amano results ia the corresponding acid ia 50% yield and 95% ee (56). Various substituents on the a-carbon (41—44) are readily tolerated by both Upases and proteases without reduction ia selectivity (57—60). The enantioselectivity of many Upases is not significantly affected by changes ia the alcohol component. As a result, activated esters may be used as a means of enhancing the reaction rate. 

Optically Active Alcohols and Esters. In addition to the hydrolysis of esters formed by simple alcohols described above, Hpases and esterases also catalyze the hydrolysis of a wide range of esters based on more complex and synthetically useful cycHc and acycHc alcohols (Table 5). Although the hydrolysis of acetates often gives the desirable resolution, to achieve maximum selectivity and reaction efficiency, comparison of various esters is recommended. 

Both saturated (50) and unsaturated derivatives (51) are easily accepted by lipases and esterases. Lipase P from Amano resolves azide (52) or naphthyl (53) derivatives with good yields and excellent selectivity. PPL-catalyzed resolution of glycidyl esters (54) is of great synthetic utiUty because it provides an alternative to the Sharpless epoxidation route for the synthesis of P-blockers. The optical purity of glycidyl esters strongly depends on the stmcture of the acyl moiety the hydrolysis of propyl and butyl derivatives of epoxy alcohols results ia esters with ee > 95% (30). 

A synthesis of the important biosynthetic intermediate mevalonic acid starts with the enzymatic hydrolysis of the diester A by pig liver esterase. The pro-R group is selectively hydrolyzed. Draw a three-dimensional structure of the product. 

You probably came up with the suggestion that by using an esterase which selectively hydrolyses the succinate ester of L-menthol, you would be able to isolate L-menthol from the mixture. This is in essence the way the process is carried out commercially. We can represent this process by ... 

A different task was pursued by the CM of CsA with various maleates 339 [ 148]. The CM demanded in this case the highly active Hoveyda catalyst D, that exhibits potency not reached by the phosphine-containing catalysts C and E. Under the conditions given in Scheme 65, metathesis with maleates 339 led (E)-selectively to the a,/J-unsaturated ester derivatives 340 in high yield. Compounds 340 still demonstrated activity comparable to that of CsA and are thus potential soft drugs via esterase-mediated biotransformation to the corresponding inactive carboxylic acids 341. 

The organophosphorons insecticides dimethoate and diazinon are mnch more toxic to insects (e.g., housefly) than they are to the rat or other mammals. A major factor responsible for this is rapid detoxication of the active oxon forms of these insecticides by A-esterases of mammals. Insects in general appear to have no A-esterase activity or, at best, low A-esterase activity (some earlier stndies confnsed A-esterase activity with B-esterase activity) (Walker 1994b). Diazinon also shows marked selectivity between birds and mammals, which has been explained on the gronnds of rapid detoxication by A-esterase in mammals, an activity that is absent from the blood of most species of birds (see Section 23.23). The related OP insecticides pirimiphos methyl and pirimiphos ethyl show similar selectivity between birds and mammals. Pyrethroid insecticides are highly selective between insects and mammals, and this has been attributed to faster metabolic detoxication by mammals and greater sensitivity of target (Na+ channel) in insects. 

In addition to ester bonds with P (Section 10.2.1, Figures 10.1 and 10.2), some OPs have other ester bonds not involving P, which are readily broken by esteratic hydrolysis to bring about a loss of toxicity. Examples include the two carboxylester bonds of malathion, and the amido bond of dimethoate (Figure 10.2). The two carboxylester bonds of malathion can be cleaved by B-esterase attack, a conversion that provides the basis for the marked selectivity of this compound. Most insects lack an effective carboxylesterase, and for them malathion is highly toxic. Mammals and certain resistant insects, however, possess forms of carboxylesterase that rapidly hydrolyze these bonds, and are accordingly insensitive to malathion toxicity. 

In media selective for enterobacteria a surface-active agent is the main selector, whereas in staphylococcal medium sodium and lithium chlorides are the selectors staphylococci are tolerant of salt concentrations to around 7.5%. Mannitol salt, Baird-Parker (BP) and Vogel-Johnson (VJ) media are three examples of selective staphyloccocal media. Beside salt concentration the other principles are the use of a selective carbon source, mannitol or sodium pyruvate together with a buffer plus acid-base indicator for visualizing metabolic activity and, by inference, growth. BP medium also contains egg yolk the lecithin (phospholipid) in this is hydrolysed by staphylococcal (esterase) activity so that organisms are surrounded by a cleared zone in the otherwise opaque medium. The United States Pharmacopeia (1990) includes a test for staphylococci in pharmaceutical products, whereas the British Pharmacopoeia (1993) does not. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Chiral multiple-coupling reagents have been prepared in enantiomerically pure form by enantio-selective saponification of diesters of meso-2-nitrocyclohexane-l,3-diols (Eq. 3.44) with pig liver esterase (PLE).69... 

The same group reported in 1986 a sensitive and selective HPLC method employing CL detection utilizing immobilized enzymes for simultaneous determination of acetylcholine and choline [187], Both compounds were separated on a reversed-phase column, passed through an immobilized enzyme column (acetylcholine esterase and choline oxidase), and converted to hydrogen peroxide, which was subsequently detected by the PO-CL reaction. In this period, other advances in this area were carried out such as the combination of solid-state PO CL detection and postcolumn chemical reaction systems in LC [188] or the development of a new low-dispersion system for narrow-bore LC [189],... 

Esmolol hydrochloride is a competitive p-adrenergic receptor antagonist it is selective for pT adrenoceptors. In contrast to pindolol, esmolol has little intrinsic sympathomimetic activity, and it differs from propranolol in that it lacks membrane stabilizing activity Of all of the p-adrenergic blocking drugs, this compound has the shortest duration of action because it is an ester, it is hydrolyzed rapidly by plasma esterases and must be used by the intravenous route Esmolol is approved only for the treatment of supraventricular arrhythmias... 

Acid- and base-sensitive lipidated peptides can be selectively deprotected by enzymatic hydrolysis of choline esters.[13al Choline esters of simple peptides, but also of sensitive peptide conjugates like phos-phorylated and glycosylated peptides,1141 nucleopep-tides1151 and lipidated peptides,113,1631 can be cleaved with acetyl choline esterase (AChE) and butyryl choline esterase (BChE) under virtually neutral conditions with complete chemoselectivity. Acid-labile farnesyl groups and base-sensitive thioesters are not attacked. 

For instance, in a synthesis of N-Ras lipopeptide 8, the choline ester in the palmitoylated tripeptide 5 was removed selectively and in high yield by means of the butyryl choline esterase (BChE). Efficient cou-... 

Furthermore, enzymes that quantitatively liberate ester-bound fatty acids of lipid A are known. Such esterases are present in the amoebae Dictyoste-liurn discoideum (177-179) and Acanthamoeba castellanii (178). Selective... 

A selection of carboxylic ester hydrolases (EC 3.1.1) of major or more-modest significance in xenobiotic metabolism is given in Table 2.5. The recommendations of the Enzyme Nomenclature Committee on the classification of esterases cannot be considered completely satisfactory, but, even after decades of debate, a more satisfactory classification system remains to be proposed [56] [57], The main difficulties with esterase classification have been summarized as follows [58],... 

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