Hydroxyl groups acylation/deacylation

The ratios of these slopes for L- and D-esters are shown in Table 12. The kL/kD values of the acylation step in the CTAB micelle are very close to those in Table 9, as they should be. It is interesting to note that the second deacylation step also occurs enantioselectively. Presumably it is due to the deacylation ocurring by the attack of a zinc ion-coordinated hydroxide ion which, in principle, should be enantioselective as in the hydroxyl group of the ligand. Alternatively, the enantioselectivity is also expected when the free hydroxide ion attack the coordinated carbonyl groups of the acyl-intermediate with the zinc ion. At any rate, the rates of both steps of acylation and deacylation for the L-esters are larger than those for the D-esters in the CTAB micelle. However, in the Triton X-100 micelle, the deacylation step for the D-esters become faster than for the L-esters. 

As noted in an earlier section of this article, the utility of the cycloamyloses as covalent catalysts is limited by the low reactivity of the catalytically active hydroxyl groups at neutral pH s and by the relatively slow rates of deacylation of the covalent intermediates. In an effort to achieve effective catalysis, several investigators have attempted to selectively modify the cycloamyloses by either (1) introducing an internal catalyst to facilitate deacylation or (2) introducing a more reactive nucleophile to speed acylation and/or deacylation. 

In lipase-catalyzed ROP, it is generally accepted that the monomer activation proceeds via the formation of an acyl-enzyme intermediate by reaction of the Ser residue with the lactone, rendering the carbonyl more prone to nucleophilic attack (Fig. 3) [60-64, 94]. Initiation of the polymerization occurs by deacylation of the acyl-enzyme intermediate by an appropriate nucleophile such as water or an alcohol to produce the corresponding co-hydroxycarboxylic acid or ester. Propagation, on the other hand, occurs by deacylation of the acyl-enzyme intermediate by the terminal hydroxyl group of the growing polymer chain to produce a polymer chain that is elongated by one monomer unit. 

Plasma-desorption mass spectrometry is another technique that has been applied successfully to the detection of readily removable fatty acyl substituents in intact glycolipids and their acylated derivatives. The specific location of the fatty acyl substituents in the ring of the glycosyl residues, as in LOS antigens, is determined by methylation under nonbasic conditions (see Section II.lb), followed successively by O-deacylation, ethylation of the exposed hydroxyl groups, and GC-MS analysis of partially alkylated alditol acetates21 ethyl groups denote the sites of previous O-acylation. 

However the nucleophiles in question—the hydroxyl group of serine for the acylation reaction and water for the deacylation reaction—are weak nucleophiles, and if the reaction is to proceed at appreciable rates, the enzyme must provide some agent to abstract a proton from the nucleophile and to donate a proton to the leaving group. 

OPs and CMs are acylating inhibitors (ABs) of AChE and BuChE. Cholinesterases react with A6 compounds in the same way as they react with substrates that is, they acylate the hydroxyl group of serine in the catalytic site. However, there is a significant quantitative difference between substrates and AB compounds in the rates of the individual reaction steps. In the reaction with. substrates, acylation and deacylation of the serine is very fast, whereas AB compounds quickly acylate the enzyme but very slowly deacylale from the enzyme, particularly when AB is an OP. The enzyme therefore stays acylaled by AB compounds for a long time and cannot hydrolyze substrates during that time. Consequently, OP and CM compounds are inhibitors of cholinesterases. 

It must be pointed out, however, that the catalytic role played by the imidazole group at the active site of serine esterases is different from tlmt of Eq. (4—1). The imidazole group at the active site helps acylation and deacylation at the seryl hydroxyl group as ageneralbase (see Fig. 2—1), whereas in Eq. (4—1) imidazole acts as a nucleophilic catalyst. 

For example, a superb mimic of the charge-relay system in serine proteases has been prepared by attaching both carboxylate and imidazole to a-, fi-, and y-CyDs [24]. The hydroxy group, the last component of the charge-relay system, is provided by the CyDs. The activity (kinetic parameters) of the -CyD-based artificial enzyme for ester hydrolysis is close to that of aartificial enzymes show acylation, deacylation, and turnover, as is observed in the reactions of chymo-trypsin. The substrate-specificity is dependent on the kind of CyD used, since it is primarily governed by the substrate-binding process. In phenyl ester hydrolysis, a- and yS-CyD-based artificial enzymes are better than the y-CyD-based artificial enzyme. For the hydrolysis of tryptophan ethyl ester, however, the y-CyD-based artificial enzyme is the best. In another serine protease model, tripeptide (Ser-His-Asp) is directly introduced to the primary hydroxyl side of f -CyD [25]. This... 

Complementary to the bioconversion studies on tylosin, chemical methods for selective esterification of its 4"-hydroxyl group were developed in order to synthesize additional esters for structure-activity studies which were not available from bioconversion methods [100,101]. Although deacylation of the 4"-0-acyl group has generally been encountered in vivo, increased resistance to such hydrolysis by mouse liver esterase has been recently reported for two 4"-esters of tylosin [101, 102]. 

---