Acylation and Deacylation Reactions

Acetylation of eucomin (3) with boiling acetic anhydride leads to 7-O-acetyleucomin (49) (9). In later experiments additional formation of di-O-acetyleucomin (50) was observed (64). This is exclusively formed in presence of pyridine (9). In the case of eucomol (10) the aliphatic hydroxyl group is also acetylated under both conditions (9). 

Benzoylation of eucomin (3) with benzoyl chloride and potassium carbonate in acetone at room temperature occurs selectively at the 7-hydroxyl group (64). This is probably due to the low solubility of 

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Baldessari, A. Mangone, C. P. Gros, E. G. Lipase-catalyzed acylation and deacylation reactions of pyridoxine, a member of the vitamin-B6 group. Helv. Chim. Acta 1998, 82, 2407-2413. 

The acylation and deacylation reactions at the serine residue can be described mechanistically as bimolecular substitutions in which a nucleophile uses an unshared electron pair to bond to a polarized carbon atom and displace the leaving group (Z) ... 

Many new, selective acylation and deacylation reactions by use of enzymes have been reported. A comparative study of the enzyme-catalysed acetylation of monosaccharides in organic solvents found a Pseudomonas lipase the most... 

These reactions represent the acylation and deacylation reactions respectively (with rate constants k2 and the constants k and k-i correspond to the processes of non-covalent binding of the substrate to the enzyme forming the Michaelis complex and its dissociation,... 

The hydroxy group undergoes 0-acylation and deacylation (79JHC689). These reactions of functionalized hydroxyfurazans are valuable methods for modification of these compounds. Thus, hydroxybifurazan 248 was aroylated with benzoyl chloride in the presence of pyridine with concomitant cleavage of the unsubstituted furazan ring to give nitrile 262 (Scheme 170) (75LA1029). 

Table 12. Enantioselectivities in the acylation and deacylation steps in the burst kinetics of the reaction of (Z)-Phe-PNP(52)...

The solvent present in biphasic reactions can still have an effect on the enzyme even though the enzyme functions primarily in an aqueous microenvironment. A particularly dramatic example is the lipase AH (lipase from Burkholderia cepac/fl)-catalysed desym-metrization of prochiral 1,4-dihydropyridine dicarboxylic esters, where either enantiomer can be accessed in high enantioselectivity by using either water-saturated cyclohexane or diisopropyl ether (DIPE) respectively (Scheme 1.60). The acyl group used in acylation and deacylation can also have a dramatic effect on enantioselectivity. " ... 

There is now convincing evidence that an acyl chymotrypsin intermediate is formed from both specific and non-specific substrates (Bender and Kezdy, 1964 Bender et al., 1964). This intermediate is undoubtedly an acylserine. Acyl- and phosphorylserine derivatives have been isolated and identified. In view of evidence such as a D2 O solvent isotope effect ( h2oAd2o) 2-3 for both acylation and deacylation (Bender and Hamilton, 1962), alcohol and amine nucleophiles showing little dependence on the p/iTa-value of the nucleophile in reaction with furoyl enzyme (Inward and Jencks, 1965), and the influence of increasing steric bulk in the acyl group (Fife and Milstien, 1967 Milstien and Fife, 1968,.1969), consistent... 

Acylation and deacylation in equation (13) proceed through similar transition states. If deacylation occurs through attack of an alcohol molecule R OH rather than water on the carbonyl carbon atom, then deacylation is the microscopic reverse of acylation. Bender and coworkers (Bender and Kezdy, 1965) have demonstrated the symmetry of the reaction about the acyl enzyme in reactions in which reversibility can be observed. 

Reaction Mechanism of Lipases and Implications for Monomer Acceptance in the Acylation and Deacylation Step... 

The strategy is to measure the rate constants k2 and k3 of the acylenzyme mechanism (equation 7.1) and to show that each of these is either greater than or equal to the value of kCM for the overall reaction in the steady state (i.e., apply rules 2 and 3 of section Al). This requires (1) choosing a substrate (e.g., an ester of phenylalanine, tyrosine, or tryptophan) that leads to accumulation of the acylenzyme, (2) choosing reaction conditions under which the acylation and deacylation steps may be studied separately, and (3) finding an assay that is convenient for use in pre-steady state kinetics. The experiments chosen here illustrate stopped-flow spectrophotometry and chromopboric procedures. 

The accumulation of the acyl intermediate (PVIm-Ac ) was established unequivocally by studying the acylation and deacylation behaviors of the polymer separately. Overberger and Glowaky (60) allowed polyvinylimidazole 1 to react with loi -chain acyl substrates SJJ" 15 and separated the partially acylated polymer by utilizing gel permeation chromatography (Sephadex LH-20). Table 4—3 gives first-order rate coie stants for acylation (kobs) and deacylation (ka) reactions, is larger than kobs fot... 

One of the key intermediates shown in this reaction scheme is the formation of a tetrahedral adduct during acylation and deacylation (84). Additional support for the formation of a tetrahedral intermedite comes from the observation already referred to— that aldehydes may act as potent inhibitors of papain. Westerik and Wolfenden (65) attribute the inhibitory eflFect of aldehydes to the formation of a stable thiol adduct (thiohemiacetal) analogous to the tetrahedral intermediate produced when papain acts on a substrate. This relationship is depicted in Figure 14. When the complete picture for the mechanism of catalysis by the thiol proteases finally emerges, it will no doubt be similar to the mechanism of action of the serine proteinases. 

Reactions such as acylation and deacylation are catalyzed by enzymes through lowering of the energy of activation. Binding and stabilization of the near tetrahedral oxyanion transition state by the active site of the enzyme is how this is accomplished. Thus, a transition state analogue inhibitor is a species, which binds the active site in a way similar to that... 

Lactamases enzymes are responsible for most bacterial resistance against 13-lac tarn antibiotics. As such, they are a serious and growing threat to the effectiveness of antibacterial chemotherapy, and are a major threat to human health. The reaction mechanism of a Class A (3-lactamase (with benzylpenicillin) has been investigated by illustrative recent QM/MM calculations (see Figure 4 and 5). Glul66 was identified as the base in both acylation and deacylation... 

In the Novozym 435 catalysed ring-opening of a (chiral) substituted lacton, both acylation and deacylation can be enantioselective. For example, it is well known that CALB shows pronounced selectivity for / -secondary alcohols in the deacylation step. Since the forward and backward reaction exhibits by definition the same selectivity, esters comprising a substituent at the alcohol side are expected to show pronounced / -selectivity in the acylation step. " This is indeed observed for 7-MeHL, 8-MeOL and 12-MeDDL (Table 1). However, the selectivity for acyl donor in the case of PBL, 5-MeVL and 6-MeCL -lactones in which the ester bond is exclusively in a cisoid conformation- is low or for the S-enantiomer. We can speculate that lactones in a cisoid conformation must attain a different orientation in the active site in order to be activated. ... 

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. 

The influence of structural and electronic parameters on the acylation and deacylation rate were separately studied (catalyst-on half cycle vs. catalyst-off half cycle cf. scheme 7.10). Through kinetic H-NMR-studies it was proven that the acylation rate of the dialkyl amino alcohol catalysts and an acyl donor (butyric anhydride) depends on the number of (carbon) spacer atoms between hydroxyl and tertiary amine, the flexibility of the molecule and the presence and position of further heteroatoms. Besides, it could be detected that the methanolysis (off-half cycle) of the formed ]3-amino ester intermediate follows a similar trend as the acylation reaction, but appeared to be rate limiting in this studies setup. The information was used for the selective auto-catalytic acylation and deacylation of complex natural antibiotics. 

If the enzyme-catalyzed hydrolysis of peptide bond involves a simple reversible reaction as shown by Equation 2.5 then, indeed, the enzyme must catalyze the rate of formation of peptide bond from amino acids (i.e., lq,-step), provided the amino acids do not react irreversibly with the enzyme. Incidentally, if the function of serine proteases is to catalyze both the rate of hydrolytic cleavage and the rate of formation of protein peptide bond, then, probably, these enzymes cannot digest the proteins that we eat and, consequently, the results would have been disastrous for all protein-eating creatures — which certainly Nature will never allow. Although the mechanisms of most of the enzyme-catalyzed reactions are unknown, even at a very rudimentary level, the mechanism of a-chymotrypsin-catalyzed hydrolysis of peptide bond has been relatively well understood. The reaction has been almost ascertained to involve acylation and deacylation of enzyme as shown by Equation 2.6. Widely accepted mechanisms for acylation and deacylation steps are shown in Scheme 2.6 and Scheme 2.7. ... 

Werber, M.M., Shahtin, Y. Reaction of tertiary amino alcohols with active esters acylation and deacylation steps. Bioorg. Chem. 1973, 2(3), 202-220. 

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