3-hydroxyl esters enzymatic hydrolysis

These hemiketalic adducts are very good mimics of the tetrahedral transition state involved in the enzymatic hydrolysis of an ester bond or a peptidic bond [71,72], The nucleophilic entity of the enzyme active site (e.g. the hydroxyl of hydrolytic serine enzymes) can easily add onto the activated carbonyl of a fluor-oketone leading to a very stable tetrahedral intermediate. The enzyme is not regenerated and is thus inhibited (Fig. 21) [73],... 

Cephalothin can be determined by means of the colored complex formed on the addition of a ferric reagent to the corresponding hy-droxamic acid produced by treatment with hy-droxylamine. The method used is essentially the same.as the procedure described for penicillins. The ferric hydroxamate procedure is not specific for cephalothin or penicillins. For example, many amides, esters, and anhydrides form hydroxamic acids when reacted with hydroxyl-amine. This type of interference is eliminated by the blank determination wherein cephalothin is rendered incapable of forming hydroxamic acid by use of basic hydrolysis or enzymatic hydrolysis with cephalosporinase. Since cephalothin degradation products having an intact (3-lactam ring react as well as the parent compound, the method measures total 3-lactam content. ... 

The extraction of CNCs can be performed by strong acid hydrolysis of cellulose or purified cellulose under controlled conditions. Different common acids were used to extract CNCs such as sulfuric acid (H SO ), hydrochloric acid (HCl), and hydrobromic acid (HBr) whereas phosphoric acid (H PO ), maleic acid (HO CCHCHCO H), and hydrogen peroxide (H O) were used to a lesser extent. However, sulfuric acid is the most common acid used for acid hydrolysis of corresponding cellulose,where it reacts with hydroxyl (-OH) groups to form sulfate esters and produces stable aqueous suspension compromising thermal stabihty [6,8,59]. Apart from acid hydrolysis, many other processes have also recently been used to extract nanocrystalline cellulose from cellulose fibers such as enzymatic hydrolysis [60-62], TEMPO-mediated oxidation [63,64], hydrolysis with gaseous acid [65], and ionic liquid [66]. 

Several 16-membered macrolides form metabolites which retain antimicrobial activity. As discussed above, 3"-esters such as rokitamycin and miokamycin produce prolonged concentrations of antibiotic in vivo due to the facile 3"- to 4"-0-acyl migration that follows enzymatic removal of the original 4 -ester [34, 269, 270], Following a different approach to overcome the lability of 4"-esters, specific 4"-0-acyl derivatives of tylosin were selected from the series of esters (15) based upon their greater stability toward liver enzymes [80], Although esterases play the most prominent role in metabolism of 16-membered macrolides, other mechanisms such as oxidative hydroxylation, A-demethylation, reduction, and hydrolysis of sugars have been reported for various compounds [91, 96, 115, 259, 270-272]. 

In contrast to other monosaccharides, activated sialic acid donors are biosynthesized from A -acetylmannosamine (ManNAc) or directly from sialic acids (Sia), including A-acetylneuraminic acid (NeuAc), via a more complex pathway (21). ManNAc is phosphorylated at the at the 6-hydroxyl group and condensed with phosphoenolpyruvate to give A -acetylneuraminic acid-9-phosphate (NeuAc-9-P). Phosphate ester hydrolysis is followed by direct condensation with CTP to give CMP-NeuAc (Figure 3). Sialic acids can intercept this pathway directly via enzymatic reaction with CTP. 

Of the six main classes of enzymes, hydrolases, oxidoreductases and transferases have been the three most useful in kinetic resolution. Among the hydrolases, lipases are extensively used. The molecular machinery of lipases consists of a catalytic triad of the amino acids serine, histidine, and aspartic (or glutamic) acid. The enzyme first transfers the acyl group of an ester to the hydroxyl group of the serine residue to form the acylated enzyme. The acyl group is subsequently transferred to an external nucleophile with the return of the enzyme to its pre-acylated state to start the process again. A variety of nucleophiles can participate in this process water results in hydrolysis, an amine results in amidation, an alcohol results in esterification or transesterification, and hydrogen peroxide results in the formation of perac-id. Another reason which favored the relatively wide applicability of lipases in enzymatic... 

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