2- acetyl group, enzymic hydrolysis

Baddiley and coworkers42 have studied the structure of S13, which is composed of D-galactose, D-glucose, 2-acetamido-2-deoxy-D-glucose, and ribitol residues, and phosphate groups in the molar proportions 2 1 1 1 1. O-Acetyl groups are also present. A neutral pentasaccharide was obtained by hydrolysis with alkali, followed by enzymic dephosphorylation. On mild, acid hydrolysis, this yielded two main products, a trisaccharide (19) and a disaccharide (20), the structures of which were determined by conventional methods. 

It is possible to speculate on a situation analogous to that shown in fig. 19(6) or fig. 20(a) for certain anticholinesterases such as T.E.P.P. or p-nitrophenyl esters (fig. 21). Here a H 0 bonding may perhaps be envisaged in place of the H F bonding. Whatever the precise mechanism of attack by the phosphorus compound on the enzyme, the fact is that the latter is phos-phorylated in contradistinction to normal acetylation. Whereas the acetyl group is readily removed by hydrolysis under normal conditions, the phosphoryl group is usually firmly attached. 

Figure 3.3 (a) Covalent catalysis the catalytic mechanism of a serine protease. The enzyme acetylcholinesterase is chosen to illustrate the mechanism because it is an important enzyme in the nervous system. Catalysis occurs in three stages (i) binding of acetyl choline (ii) release of choline (iii) hydrolysis of acetyl group from the enzyme to produce acetate, (b) Mechanism of inhibition of serine proteases by diisopropylfluorophosphonate. See text for details. 

Hydrolysis involves nucleophilic attack by the serine hydroxyl onto the ester carbonyl (see Box 7.26). This leads to transfer of the acetyl group from acetylcholine to the enzyme s serine hydroxyl, i.e. formation of a transient acetylated enzyme, and release of choline. We have met this type of reaction before under transesterification (see Section 7.9.1). Hydrolysis of the acetylated enzyme then occurs rapidly, releasing acetate and regenerating the free enzyme. 

Conversion of the carboxyl groups in penicillins confers partial resistance to /3-lactamase (85-87). In cephalosporins, replacement of the acetyl group in position 3 of the dihydrothiazine ring [R in Fig. 1, (III)] by pyridine causes increased susceptibility to hydrolysis by /3-lactamases of Pseudomonas pyocyanea (31, 88), Enterobacter cloacae (45), E. coli (42, 88), and both the extracellular and cell-bound enzymes of B. cereus (38). 

The acetyl group is removed from the enzyme by hydrolysis and the enzyme is regenerated... 

The mechanism for the hydrolysis that is catalyzed by the enzyme involves the hydroxy group of a serine amino acid residue in the protein acting as a nucleophile and attacking the carbonyl carbon of the acetylcholine ester. The ester is cleaved, and the acetyl group becomes bonded to the enzyme. Then the acetyl group is hydrolyzed off the enzyme. enabling it to perform another catalytic cycle. This hydrolysis is very facile, so a single enzyme molecule can catalyze the hydrolysis of many acetylcholine molecules ... 

Acetylated cottonseed protein demonstrated significantly higher water and oil holding capacities and improved foaming properties (38) compared to unmodified proteins (Table III). Thus, while acetylation does not significantly enhance functional properties of proteins, it improves thermal stability and since acetylated proteins are susceptible to enzyme hydrolysis in vivo it affords a useful reagent for protection of e-NH groups of lysine (11). 

Figure 9.15. Catalytic mechanism of acetylcholinesterase, a A catalytic triad yields a deprotonated serine, which attacks acetylcholine and winds np as an acetylated intermediate. Transfer of the acetyl gronp to the enzyme involves a tetrahedral transition state, b Hydrolysis of the acetyl group is facilitated by an analogous mechanism, which involves a hydroxide anion as the nucleophile.

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