Nucleophilic transacylase

Nucleophilic Catalysts with Transacylase Activity I 119 ArOAc ArO ... 

A first example of nucleophilic catalyst with transacylase activity is given by an equimolar mixture of a Ba + salt (either bromide or perchlorate) and p-tert-butylcalix [4]arene-crown-5 (1), which catalyzes the methanolysis of aryl acetates in MeCN— MeOH (1 1, v/v) under slightly basic conditions (3 1 diisopropylethylamine-per-chlorate salt buffer) at 25°C [19,20]. 

Nucleophilic Catalysts ivith Transacylase Activity 125 Table 5.4 Methanolysis of 12 in MeCN-MeOH (9 1) at 25"C. 

We saw in Section 12.3.1 the use of the cyclodextrins as mimics for transacylases, a well-understood class of enzymes that perform the task of transferring an acyl group from one substrate to another (e.g. from an ester to water). Transacylase chemistry has also been addressed by Cram,5 who used chiral corands, such as 12.11, related to 3.106 bearing thiolate nucleophiles situated above and below the plane of the macrocycle. An acyclic analogue 12.12 was also prepared for comparison. The salient features of 12.11 are shown diagrammatically in Figure 12.4. 

Steps 1-2 of Figure 29.5 Acyl Transfers The starting material for fatty-acid synthesis is the thioester acetyl CoA, the ultimate product of carbohydrate breakdown, as we ll see in Section 29.6. The synthetic pathway begins with several priming reactions, which transport acetyl CoA and convert it into more reactive species. The first priming reaction is a nucleophilic acyl substitution reaction that converts acetyl CoA into acetyd ACP (acyl carrier protein). The reaction is catalyzed by ACP transacylase. 

Serine carbohydrate esterases and transacylases. The commonest reaction mechanism is the standard serine esterase /protease mechanism, demonstrated paradigmally for chymotrypsin, involving an acyl-enzyme intermediate. The enzyme nucleophile is a serine hydroxyl, which is hydrogen bonded the imidazole of a histidine residue, whose other nitrogen is hydrogen bonded to a buried, but ionised, aspartate residue (Figure 6.28),... 

For the malonate group to be used for fatty acid synthesis, it must first be transferred from malonyl-CoA to malonyl-ACP by the 32.4-kDa monomeric malonyl-CoA ACP transacy-lase, the product of the fabD gene (Fig. 2). A stable malonyl-serine enzyme intermediate is formed during the course of the FabD reaction, and subsequent nucleophilic attack on this ester by the sulfhydryl of ACP yields malonyl-ACP. The high reactivity of the serine in malonyl-ACP transacylase is due to the active site being composed of a nucleophilic elbow as observed in alpha/beta hydrolases. The serine is hydrogen bonded to His-201 in a fashion similar to serine hydrolases. 

Cram s group, like that of Lehn, is attempting to imitate enzyme catalysis via kinetic acceleration through host-guest complexation. A series of transacylase mimics have been synthesized [44] which contain the complexing site, the proton transfer catalyst and the nucleophile found in the chymotrypsin active site. Relatively rapid rates of acylation were observed. It is obvious, however, that additional information will have to be acquired and more model compounds synthesized before the mode of action of an enzyme such as chymotrypsin can be clarified. 

A fairly efficient nucleophile catalyst with transacylase activity (10—Ba cat) was developed by us [9]. Previous investigations indicated that alkaline-earth metal ions complexed by crown ethers are efficient promoters of acyl transfer reactions from esters to anionic nucleophiles [10]. The barium complex 10—Ba acts as a turnover catalyst in the methanolysis of aryl acetates, (diisopropylethylamine buffer in MeCN-MeOH 9 1, (v/v), 25 °C). Experimental evidence points to the operation of a ping-pong mechanism as outlined in Scheme 26.1. 

---