Transacylases

It was almost immediately recognised that the deacylated product, 7-aminocephalosporanic add (7-ACA, Figure 6.16), would be of similar importance as was 6-APA in the development of new penidllins. However, 7-ACA, the cephalosporin equivalent of 6-APA, could not be found in fermentations of Cephalosporin acremonium. In Figure 6.15 we have shown that penicillin acylase hydrolyses the acyl residue from natural cephalosporins. Up to a point this is true. These acylases will, however, only work with a limited range of acyl residues. It now seems that nature does not provide for acylases or transacylases that have the capacity to remove or change the D-a-aminoadipyl side chain of cephalosporin C efficiently in a single step. Widespread search for such an enzyme still remains unsuccessful. 

CLC Charcot-Leyden crystal CMC Critical micellar concentration CMI Cell mediated immunity CML Chronic myeloid leukaemia CMV Cytomegalovirus CNS Central nervous system CO Cyclooxygenase CoA Coenzyme A CoA-IT Coenzyme A - independent transacylase... 

Malonyl-CoA Acyl carrier protein transacylase (MCAT or FabD) catalyzes the transfer of a malonyl group between coenzyme A and acyl carrier proteins that are the privileged transporters of the FASH system. Although essential, this enzyme is present in excess and does not have a regulatory role in the FASH pathway, which might explain the relative paucity of reported inhibitors [1],... 

Fig. 9. The transacetylase ribozyme. A Secondary structure of the clone 11 transacylase ribozyme based on the Zuker RNA folding algorithm Mfold. The oligonucleotide substrate is shaded in gray. The 2 -OH group of cytosine 147 (arrow) is the site of modification of the oligonucleotide substrate. B Reaction catalyzed by the clone 11 transacylase ribozyme. Note that the equilibrium of the reaction lies strongly on the side of the Bio-Phe-AMP substrate...

Selected entries from Methods in Enzymology [vol, page(s)] Assay, 1, 611 3, 935-938 63, 33 separation by HPLC, 72, 45 extraction from tissues, 13, 439 formation of, 1, 486, 518, 585 5, 466 free energy of hydrolysis, 1, 694 substrate for the following enzymes [acetyl-coenzyme A acyl carrier protein transacylase, 14, 50 acetyl-coenzyme A carboxylase, 14, 3, 9 acetyl-coenzyme A synthetase, 13, 375 N-acetyltransferase, 17B, 805 aminoacetone... 

ACETYL-CoA ACYLTRANSFERASE ACETYL-CoAACP TRANSACYLASE ACETYL-CoA CARBOXYLASE ACETYL-CoA SYNTHETASE N-ACETYLGLUTAMATE SYNTHASE... 

MALEATE ISOMERASE MALONYL-C0A.ACP TRANSACYLASE MALYL-CoA LYASE... 

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. 

Cacciapaglia, R., Casnati, A., Mandolini, L. and Ungaro, R. (1992) The barium(II) complex of p-tert-butylcalix[4]arene-crown-5 a novel nuclephilic catalyst with transacylase activity. J. Am. Chem. Soc., 114, 10956. 

The now-vacant ACP accepts a three-carbon malonate unit from malonyl CoA. Domain Malonyl CoA-ACP-transacylase. 

Knudsen and Grunnet (1982) have proposed an interesting system for the control of medium-chain fatty acid synthesis by ruminant mammary tissue. Their proposal is based on their observations that ruminant mammary tissue fatty acid-synthetase exhibits both medium-chain thioesterase (Grunnet and Knudsen 1978) and transacylase (Knudsen and Grunnet 1980) activity and that medium-chain fatty acids synthesized de novo can be incorporated into TG without an intermediate activation step (Grunnet and Knudsen 1981). They proposed that the synthesis of the medium-chain fatty acids is controlled by their incorporation into TG (Grunnet and Knudsen 1981). Further work will be needed to substantiate transacylation as a chain-termination mechanism in fatty acid synthesis by ruminant mammary tissue. 

Electron microscopy of the core dihydrolipoyl transacylase from E. coli reveals a striking octahedral symmetry which has been confirmed by X-ray diffraction.306 3073 The core from pyruvate dehydrogenase has a mass of 2390 kDa and contains 24 identical 99.5-kDa E2 subunits. The 2-oxoglutarate dehydrogenase from E. coli has a similar but slightly less symmetric structure. Each core subunit is composed of three domains. A lipoyl group is bound in amide linkage to lysine 42 and protrudes from one end of the domain. 

Mitochondrial pyruvate dehydrogenase, which contains a 60-subunit icosohedral core of dihydrolipoyl-transacylase (Fig. 15-14), is associated with three molecules of a two-subunit kinase as well as six molecules of a structural binding protein which contains a... 

After malonyl-CoA synthesis, the remaining steps in fatty acid synthesis occur on fatty acid synthase, which exists as a multienzyme complex. In the initial reactions acetyl-CoA and malonyl-CoA are transferred onto the protein complex by acetyl-CoA transacylase and malonyl-CoA transacylase (step 1 and step 2 in fig. 18.12a). The acceptor for the acetyl and malonyl groups is acyl carrier protein (ACP). ACP also carries all of the intermediates during fatty acid biosynthesis. The prosthetic group that binds these intermediates is... 

Fatty acid synthesis begins when the substrates, acetyl-CoA and malonyl-CoA, are transferred onto the protein by malonyl-CoA acetyl-CoA-ACP transacylase (MAT, steps 1 and 2 in fig. 18.12a). The numbers in parentheses below the abbreviation of the enzyme in this figure refer to the reactions shown in fig. 18.12. (Whereas E. coli has separate enzymes that catalyze the transfer of acetyl- and malonyl-CoA to ACP, both reactions are catalyzed by the same enzymatic activity (MAT) on the animal fatty acid synthase.) Subsequently, /3-ketobutyryl-ACP and CO2 are formed in a condensation reaction catalyzed by /3-ketoacyl-ACP synthase (KS, step 3 in fig. 18.12a). 

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