2- acetyl group, enzymic

All the individual steps are catalyzed by enzymes NAD" (Section 15 11) is required as an oxidizing agent and coenzyme A (Figure 26 16) is the acetyl group acceptor Coen zyme A is a thiol its chain terminates m a sulfhydryl (—SH) group Acetylation of the sulfhydryl group of coenzyme A gives acetyl coenzyme A... 

We can descnbe the major elements of fatty acid biosynthesis by considering the for mation of butanoic acid from two molecules of acetyl coenzyme A The machinery responsible for accomplishing this conversion is a complex of enzymes known as fatty acid synthetase Certain portions of this complex referred to as acyl carrier protein (ACP), bear a side chain that is structurally similar to coenzyme A An important early step m fatty acid biosynthesis is the transfer of the acetyl group from a molecule of acetyl coenzyme A to the sulfhydryl group of acyl carrier protein... 

In the chymotrypsiii mechanism, the nitrophenylacetate combines with the enzyme to form an ES complex. This is followed by a rapid second step in which an acyl-enzyme intermediate is formed, with the acetyl group covalently bound to the very reactive Ser . The nitrophenyl moiety is released as nitrophenolate (Figure 16.22), accounting for the burst of nitrophenolate product. Attack of a water molecule on the acyl-enzyme intermediate yields acetate as the second product in a subsequent, slower step. The enzyme is now free to bind another molecule of nitrophenylacetate, and the nitrophenolate product produced at this point corresponds to the slower, steady-state formation of product in the upper right portion of Figure 16.21. In this mechanism, the release of acetate is the rate-llmitmg step, and accounts for the observation of burst kinetics—the pattern shown in Figure 16.21. 

The first domain of one subunit of the fatty acid synthase interacts with the second and third domains of the other subunit that is, the subunits are arranged in a head-to-tail fashion (Figure 25.9). The first step in the fatty acid synthase reaction is the formation of an acetyl-O-enzyme intermediate between the acetyl group of an acetyl-CoA and an active-site serine of the acetyl trails-... 

Enzymes work by bringing reactant molecules together, holding them, in the orientation necessary for reaction, and providing any necessary acidic or basic sites to catalyze specific steps. As an example, let s look at citrate synthase, an enzyme that catalyzes the aldol-like addition of acetyl CoA to oxaloacetate to give citrate. The reaction is the first step in the citric acid cycle, in which acetyl groups produced by degradation of food molecules are metabolized to yield C02 and H20. We ll look at the details of the citric acid cycle in Section 29.7. 

Step 1 of Figure 27.7 Claisen Condensation The first step in mevalonate biosynthesis is a Claisen condensation (Section 23.7) to yield acetoacetyl CoA, a reaction catalyzed by acetoacetyl-CoA acetyltransferase. An acetyl group is first bound to the enzyme by a nucleophilic acyl substitution reaction with a cysteine —SH group. Formation of an enolate ion from a second molecule of acetyl CoA, followed by Claisen condensation, then yields the product. 

Acetyltransferase is an enzyme that catalyses the transfer of an acetyl group from one substance to another. 

The major mechanism of resistance to chloramphenicol is mediated by the chloramphenicol acetyltransferases (CAT enzymes) which transfer one or two acetyl groups to one molecule of chloramphenicol. While the CAT enzymes share a common mechanism, different molecular classes can be discriminated. The corresponding genes are frequently located on integron-like structures and are widely distributed among Gramnegative and - positive bacteria. 

An enzyme activity ascribed to many coactivators, which transfers acetyl groups to lysine residues of histone tails of the nucleosomes and thereby facilitate their disruption and the opening of the chromatin. 

Enzyme activity ascribed to corepressors, which is the removal of acetyl groups from lysine residues of histone tails. Thereby the assembly of nucleosomes is maintained, which leads to a dense, transcriptional inactive chromatin structure. 

The enzymes in peroxisomes do not attack shorter-chain fatty acids the P-oxidation sequence ends at oc-tanoyl-CoA. Octanoyl and acetyl groups are both further oxidized in mitochondria. Another role of peroxisomal P-oxidation is to shorten the side chain of cholesterol in bile acid formation (Chapter 26). Peroxisomes also take part in the synthesis of ether glycerolipids (Chapter 24), cholesterol, and dolichol (Figure 26-2). 

The same enzyme (RGAE) could be purified from A. niger, together with two other esterases a feruloyl esterase (FAE) and an acetyl esterase (PAE) specific for the removal of one type of acetyl group present in the smooth regions of sugar-beet pectin. 

AE catalyses the cleavage of acetyl groups from different substrates. The enzyme activity was determined by measuring the release of acetic acid. The amount of acetic acid was measured spectrophotometrically using an acetic acid analysis kit (Boehringer, Mannheim). The activity of AE was measured in 0.6% sugar beet pectin solubilised in 25 mM Na-succinate pH 6.2 and incubated with enzyme fraction in total 500 nl assay. The samples were incubated at 40°C and aliquots were examined after 0, 1, 2 and 3 hours of incubation. The enzyme reaction was stopped by incubating the samples at lOO C for 5 min. Precipitated... 

Only PAE, a novel acetyl esterase, could remove acetyl from beet pectin, to a maximum of 30%. This was shown to be one specific acetyl group in theJiomogalacturonan chain of pectin (smooth region) by NMR spectroscopy. PAE activity was influenced by buffer salts and the addition of bivalent cations. PAE worked cooperatively with pectolytic enzymes. 

FAE was specific for esterified xylan-oligomers, but did not show selectivity towards a specific ester. This enzyme could release ferulic acid as well as acetyl groups from esterified arabinoxylans in the presence of an endoxylanase. 

This novel enzyme was the only esterase able to release acetyl from sugar beet pectin and removed about 30% of the total acetyl groups present. It also caused the release of acetyl groups from a range of other acetylated substrates, either synthetic or extracted from plants, in small amounts. PAE had an apparent molecular weight of 60 kDa and showed optimal activity at pH 5.5 and a temperature of 50 C. The enzyme is sensitive to buffer composition and requires a bivalent cation for optimal activity and stability. In purified form this enzyme proved unstable, especially in phosphate buffers. 

As this enzyme proved specific for the release of acetyl groups from MHR, to a maximum of 70 %, and is essential for the degradation of MHR by RG ( Figure 3), it was concluded to be comparable to that isolated previously from A. aculeatus[8,9]. The importance of this enzyme in the application of tailormade commercial pectinases is discussed elsewhere in these proceedings as well (H.P. Heldt-Hansen et al.). 

The chain shortening pathway has not been characterized in detail at the enzymatic level in insects. It presumably is similar to the characterized pathway as it occurs in vertebrates. These enzymes are a partial P-oxidation pathway located in peroxisomes [29]. The key enzymes involved are an acyl-CoA oxidase (a multifunctional protein containing enoyl-CoA hydratase and 3-hy-droxyacyl-CoA dehydrogenase activities) and a 3-oxoacyl-CoA thiolase [30]. These enzymes act in concert to chain shorten acyl-CoAs by removing an acetyl group. A considerable amount of evidence in a number of moths has accumulated to indicate that limited chain shortening occurs in a variety of pheromone biosynthetic pathways. 

An enzyme system in pigeon liver is able to transfer the acetyl group from... 

---