Acetyl CoA utilization

The sequence of events known as the Krebs cycle is indeed a cycle ox-aloacetate is both the first reactant and the final product of the metabolic pathway (creating a loop). Because the Krebs cycle is responsible for the ultimate oxidation of metabolic intermediates produced during the metabolism of fats, proteins, and carbohydrates, it is the central mechanism for metabolism in the cell. In the first reaction of the cycle, acetyl CoA condenses with oxaloacetate to form citric acid. Acetyl CoA utilized in this way by the cycle has been produced either via the oxidation of fatty acids, the breakdown of certain amino acids, or the oxidative decarboxylation of pyruvate (a product of glycolysis). The citric acid produced by the condensation of acetyl CoA and oxaloacetate is a tricarboxylic acid containing three car-boxylate groups. (Hence, the Krebs cycle is also referred to as the citric acid cycle or tricarboxyfic acid cycle.)... 

Long chain acyl-CoA esters inhibit the activity of isolated citrate synthase specifically (Wieland, 1968 Hsu and Powell, 1975 Caggiano and Powell, 1979) but this effect has not been demonstrated with intact mitochondria and its possible involvement in the control of acetyl-CoA utilization for citrate formation in vivo remains uncertain. Similarly an elevation of palmi-toyl-CoA generation at the outside of mitochondrial membrane in vitro increases the relative rates of ketogenesis and p-hydroxybutyrate to acetoacetate ratio, and these events can be rationalized in terms of the known inhibition of mitochondrial adenine nucleotide translocase by long chain acyl-CoA esters (Pande and Blanchaer, 1971 Shug et al., 1971) but whether this inhibition is exerted in intact cells is equivocal (Hansford,... 

It may seem surprising that isocitrate dehydrogenase is strongly regulated, because it is not an apparent branch point within the TCA cycle. However, the citrate/isocitrate ratio controls the rate of production of cytosolic acetyl-CoA, because acetyl-CoA in the cytosol is derived from citrate exported from the mitochondrion. (Breakdown of cytosolic citrate produces oxaloacetate and acetyl-CoA, which can be used in a variety of biosynthetic processes.) Thus, isocitrate dehydrogenase activity in the mitochondrion favors catabolic TCA cycle activity over anabolic utilization of acetyl-CoA in the cytosol. 

Succinyl-CoA derived from propionyl-CoA can enter the TCA cycle. Oxidation of succinate to oxaloacetate provides a substrate for glucose synthesis. Thus, although the acetate units produced in /3-oxidation cannot be utilized in glu-coneogenesis by animals, the occasional propionate produced from oxidation of odd-carbon fatty acids can be used for sugar synthesis. Alternatively, succinate introduced to the TCA cycle from odd-carbon fatty acid oxidation may be oxidized to COg. However, all of the 4-carbon intermediates in the TCA cycle are regenerated in the cycle and thus should be viewed as catalytic species. Net consumption of succinyl-CoA thus does not occur directly in the TCA cycle. Rather, the succinyl-CoA generated from /3-oxidation of odd-carbon fatty acids must be converted to pyruvate and then to acetyl-CoA (which is completely oxidized in the TCA cycle). To follow this latter route, succinyl-CoA entering the TCA cycle must be first converted to malate in the usual way, and then transported from the mitochondrial matrix to the cytosol, where it is oxida-... 

Acetyl-CoA is also utilized as a cofactor to modify chloramphenicol by O-acetyltranferases (CATs). These enzymes have been found in many different bacterial genera and are usually plasmid encoded in clinical isolates. Furthermore, streptogramin type A antibiotics are acetylatedby Vat enzymes that occur on plasmids in staphylococci and enterococci. 

The rate of mitochondrial oxidations and ATP synthesis is continually adjusted to the needs of the cell (see reviews by Brand and Murphy 1987 Brown, 1992). Physical activity and the nutritional and endocrine states determine which substrates are oxidized by skeletal muscle. Insulin increases the utilization of glucose by promoting its uptake by muscle and by decreasing the availability of free long-chain fatty acids, and of acetoacetate and 3-hydroxybutyrate formed by fatty acid oxidation in the liver, secondary to decreased lipolysis in adipose tissue. Product inhibition of pyruvate dehydrogenase by NADH and acetyl-CoA formed by fatty acid oxidation decreases glucose oxidation in muscle. 

Fig. 11. Active sites and reactions of the bifunctional CODH/ACS. For synthesis of acetyl-CoA, two electrons are transferred from external electron donors to Cluster B of the CODH subunit. Electrons are relayed to Cluster C which reduces CO2 to CO. The CO is proposed to be channeled to Cluster A of the ACS subunit to form a metal-CO adduct that combines with the methyl group of the CFeSP and CoA to form acetyl-CoA. For utilization of acetyl-CoA, these reactions are reversed. The abbreviations are CODH, CO dehydrogenase ACS, acetyl-CoA synthase CFeSP, the corrinoid iron-sulfur protein CoA, Coenzyme A.

Production of acetate ester pheromone components utilizes an enzyme called acetyl-CoA fatty alcohol acetyltransferase that converts a fatty alcohol to an acetate ester. Therefore, alcohols could be utilized as substrates for both aldehyde and acetate ester formation. In some tortricids an in vitro enzyme assay was utilized to demonstrate specificity of the acetyltransferase for the Z isomer of ll-14 OH [66]. This specificity contributes to the final ratio of... 

In the pathway, the 3HA-CoA is produced from the carbon source by the fatty acid /1-oxidation route so that the 3HA-CoA so formed can be utilized either for the production of a PHA directly or for the production of acetyl-CoA, which results in the formation of 3HA-CoA containing two carbons less than the original 3HA. 3HA-CoA thus formed can similarly be utilized for the production of... 

Ketone bodies are utilized in other tissues (but not the liver) by converting the acetoacetate to acetoacetyl-CoA and then converting the acetoacetyl-CoA to 2 acetyl-CoA, which are burned in the muscle mitochondria. 

The actual pathway by which fatty acid oxidation occurred was established by Lynen (1952-1953). Its unique and characteristic reaction was the thioclastic attack by coenzyme A on the B-ketoacyl CoA derivative, splitting off the 2C fragment, acetyl CoA. Free coenzyme A was very difficult to isolate and although it was synthesized in Todd s laboratory in Cambridge in the mid-1950s, much of the early work from Lynen s laboratory utilized A-acetyl cysteamine as a not very efficient (ca.1%) coenzyme A analogue. It carried the essential thiol group of the B-mercaptoethylamine end of CoA and could be used in most, but not all, of the steps in the spiral. 

Because glucose is the preferred fuel for the brain, an individual who experiences a rapid fall in glucose concentration leading to acute neuroglycopenia will initially feel confusion and may progress to coma and even death. In the event that the person survives 3-4 days, the brain can adapt its metabolism to utilize ketone bodies, metabolically derived from acetyl-CoA (see Figure 6.17), as a source of energy. 

The situation is simpler for odd numbered fatty acyl derivatives as [3-oxidation proceeds normally until a 5-carbon unit remains, rather than the usual 4-carbon unit. The C5 moiety is cleaved to yield acetyl-CoA (C2) and propionyl-CoA (C3). Propionyl CoA can be converted to succinyl CoA and enter the TCA cycle so the entire molecule is utilized but with a slight reduction in ATP yield as the opportunity to generate two molecules of NADH by isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase is lost because succinyl-CoA occurs after these steps in the Krebs cycle (Figure 7.18). 

In a muscle at rest, most of the 2-oxo acids produced from transamination of branched chain amino acids are transported to the liver and become subject to oxidation in reactions catalysed by branched-chain 2-oxo acid dehydrogenase complex. During periods of exercise, however, the skeletal muscle itself is able to utilize the oxo-acids by conversion into either acetyl-CoA (leucine and isoleucine) or succinyl-CoA (valine and isoleucine). 

The nucleophile in biological Claisen reactions that effectively adds on acetyl-CoA is almost always malonyl-CoA. This is synthesized from acetyl-CoA by a reaction that utilizes a biotin-enzyme complex to incorporate carbon dioxide into the molecule (see Section 15.9). This has now flanked the a-protons with two carbonyl groups, and increases their acidity. The enzymic Claisen reaction now proceeds, but, during the reaction, the added carboxyl is lost as carbon dioxide. Having done its job, it is immediately removed. In contrast to the chemical analogy, a carboxylated intermediate is not formed. Mechanistically, one could perhaps write a concerted decarboxylation-nucleophilic attack, as shown. An alternative rationalization is that decarboxylation of the malonyl ester is used by the enzyme to effectively generate the acetyl enolate anion without the requirement for a strong base. 

Commonly, in vitro determination of HDAC activity is a manual assay utilizing a coupled two-step process, including enzymatic deacetylation of a substrate followed by reaction termination and readout [10]. Assays utilize nuclear extracts and substrates containing labeled (radioactive or fluorescent) acetylated histones. For the isotope-based assays, the enzymes are incubated with acetate-radiolabled histones prepared from chicken reticulocytes or chemically [ Hjacetylated peptide substrates, and the enzymatic activity is determined by liquid scintillation counting [11]. Alternatively, histones may be obtained from cells following treatment with [ H]acetyl-CoA [12]. The caveats of these approaches include the variability of prelabeled acetylated core histones within preparations, potential high costs, their labor-intensive nature and the presence of radioactive waste. 

This enzyme [EC 6.2.1.1], also referred to as acetate-CoA ligase or acetate thiokinase, catalyzes the reaction of acetate, coenzyme A, and ATP to form acetyl-CoA, AMP, and pyrophosphate. The enzyme will also utilize propanoate and propenoate as substrates. 

This enzyme [EC 2.3.1.6], also known as choline ace-tylase, catalyzes the reaction of acetyl-CoA with choline to produce coenzyme A and O-acetylcholine. The enzyme can also utilize propionyl-CoA as a substrate, albeit as a weaker reactant. 

This enzyme [EC 4.1.3.25] catalyzes the conversion of (35)-citramalyl-CoA to acetyl-CoA and pyruvate. The (35)-citramalyl thioacyl-carrier protein can also be utilized as a substrate. This enzyme has been reported to be a component of citramalate lyase [EC 4.1.3.22]. 

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