Active acetate acetyl CoA

Squalene [(a//- )-2,6,10,15,19,23-hexamethyl-2.6, 10,14,18,22-tetracosahexaene]. The most important aliphatic, acyclic triterpene, C30H50, Mr 410.73, mp. -4.8 to -5.2 C, bp. 284-285°C, formula, see steroids. S. was first isolated from fish liver oils and later detected in plant oils and human fat. It is composed of 6 isoprene units and is formed from activated acetate ( acetyl-CoA) via mevalonic acid. It is an intermediate in the biosynthesis of all cyclic triterpenoids and thus also of the steroids. Its enzymatic cyclization to IanosteroI or cycloartenoI requires molecular oxygen and proceeds through (35)-squalene 2,3-epoxide. lit. Annu. Rev. Biochem. 14,555-585 (1982)"Chem. Soc. Rev. 20,129-147 (1991) - Kaiier, No. 34 Nat. Prod. Rep. 2, 525 - 5W (1985) Phytochemisby 27,628 (1988) (biosy nthe-sis) Stryer 1995,692-695.-/HS290/29 CASHI-02-4]... 

ACh synthesis is carried out in the neuron by means of the coenzyme A (CoA) and a specific enzyme, choline-acetylase (ChAc, choline-acetyltransferase), which transfers the acetyl radical from active acetate (acetyl CoA) to choline (see literature in ref. 16) ... 

The reactions of activated acetate (acetyl-CoA) can be indicated here only sketchily. There are two groups (1) reactions of the carboxyl group, and (2) reactions of the methyl group (or, in the case of higher homologs, of the a-methylene group). 

This pathway is followed by alanine, valine, isoleucine, and leucine. The prototype for all is the metabolism of alanine, which goes to activated acetate (acetyl-CoA) in two steps. Acetyl-CoA, of course, then can undergo a multitude of reactions. The first step is transamination, yielding in the usual manner the a-keto... 

Syntheses with Active Acetate. Acetyl-CoA is an important starting material for biosynthetic reactions. First of all, fatty acids can be formed from it. This is the major pathway followed during the conversion of carbohydrate to fat. 

The active form of acetate, acetyl CoA, was finally isolated by Lynen and Reichert in 1951 following studies of fatty acid oxidation (Chapter... 

Cleavage of ATP to AMP and PP, during Metabolism The synthesis of the activated form of acetate (acetyl-CoA) is carried out in an ATP-dependent process ... 

In the ruminant mammary tissue, it appears that acetate and /3-hydroxybutyrate contribute almost equally as primers for fatty acid synthesis (Palmquist et al. 1969 Smith and McCarthy 1969 Luick and Kameoka 1966). In nonruminant mammary tissue there is a preference for butyryl-CoA over acetyl-CoA as a primer. This preference increases with the length of the fatty acid being synthesized (Lin and Kumar 1972 Smith and Abraham 1971). The primary source of carbons for elongation is malonyl-CoA synthesized from acetate. The acetate is derived from blood acetate or from catabolism of glucose and is activated to acetyl-CoA by the action of acetyl-CoA synthetase and then converted to malonyl-CoA via the action of acetyl-CoA carboxylase (Moore and Christie, 1978). Acetyl-CoA carboxylase requires biotin to function. While this pathway is the primary source of carbons for synthesis of fatty acids, there also appears to be a nonbiotin pathway for synthesis of fatty acids C4, C6, and C8 in ruminant mammary-tissue (Kumar et al. 1965 McCarthy and Smith 1972). This nonmalonyl pathway for short chain fatty acid synthesis may be a reversal of the /3-oxidation pathway (Lin and Kumar 1972). 

Biotin enzymes are believed to function primarily in reversible carboxvlahon-decarboxylation reactions. For example, a biotin enzyme mediates the carboxylation of propionic acid to methylmalonic add, which is subsequently converted to succinic acid, a dtric acid cycle intermediate. A vitamin Bl2 coenzyme and coenzyme A are also essential to this overall reaction, again pointing out the interdependence of the B vitamin coenzymes. Another biotin enzyme-mediated reaction is the formation of malonyl-CoA by carboxylation of acetyl-CoA ( active acetate ). Malonyl-CoA is believed lo be a key intermediate in fatly add synthesis. 

Answer TPP thiazolium ring adds to a carbon of pyruvate, then stabilizes the resulting car-banion by acting as an electron sink. Lipoic acid oxidizes pyruvate to level of acetate (acetyl-CoA), and activates acetate as a thioester. CoA-SH activates acetate as thioester. FAD oxidizes lipoic acid. NAD+ oxidizes FAD. (See Fig. 16-6.)... 

The de novo synthesis of fatty acids in the mammary gland utilizes mainly acetate and some (3-hydroxybutyrate. These precursors arise from the microbial fermentation of cellulose and related materials in the rumen. Once in the mammary gland, acetate is activated to acetyl-CoA. The mechanism of fatty acid synthesis essentially involves the carboxylation of acetyl-CoA to malonyl-CoA, which is then used in a step-wise chain elongation process. This leads to a series of short-chain and medium-chain length fatty acids, which differ by two CH2 groups (e.g., 4 0, 6 0, 8 0, etc.) (Hawke and Taylor, 1995). These are straight-chain, even-numbered carbon fatty acids. However, if a precursor such as propionate, valerate or isobutyrate, rather than acetate, is used, branched-chain or odd-numbered carbon fatty acids are synthesised (Jenkins, 1993 see Chapter 2). 

System Interaction of Glia and Neurons Acetate is metabolized in astrocytes nearly 18 times faster than in cortical synaptosomes, though activity of acetyl-CoA synthase in synaptosomes is almost double that in astrocytes (5.0 and 2.9nmol/min per mg of protein, respectively). The principal difference in the acetate metabolism rates is explained by differences in the kinetics of its transport, which is mediated by a monocarboxylate carrier (Hosoi et ah, 2004) acetate uptake by astrocytes, unlike synaptosomes, rapidly increases and follows saturation kinetics = 498nmol/mg protein/min, =... 

Cell extracts of Methanothrix soehngenii contain high activities of acetyl-CoA synthetase rather than acetate kinase and phosphate acetyltransferase [240,241] (Fig. 10). This indicates that in this organism acetate is activated to acetyl-CoA by acetyl-CoA synthase (AS). Since AMP is converted to ADP via adenylate kinase (AK), and pyrophosphate (PPj) is completely hydrolyzed via pyrophosphatase (PPj-ase), acetate activation by acetyl-CoA synthetase requires the input of 2 ATP equivalents ... 

At least three acyl-CoA synthases, each specific for a particular size of fatty acid, exist acetyl-CoA synthase acts on acetate and other low-molecular-weight carboxylic acids, medium-chain acyl-CoA synthase on fatty acids with 4-11 carbon atoms, and acyl-CoA synthase on fatty acids with 6-20 carbon atoms. The activity of acetyl-CoA synthase in muscle is restricted to the mitochondrial matrix. Medium-chain acyl-CoA synthase occurs only in liver mitochondria, where medium-chain fatty acids obtained from digestion of dietary triacylglycerols and transported by the portal blood are metabolized. Acyl-CoA synthase, the major activating enzyme, occurs on the outer mitochondrial membrane surface and in endoplasmic reticulum. The overall reaction of activation is as follows ... 

Acetate in cells can be converted to a more activated form (acetyl-CoA) by the following reaction ... 

Acetate (acetic acid) is present in the diet, and can be produced from the oxidation of ethanol. Roman soldiers carried vinegar, a dilute solution of acetic acid. The acidity of the vinegar made it a relatively safe source of drinking water because many kinds of pathogenic bacteria do not grow well in acid solutions. The acetate, which is activated to acetyl CoA, provided an excellent fuel for muscular exercise. 

Ethanol is a dietary fuel that is metabolized to acetate principally in the liver, with the generation ofNADH. The principal route for metabolism of ethanol is through hepatic alcohol dehydrogenases, which oxidize ethanol to acetaldehyde in the cytosol (Fig. 25.1). Acetaldehyde is further oxidized by acetaldehyde dehydrogenases to acetate, principally in mitochondria. Acetaldehyde, which is toxic, also may enter the blood. NADH produced by these reactions is used for adenosine triphosphate (ATP) generation through oxidative phosphorylation. Most of the acetate enters the blood and is taken up by skeletal muscles and other tissues, where it is activated to acetyl CoA and is oxidized in the TCA cycle. 

Approximately 90% of the acetaldehyde that is generated is further metabolized to acetate in the hver. The major enzyme involved is a low mitochondrial acetaldehyde dehydrogenase (ALDH), which oxidizes acetaldehyde to acetate with generation of NADH (see Fig. 25.2). Acetate, which has no toxic effects, may be activated to acetyl CoA in the hver (where it can enter either the TCA cycle or the pathway for fatty add synthesis). However, most of the acetate that is generated enters the blood and is activated to acetyl CoA in skeletal muscles and other tissues (see Fig. 25.1). Acetate is generally considered nontoxic and is a normal constituent of the diet. 

Metabolism of acetate requires activation to acetyl CoA by acetyl CoA synthetase in a reaction similar to that catalyzed by fatty acyl CoA synthetases (Fig. 25.4). In liver, the principle isoform of acetyl CoA synthetase (ACS I) is a cytosolic enzyme that generates acetyl CoA for the cytosolic pathways of cholesterol and fatty acid synthesis. Acetate entry into these pathways is under regulatory control by mechanisms involving cholesterol or insulin. Thus, most of the acetate generated enters the blood. 

Acetate is an excellent fuel for skeletal muscle. It is treated by the muscle as a very-short-chain fatty acid. It is activated to acetyl CoA in the cytosol and then transferred into the mitochondria via acetylcamitine transferase, an isozyme of carnitine palmitoyl transferase. Sources of acetate include the diet (vinegar is acetic acid) and acetate produced in the liver from alcohol metabolism. Certain commercial power bars for athletes contain acetate. 

An active enzyme fraction was isolated according to H0j and Mikkelsen (1982) with some modifications. Details of plant cultivation, enzyme isolation, and enzyme assay were described before (Focke and Lichtenthaler 1987, Kobek et al. 1988). 6-day-old etiolated barley (var. Alexis) were illuminated for six hours. The 40-70% ammonium sulphate saturation fraction of the chloroplast stroma was dialyzed and concentrated against polyethyleneglycol. Enzymic activity was measured by the incorporation of acetate, acetyl-CoA, malonate, or malonyl-CoA into the fatty acid fraction. Herbicides were added in a... 

Howard (1968b) studied fatty acid synthetic systems in cell-free preparations from squirrel monkey aortas, and the data were similar to those for the rabbit aorta with regard to the mitochondrial system. Acetate or acetyl-CoA was a more efficient precursor than malonyl-CoA, and the Schmidt degradation data indicated that it was primarily an elongation system. The cytosol or HSS was examined, and malonyl-CoA was found to be incorporated into fatty acids 55-200 times more actively than acetyl-CoA, a finding that had been noted previously in liver HSS by Abraham et al. (1962a). Majerus and Lastra (1967) noted that malonyl-CoA was incorporated into fatty acids six or seven times as fast as acetyl-CoA by human leukocytes. The latter authors were unable to find any acetyl-CoA carboxylase activity in leukocytes and reasoned that these cells possess only the fatty acid synthetase. As they pointed out, in the absence of any acetyl-CoA carboxylase, the synthetase alone uses 1 mole of acetyl-CoA plus 7 moles of malonyl-CoA to make 1 mole of palmitate (Wakil... 

We are familiar with several examples of chemical activation as a strategy for group transfer reactions. Acetyl-CoA is an activated form of acetate, biotin and tetrahydrofolate activate one-carbon groups for transfer, and ATP is an activated form of phosphate. Luis Leloir, a biochemist in Argentina, showed in the 1950s that glycogen synthesis depended upon sugar nucleotides, which may be... 

Fatty acids are synthesized by an extramitochondrial system, which is responsible for the complete synthesis of palmitate from acetyl-CoA in the cytosol. In the rat, the pathway is well represented in adipose tissue and liver, whereas in humans adipose tissue may not be an important site, and liver has only low activity. In birds, lipogenesis is confined to the liver, where it is particularly important in providing lipids for egg formation. In most mammals, glucose is the primary substrate for lipogenesis, but in ruminants it is acetate, the main fuel molecule produced by the diet. Critical diseases of the pathway have not been reported in humans. However, inhibition of lipogenesis occurs in type 1 (insulin-de-pendent) diabetes mellitus, and variations in its activity may affect the nature and extent of obesity. 

The activity of carbamoyl phosphate synthase I is determined by A -acetylglutamate, whose steady-state level is dictated by its rate of synthesis from acetyl-CoA and glutamate and its rate of hydrolysis to acetate and glutamate. These reactions are catalyzed by A -acetylglu-tamate synthase and A -acetylglutamate hydrolase, respectively. Major changes in diet can increase the concentrations of individual urea cycle enzymes 10-fold to 20-fold. Starvation, for example, elevates enzyme levels, presumably to cope with the increased production... 

Figure 6.1 Synthesis and metabolism of acetylcholine. Choline is acetylated by reacting with acetyl-CoA in the presence of choline acetyltransferase to form acetylcholine (1). The acetylcholine binds to the anionic site of cholinesterase and reacts with the hydroxy group of serine on the esteratic site of the enzyme (2). The cholinesterase thus becomes acetylated and choline splits off to be taken back into the nerve terminal for further ACh synthesis (3). The acetylated enzyme is then rapidly hydrolised back to its active state with the formation of acetic acid (4)...

Acetyl-CoA metabolized through the TCA cycle yields 3 NADH, 1 FADH2, and 1 GTP—a total of 12 ATP equivalents (3 from each NADH, 2 from each FADH2, and 1 GTP—12 in all). Four O s are used, 1 for each NADH and FADH2. The P in this case is 10 (12 — 2 for the activation of acetate to acetyl-CoA). The P/O = 10/4 = 2.5. P/O ratios for anything else are calculated in the same way. 

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