Acetyl CoA, in fatty acid

Expiain the roie of acetyl CoA in fatty acid metabolism. 

Propionyl-CoA is the key intermediate in the formation of the majority of the abnormal urinary metabolites observed in propionic acidaemia and is also responsible for the accumulation of odd-carbon-number fatty acids and abnormal triglycerides and lipids in the disease by competition with acetyl-CoA in fatty acid biosynthesis. The metabolite may also inhibit other enzyme systems, particularly in mitochondria, giving rise to other symptoms. Inhibition of A -acetylglutamate synthetase has been used to explain the hyper-ammonaemia that is frequently observed in patients with propionic acidaemia (Coude et al., 1979), sometimes occurring as the major presenting biochemical abnormality (Harris et ai, 1980). Inhibition of other enzyme systems and of mitochondrial function by propionyl-CoA may well also be responsible for the occasional occurrence of hypoglycaemia in the diseases. Propionyl-CoA accumulation is also Important in the biochemical and clinical presentation of patients with methylmalonic aciduria, the disease described in the next section (11.2). 

FIGURE 20.23 Export of citrate from mitochondria and cytosolic breakdown produces oxaloacetate and acetyl-CoA. Oxaloacetate is recycled to malate or pyruvate, which re-enters the mitochondria. This cycle provides acetyl-CoA for fatty acid synthesis in the cytosol. 

Citrate is isomerized to isocitrate by the enzyme aconitase (aconitate hydratase) the reaction occurs in two steps dehydration to r-aconitate, some of which remains bound to the enzyme and rehydration to isocitrate. Although citrate is a symmetric molecule, aconitase reacts with citrate asymmetrically, so that the two carbon atoms that are lost in subsequent reactions of the cycle are not those that were added from acetyl-CoA. This asymmetric behavior is due to channeling— transfer of the product of citrate synthase directly onto the active site of aconitase without entering free solution. This provides integration of citric acid cycle activity and the provision of citrate in the cytosol as a source of acetyl-CoA for fatty acid synthesis. The poison fluo-roacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate. 

Fatty acids are degraded by two-carbon units in a reverse manner analogous to their biosynthesis. The acyl-CoAs are first dehydrogenated to a,(3-unsaturated acyl-CoA, and then hydrated to (3-hydroxyacyl-CoA, followed by oxidation to (3-ketoacyl-CoA. The C-C bond between C-2 and C-3 of the latter compound is broken by a free CoA molecule via thiolysis to form an acyl-CoA that is two carbons shorter and acetyl-CoA. Unlike fatty acid biosynthesis, each step of the (3 oxidation of fatty acids is... 

Although the acetyl CoA from fatty acids cannot he converted to glucose, it can be converted to ketone bodies as an alternative fuel for cells, induding the brain. Chronic hypoglycemia is thus often accompanied physiologically by an increase in ketone bodies. 

The tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

Lipid metabolism in the liver is closely linked to the carbohydrate and amino acid metabolism. When there is a good supply of nutrients in the resorptive (wellfed) state (see p. 308), the liver converts glucose via acetyl CoA into fatty acids. The liver can also take up fatty acids from chylomicrons, which are supplied by the intestine, or from fatty acid-albumin complexes (see p. 162). Fatty acids from both sources are converted into fats and phospholipids. Together with apoproteins, they are packed into very-low-density lipoproteins (VLDLs see p.278) and then released into the blood by exocytosis. The VLDLs supply extrahepatic tissue, particularly adipose tissue and muscle. 

Note that the C02 added to pyruvate in the pyruvate carboxylase step is the same molecule that is lost in the PEP carboxykinase reaction (Fig. 14-17). This carboxylation-decarboxylation sequence represents a way of activating pyruvate, in that the decarboxylation of oxaloacetate facilitates PEP formation. In Chapter 21 we shall see how a similar carboxylation-decarboxylation sequence is used to activate acetyl-CoA for fatty acid biosynthesis (see Fig. 21-1). 

Consumption of excess protein There is no physiologic advantage to the consumption of more protein than the RDA. Protein consumed in excess of the body s needs is deaminated, and the resulting carbon skeletons metabolized to provide energy or acetyl CoA for fatty acid synthesis. When excess protein is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, increasing the risk of nephrolithiasis and osteoporosis. 

Eight enzyme-catalyzed reactions are involved in the conversion of acetyl-CoA into fatty acids. The first reaction is catalyzed by acetyl-CoA carboxylase and requires ATP. This is the reaction that supplies the energy that drives the biosynthesis of fatty acids. The properties of acetyl-CoA carboxylase are similar to those of pyruvate carboxylase, which is important in the gluconeogenesis pathway (see chapter 12). Both enzymes contain the coenzyme biotin covalently linked to a lysine residue of the protein via its e-amino group. In the last section of this chapter we show that the activity of acetyl-CoA carboxylase plays an important role in the control of fatty acid biosynthesis in animals. Regulation of the first enzyme in a biosynthetic pathway is a strategy widely used in metabolism. 

The cycle oxidizes pyruvate (formed during the glycolytic breakdown of glucose) to C02 and H20, with the concomitant production of energy. Acetyl CoA from fatty acid breakdown and amino acid degradation products are also oxidized. In addition, the cycle has a role in producing precursors for biosynthetic pathways. 

Acetyl-CoA from fatty acid oxidation enters the TCA cycle in the same way as does acetyl-CoA derived from glucose addition to oxaloacetate to make citrate. This can cause complications if an individual is metabolizing only fat, because the efficient metabolism of fat requires a supply of TCA-cycle intermediates, especially dicar-boxylic acids, which can t (usually) be made from fatty acids. These intermediates must be supplied by the metabolism of carbohydrates, or more often, amino acids derived from muscle tissue. 

For the conversion of pyruvate to oxaloacetate and the formation of citrate in the mitochondrion, see Chap. 12. Acetyl-CoA for fatty acid synthesis is converted to malonyl-CoA this reaction is catalyzed by acetyl-CoA carboxylase. Seven molecules of acetyl-CoA are converted to malonyl-CoA for the synthesis of one molecule of palmitic acid. 

The depletion of NAD+ (and the change to the NADH to NAD ratio) slows the TCA cycle, resulting in a build-up of pyruvate and acetyl-CoA. Excess acetyl-CoA increases fatty acid synthesis and fat deposits in the liver (fatty hver). An accumulation of fat in the liver can be observed after just a single night of heavy drinking. 

Pyruvate carboxylase is also important in lipogenesis. Citrate is transported out of mitochondria and cleaved in the cytosol to provide acetyl CoA for fatty acid synthesis the resultant oxaloacetate is reduced to malate, which undergoes oxidative decarboxylation to pyruvate, a reaction that provides at least half of the NADPH required for fatty acid synthesis. Pyruvate reenters the mitochondria and is carboxylated to oxaloacetate to maintain the process. 

This is a key reaction as it generates the C2 building unit (acetyl-CoA) for fatty acid biosynthesis. Without this enzyme being present, there would be no abundant supply of the acetyl-CoA units and, indeed, many if not all of the nonoleaginous yeasts do not possess this enzyme. The oxaloacetate generated in this cleavage reaction is immediately converted to malate by malate dehydrogenase and then the malate, in turn, is converted to pyruvate by the action of malic enzyme (2). 

In known metabolic states and disorders, the nature of metabolites excreted at abnormal levels has been identified by GC-MS. Examples of this are adipic and suberic acids found in urine from ketotic patients [347], 2-hydroxybutyric acid from patients with lactic acidosis [348], and methylcitric acid (2-hydroxybutan-l,2,3-tricarboxylic acid) [349] in a case of propionic acidemia [350,351]. In the latter instance, the methylcitric acid is thought to be due to the condensation of accumulated propionyl CoA with oxaloacetate [349]. Increased amounts of odd-numbered fatty acids present in the tissues of these patients due to the involvement of the propionyl CoA in fatty acid synthesis, have also been characterised [278]. A deficiency in a-methylacetoacetyl CoA thiolase enzyme in the isoleucine pathway prevents the conversion of a-methylacetoacetyl CoA to propionyl CoA and acetyl CoA [352,353]. The resultant urinary excretion of large amounts of 2-hydroxy-3-methylbutanoic acid (a-methyl-/3-hydroxybutyric acid) and an excess of a-methylacetoacetate and often tiglyl glycine are readily detected and identified by GC-MS. 

The reactions of fatty acid synthesis all take place in the cytosol, but acetyl-CoA is made in the mitochondria and can t cross the inner membrane. The Pyruvate-Malate Cycle (Citrate-Pyruvate Cycle) is used to take acetyl- groups to the cytosol while simultaneously providing a source of NADPH from NADH, and thus, coupling fatty acid synthesis to Glycolysis (Fig. 10.7). Note that the acetyl-CoA is first joined to oxaloacetate to make citrate which is readily transported out of the mitochondria using a co-transporter. The citrate is then cleaved to acetyl-CoA and oxaloacetate, a process requiring ATP to make it favourable (recall the condensation was spontaneous). Acetyl-CoA for fatty acid synthesis is now available in the cytosol, but oxaloacetate must be regenerated for the mitosol. 

The buildup of methylmalonyl-CoA in the cell may lead to reversal of the reaction of propionyl-CoA carboxylase and, as a consequence, an increase in the levels of propionyl-CoA. Increased levels of propionyl-CoA can lead to its use, in place of acetyl-CoA, by fatty acid synthase. Use of the 3-carbon propionyl group, rather than the 2-carbon acetyl group, by this enzyme can result in the production of small amounts of odd-chain fatty acids. These fatty acids contain an odd number of carbons, that is, 15,17, or 19 carbons. The addition of large amounts of propionic add to the diet during B 2 deficiency can be used to artifidally enhance the production of odd-chain fatty adds. 

Carbon dioxide is required for the conversion of acetyl CoA into fatty acids. Yet when carbon dioxide labeled with is used, none of the labeled carbon appears in the fatty acids that are formed. How do you account for these facts ... 

I- oxaloacetate + ADP + Pj. This reaction takes place in the cytoplasm and is a source of acetyl-CoA for fatty acid biosynthesis. 

Biotin is the coenzyme required by enzymes that catalyze carboxylation of a carbon adjacent to a carbonyl group. For example, pymvate carboxylase converts pyruvate—the end product of carbohydrate metabolism—to oxaloacetate, a citric acid cycle intermediate (Figure 25.2). Acetyl-CoA carboxylase converts acetyl-CoA into malonyl-CoA, one of the reactions in the anabolic pathway that converts acetyl-CoA into fatty acids (Section 19.21). Biotin-requiring enzymes use bicarbonate (HCOs ) for the source of the carboxyl group that becomes attached to the substrate. 

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