Tricarboxylic acid cycle pyruvate carboxylation

Alternate fates of pyruvate Compounds other than lactate to which pyruvate can be converted ALTERNATE FATES OF PYRUVATE (p. 103) Pyruvate can be oxidatively decarboxylated by pyruvate dehydrogenase, producing acetyl CoA—a major fuel for the tricarboxylic acid cycle (TCA cycle) and the building block for fatty acid synthesis. Pyruvate can be carboxylated to oxaloacetate (a TCA cycle intermediate) by pyruvate carboxylase. Pyruvate can be reduced by microorganisms to ethanol by pyruvate decarboxylase. 

The second metabolic pathway which we have chosen to describe is the tricarboxylic acid cycle, often referred to as the Krebs cycle. This represents the biochemical hub of intermediary metabolism, not only in the oxidative catabolism of carbohydrates, lipids, and amino acids in aerobic eukaryotes and prokaryotes, but also as a source of numerous biosynthetic precursors. Pyruvate, formed in the cytosol by glycolysis, is transported into the matrix of the mitochondria where it is converted to acetyl CoA by the multi-enzyme complex, pyruvate dehydrogenase. Acetyl CoA is also produced by the mitochondrial S-oxidation of fatty acids and by the oxidative metabolism of a number of amino acids. The first reaction of the cycle (Figure 5.12) involves the condensation of acetyl Co and oxaloacetate to form citrate (1), a Claisen ester condensation. Citrate is then converted to the more easily oxidised secondary alcohol, isocitrate (2), by the iron-sulfur centre of the enzyme aconitase (described in Chapter 13). This reaction involves successive dehydration of citrate, producing enzyme-bound cis-aconitate, followed by rehydration, to give isocitrate. In this reaction, the enzyme distinguishes between the two external carboxyl groups... 

Aerobic glycolysis first involves a ten-step conversion of glucose to pyruvic acid or pyruvate, called the Embden-Meyerhoff-Pamas pathway, followed by its further conversion to carbon dioxide and water via what is variously called the tricarboxylic acid cycle, or citric acid cycle, or Krebs cycle after its discoverer. The net products discharged from the cycle are carbon dioxide and water, with recycle of a further product called oxaloacetic acid or oxaloacetate. Successive organic acids that contain three carboxyl groups (-COOH), are initially involved in the cycle starting with citric acid or a neutral salt of citric acid (citrate). Hence the designator tricarboxylic. 

Since pyruvic acid carboxylation via the malic enzyme is the main source of oxaloacetate, slow glycolysis may result in deceleration of the Krebs cycle. The slowing down of glycolysis may result from reduced enzyme activity or from insufficient amounts of substrate the former possibility has been eliminated by the experiments of Chaikoff and his group. These investigators injected lactate, pyruvate, and acetate, and measured their conversion to CO2. They observed that CO2 formation was the same in diabetics as in nondiabetics. Insufficiency of Krebs cycle substrate is also unlikely, because CO2 production, which is derived mainly from the tricarboxylic acid cycle, is unimpaired in diabetes. In addition to being used for citric acid formation, acetyl CoA is also a key building block for fatty acids. 

When a molecule of glycerol is formed, a molecule of pyruvate cannot be transformed into ethanol following its decarboxylation into ethanal. In anaerobic conditions, oxaloacetate is the means of entry of pyruvate into the cytosolic citric acid cycle. Although the mitochondria are no longer functional, the enzymes of the tricarboxylic acids cycle are present in the cytoplasm. Pyruvate carboxylase (PC) catalyzes the carboxylation of pyruvate into oxaloacetate. The prosthetic group of this enzyme is biotin it serves as a CO2 transporter. The reaction makes use of an ATP molecule ... 

In all aerobic organisms, FA biosynthesis occurs via two different metabolic pathways (1) tricarboxylic acid cycle (TCA) or Krebs cycle and (2) reductive carboxylation pathway. Albert Szent-Gyorgyi (1893-1986, Hxmgary) discovered FA catalysis during his study (including on vitamin C) on cellular combustion process (TCA cycle) for which he was awarded the Noble Prize in Physiology or Medicine in 1937 (www.nobelprize.org). TCA involves CO2 fixation coupled with the conversion of pyruvate to oxaloacetate, the precursor to malate and fuma-rate (Fig. 8.3). Reductive CO2 fixation catalyzed by the enzyme pyruvate carboxylase xmder... 

In tissues other than the RBC, pyruvate has alternative metabolic fates that, depending on the tissue, include gluconeogenesis, conversion to acetyl-CoA by pyruvate dehydrogenase for further metabolism to CO in the tricarboxylic acid (TCA) cycle, transamination to alanine or carboxylation to oxaloacetate by pyruvate carboxylase (Table 23-1). In the RBC, however, the restricted enzymatic endowment precludes all but the conversion to lactate. The pyruvate and lactate produced are end products of RBC glycolysis that are transported out of the RBC to the liver where they can undergo the alternative metabolic conversions described above. 

Figures 4 and 5 illustrates the use of NMR spectroscopy to study the metabolism of (l-i C)glu-cose in primary cultures of neurons and astrocytes. A simplified scheme of the metabolism of (l-i C)glu-cose in neural cells is given in Figure 4. Briefly, (1-i C)glucose is metabolized to (3- C)pyruvate through the Embden Meyerhoff glycolytic pathway. The (3-i C)pyruvate produced can be transaminated to (3- C)alanine, reduced to (3-i C)lactate or enter the tricarboxylic acid (TCA) cycle through the pyruvate dehydrogenase (PDH) or pyruvate carboxylase (PC) activities. A net increase in (3-i C)lactate reveals increased aerobic glycolysis and is normally observed under hypoxic conditions in normal cells. If (3-i C)pyruvate enters the TCA cycle though PDH it produces (2-i C)acetyl-coenzyme A first, and subsequently (4-i C)a-ketoglutarate. In contrast, if (3-i C)pyruvate is carboxylated to (3-i C)oxalacetate by...

Malic acid is mainly produced as an acidulant and taste enhancer in the beverage and food industries. The most preferred metabolic pathway for malate production starts from glucose and proceeds with the carboxylation of pyruvate, followed by the reduction of oxaloacetate to malate. These pathways have been identified in bacterial, yeast, and fungal species (Werpy et al., 2004). In the microalgae reduction of oxaloacetate to malate by NADP malate dehydrogenase (Ouyang et al., 2013 Kuo et al., 2013), the condensation of oxaloacetate and acetyl-coenzyme A (acetyl-CoA) to citric acid is followed by oxidation steps of the tricarboxylic acid (TC A) cycle or glyoxyl-ate shunt to malate (Steinhauser et al., 2012 Pearce et al., 1969 Woodward and Merrett, 1975). 

COs to form oxalacetate which under anaerobic conditions is reduced to malate. The malate in turn may be converted to fumarate and succinate (Fig, 5). The last step in this series of reactions is blocked by malonate. The second pathway involves the aerobic condensation of pyruvate and oxalacetate followed by oxidation of the condensation product to form -ketoglutarate and succinate. Wood has proposed that the first condensation product of the aerobic tricarboxylic cycle is cfs-aconitic acid which is then converted to succinate by way of isocitric, oxalosuccinic, and a-ketoglutaric acids. The a-ketoglutarate is decarboxylated and oxidized to succinic acid. Isotopic a-ketoglutarate containing isotopic carbon only in the carboxyl group located a to the carbonyl would be expected to yield non-isotopic succinate after decarboxylation. This accounts for the absence of isotopic carbon in succinate isolated from malonate-poisoned liver after incubation with pyruvate and isotopic bicarbonate. 

A pathway by which acetate and acetoacetate are oxidized is indicated in Fig. 8. The similarity to the mechanism of oxidation of pyruvate is striking. According to this proposed scheme acetate and acetoacetate are converted to a hypothetical two-carbon intermediate which is capable of condensing with oxalacetate or one of the other four-carbon dicarboxylic acids to yield cis-aconitate or isocitrate. The rest of the cycle is identical with the carbohydrate tricarboxylic cycle. Reference to this cycle illustrates several points of interest. It makes clear the pathway by which labelled carbon in acetate or acetoacetate may be transformed into D-glucose or glycogen. Since oxalacetate labelled in either carboxyl position is generated in the cycle, carboxyl-labelled... 

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