Carbon dioxide pyruvate conversion

This enzyme [EC 4.1.1.11] catalyzes the conversion of aspartate to j8-alanine and carbon dioxide. Pyruvate is a cofactor for the E. coli enzyme. 

Phosphoenolpyruvate.—Phosphoenolpyruvate carboxytransphosphorylase catalyses two separate conversions of phosphoenolpyruvate (6) (Scheme 3). In the absence of carbon dioxide pyruvate and inorganic pyrophosphate may be formed enzymic dephosphorylation of (6) to enolpyruvate is probably followed by the non-enzymic protonation of the latter giving rise to pyruvate. 

This enzyme [EC 4.1.1.50] catalyzes the pyruvate-dependent conversion of -adenosylmethionine to (5-deoxy-5-adenosyl)(3-aminopropyl)methylsulfonium salt and carbon dioxide. 

The literature concerning malo—lactic fermentation—bacterial conversion of L-malic acid to L-lactic acid and carbon dioxide in wine—is reviewed, and the current concept of its mechanism is presented. The previously accepted mechanism of this reaction was proposed from work performed a number of years ago subsequently, several workers have presented data which tend to discount it. Currently, it is believed that during malo-lactic fermentation, the major portion of malic acid is directly decarboxylated to lactic acid while a small amount of pyruvic acid (and reduced coenzyme) is formed as an end product, rather than as an intermediate. It is suspected that this small amount of pyruvic acid has extremely important consequences on the intermediary metabolism of the bacteria. 

The possibility now arises that if, in fact, there is an intermediate involved in the conversion of malic acid to lactic acid, the cell may, in some way, be capable of deriving energy from it. In 1950, Korkes et ah (14), working with the malo-lactic system of Lactobacillus plantarum, demonstrated the production of a very small amount of pyruvic acid (0.2% ) from malic acid. However, 98% of the malic acid was recovered in lactic acid, and the recovery of carbon dioxide was consistent with conversion of 90% of the malic acid. The pyruvic acid recovery was attributed to spillage from the enzyme surface. We will see below that a small amount of pyruvic acid and NADH are indeed produced during... 

The reasons for the confusion surrounding the mechanism of the malo-lactic fermentation are now apparent. In the malate system from Lactobaccillus plantarum, Korkes et al. (14) demonstrated carbon dioxide and lactic acid production from malic acid, but they were unable to show a large amount of pyruvic acid production. However, the cofactor requirement for the system indicated the need for an intermediate between malic acid and lactic acid, and pyruvic acid was the logical choice. At this time, the occurrence of enzymes requiring NAD in a function other than reduction-oxidation was not realized, so it was logical to conclude that the malic acid to lactic acid conversion involved a redox reaction. The later information, however, indicates that this is probably not the case. 

Summary diagram of the breakdown of glucose to carbon dioxide and water in a eukaryotic cell. As depicted here, the process starts with the absorption of glucose at the plasma membrane and its conversion into glucose-6-phosphate. In the cytosol, this six-carbon compound is then broken down by a sequence of enzyme-catalyzed reactions into two molecules of the three-carbon compound pyruvate. After absorption by the mitochondrion, pyruvate is broken down to carbon dioxide and water by a sequence of reactions that requires molecular oxygen. 

Catabolism of amino acids usually entails their conversion to intermediates in the central metabolic pathways. All amino acids can be degraded to carbon dioxide and water by appropriate enzyme systems. In every case, the pathways involve the formation, directly or indirectly, of a dicarboxylic acid intermediate of the tricarboxylic acid cycle, of pyruvate, or of acetyl-CoA (fig. 22.11). 

Proteinoids accelerate the conversion of pyruvic acid to acetic acid and carbon dioxide. In a typical experiment, a mixture of 20 mg of proteinoid and 0.4 mg of radioactive pyruvic acid in 20 ml of 0.2 NTris buffer, pH 8.3, is incubated at 37.5 °C for 1, 2 or 3 days. The evolved 14C02 is absorbed in NaOH solution, and acetic acid is identified by paper chromatography and by preparing a derivative of acetic acid, namely p-bromophenacyl acetate 20). 

Cellular respiration, which occurs in the presence of molecular oxygen, 02, and involves the conversion of pyruvate to carbon dioxide, C02, with the release of relatively large amounts of energy by way of intermediate chemical species... 

In this way Solomon and coworkers attempted to explain the conversion of lactate, pyruvate and carbon dioxide to glycogen. According to the proposed mechanism, carboxyl-labelled pyruvate must first undergo the Wood-Werkman reaction to form oxalacetate before carboxyl-labelled phosphopyruvate can be formed by way of fumarate, phosphomalate and phospho-oxalacetate. This hypothesis is no longer necessary in view of Lardy and Ziegler s results described above. It is possible, however, that both pathways are utilized. 

FIGURE 8.10 Positions of entry of amino acid carbon skeletons into the Krebs cycle. Amino acids that are broken down to three-carbon skeletons may enter the Krebs cycle at the point of pyruvate. Glycine, after conversion to serine, can enter the Krebs cycle as pyruvate. Alternatively, glycine can be broken down by the glycine cleavage system (see Folate section). The products of this reaction are carbon dioxide and an ammonium ion. Five-carbon skeletons enter the Krebs cycle at the point of a-ketoglutarate, whereas four-carbon skeletons enter at the points of succinyl-CoA and oxaloacetic acid. 

TCA cycle. (tricarboxylic acid cycle Krebs cycle citric acid cycle). A series of enzymatic reactions occurring in living cells of aerobic organisms, the net result of which is the conversion of pyruvic acid, formed by anaerobic metabolism of carbohydrates, into carbon dioxide and water. The metabolic intermediates are degraded by a combination of decarboxylation and dehydrogenation. It is the major terminal pathway of oxidation in animal, bacterial, and plant cells. Recent research indicates that the TCA cycle may have predated life on earth and may have provided the pathway for formation of amino acids. 

The sequence of reactions from glucose to pyruvate is similar in most organisms and most types of cells. In contrast, the fate of pyruvate is variable. Three reactions of pyruvate are of primary importance conversion into ethanol, lactate, or carbon dioxide (Figure 16.9). The first two reactions are fermentations that take place in the absence of oxygen. In the presence of... 

Thiamin pyrophosphate acts as a coenzyme in several biochemical processes and, in each case, its mode of action depends on the intermediacy of a C-2-deprotonated species - an ylide (24.1.2.1 and 24.10). For example, in the later stages of alcoholic fermentation, which converts glucose into ethanol and carbon dioxide, the enzyme pyravate decarboxylase catalyses the conversion of pyruvate into ethanal... 

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. 

As for human cells or enkaryotes, the metabolic path or seqnence involves the conversion of glucose by what is labeled glycolysis. In a series of ten steps there is first produced a compound called pyruvic acid or pyruvate (the latter designating a so-called salt or componnd of pyruvic acid). In the presence of oxygen, this is fnrther converted to carbon dioxide and water in what is variously called the tricarboxylic acid cycle, or citric acid cycle, or Krebs cycle. The overall operation may be conveniently called aerobic glycolysis, with each step requiring a particnlar enzyme, and most usually requiring snpportive reactions. 

What enzymes catalyze the conversion of pyruvate to ethanol and carbon dioxide ... 

Pyruvate dehydrogenase complex System of proteins responsibie for the conversion of pyruvate into acetyi-CoA and carbon dioxide. 

While this book is not the appropriate place for a detailed discussion of the ADP-ATP cycle, perhaps a bit more mechanistic detail is useful. The aerobic metabolism of glucose to produce ATP from ADP can be-considered to consist of three parts the fermentation of glucose to form pyruvate (and a small amount of ATP from ADP), the conversion of the pyruvate to carbon dioxide in the citric acid cycle in which NAD" " and FAD (the oxidized form of flavin adenine dinucleotide) are converted to NADH and FADH2, and their oxidation in the respiratory chain, resulting in the formation of a larger quantity of ATP from ADP. If oxygen is not available, so the reaction is anaerobic, only the first step, formation of pyruvate, occurs with a small amount of ATP production. 

In animals TPP-dependent decarboxylation reactions are essential to the production of energy needed for cell metahohsm. In these reactions a-ketoacids are converted to acyl CoA molecules and carbon dioxide. The reactions (e.g., the conversion of pyruvate to acetyl CoA) are an important part of the breakdown of carbohydrates, and of the conversion of several classes of molecules (carbohydrates, fats, and proteins) to energy, carbon dioxide, and water in the citric acid cycle. In other organisms, in addition to its participation in the above reactions, TPP is a required coenzyme in alcohol fermentation, in the carbon fixation reactions of photosynthesis, and in the hiosynthesis of the amino acids leucine and valine. 

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