Citric acid cycle, reactions decarboxylation

Since citric acid cycle intermediates are quickly labeled it is assumed that much subsequent metabolism of malate is by citric acid cycle reactions. Of course, the malate itself could enter the mitochondria, and no doubt does (Lips and Beevers, 1966 a, b MacLennan et al., 1963). Pyruvate and/or P-enolpyruvate resulting from decarboxylation probably enter the citric acid cycle via acetate. Bradbeer and Ranson (1963) found that " C-pyruvate supplied to Kalanchoe leaves in the dark was consumed by the citric acid cycle. Some was also found as free sugars. Hence, there is a slow equilibration of malate and malate degradation products with other metabolic events in the cell. Measurements of oxygen consumption during CAM metabolism can leave no doubt that the citric acid cycle is active in succulents (e.g., Szarek and Ting, 1974b Kinraide and Behan, 1975 Kaplan et al., 1976 a, b 1977). 

Step 2 of Figure 29.13 Decarboxylation and Phosphorylation Decarboxylation of oxaloacetate, a jB-keto acid, occurs by the typical retro-aldol mechanism like that in step 3 in the citric acid cycle (Figure 29.12), and phosphorylation of the resultant pyruvate enolate ion by GTP occurs concurrently to give phosphoenol-pyruvate. The reaction is catalyzed by phosphoenolpyruvate carboxykinase. 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

Pyridoxal phosphate is a required coenzyme for many enzyme-catalyzed reactions. Most of these reactions are associated with the metabolism of amino acids, including the decarboxylation reactions involved in the synthesis of the neurotransmitters dopamine and serotonin. In addition, pyridoxal phosphate is required for a key step in the synthesis of porphyrins, including the heme group that is an essential player in the transport of molecular oxygen by hemoglobin. Finally, pyridoxal phosphate-dependent reactions link amino acid metabolism to the citric acid cycle (chapter 16). 

FIGURE 16-13 Products of one turn of the citric acid cycle. At each turn of the cycle, three NADH, one FADH2/ one GTP (or ATP), and two C02 are released in oxidative decarboxylation reactions. Here and in several following figures, all cycle reactions are shown as proceeding in one direction only, but keep in mind that most of the reactions are reversible (see Fig. 16-7). 

In seven sequential reactions, including two decarboxylations, the citric acid cycle converts citrate to oxaloacetate and releases two C02. The pathway is cyclic in that the intermediates of the cycle are not used up for each oxaloacetate consumed in the path, one is produced. 

We have now seen how the 20 common amino acids, after losing their nitrogen atoms, are degraded by dehydrogenation, decarboxylation, and other reactions to yield portions of their carbon backbones in the form of six central metabolites that can enter the citric acid cycle. Those portions degraded to acetyl-CoA are completely oxidized to carbon dioxide and water, with generation of ATP by oxidative phosphorylation. 

One of the first persons to study the oxidation of organic compounds by animal tissues was T. Thunberg, who between 1911 and 1920 discovered about 40 organic compounds that could be oxidized by animal tissues. Salts of succinate, fumarate, malate, and citrate were oxidized the fastest. Well aware of Knoop s (3 oxidation theory, Thunberg proposed a cyclic mechanism for oxidation of acetate. Two molecules of this two-carbon compound were supposed to condense (with reduction) to succinate, which was then oxidized as in the citric acid cycle to oxaloacetate. The latter was decarboxylated to pyruvate, which was oxidatively decarboxylated to acetate to complete the cycle. One of the reactions essential for this cycle could not be verified experimentally. It is left to the reader to recognize which one. 

As mentioned in Section 4, glyoxylate can be converted to oxaloacetate by condensation with acetyl-CoA (Fig. 17-16) and the oxaloacetate can be decarboxylated to pyruvate. This sequence of reactions resembles that of the conversion of oxaloacetate to 2-oxoglutarate in the citric acid cycle (Fig. 17-4). Doth... 

It may be protested that the reaction of the citric acid cycle by which oxaloacetate is converted to oxo-glutarate does not follow exactly the pattern of Fig. 17-18. The carbon dioxide removed in the decarboxylation step does not come from the part of the molecule donated by the acetyl group but from that formed from oxaloacetate. However, the end result is the same. Furthermore, there are two known citrate-forming enzymes with different stereospecificities (Chapter 13), one of which leads to a biosynthetic pathway strictly according to the sequence of Fig. 17-18. 

Vitamin Influences. The involvement of NAD and NADP in many carbohydrate reactions explains the importance of nicotinamide in carbohydrate melaholism. Thiamine, in the form or thiamine pyrophosphate (cocarboxylase), is the cofaclor necessary in the decarboxylation of pyruvic acid, in the iraru-kelolase-calalyzed reactions of the pentose phosphaie cycle, and in the decarboxylation of alpha-keloglutaric acid in the citric acid cycle, among other reactions. Biotin is a hound cofaclor in the fixation of carbon dioxide to form nxalacetic acid from pyruvic acid. Pantothenic acid is a part of the C oA molecule. There are separate alphabetical entries in this volume on the various specific vitamins as well as a review entry on Vitamin. 

TPP functions as a coenzyme which participates in decarboxylation of or-keto acids. Dehydrogenation and decarboxylation must precede the formation of active acetate in the initial reaction of the TCA cycle (citric acid cycle) ... 

Following this route under aerobic conditions, pyruvate is converted to acetyl CoA by the enzyme pyruvate dehydrogenase and the acetyl CoA then enters the citric acid cycle. The pyruvate dehydrogenase reaction is an oxidative decarboxylation (see Topic LI for details) ... 

These reactions produce two important intermediate compounds, succinate and malate (which is converted into oxaloacetate). The two decarboxylation steps of the citric acid cycle are bypassed, and so there is no oxidation of acetyl-CoA to C02. Two molecules of acetyl-CoA are used, but all the carbon atoms are retained. 

In the citric acid cycle, how many steps involve (a) oxidation-reduction, (6) hydration-dehydration, (c) substrate-level phosphorylation, and (d) decarboxylation List the enzymes responsible for these reactions. 

The formation of acetyl-CoA from pyruvate in animals is via the pyruvate dehydrogenase complex, which catalyzes the irreversible decarboxylation reaction. Carbohydrate is synthesized from oxaloacetate, which in turn is synthesized from pyruvate via pyruvate carboxylase. Since the pyruvate dehydrogenase reaction is irreversible, acetyl-CoA cannot be converted to pyruvate, and hence animals cannot realize a net gain of carbohydrate from acetyl-CoA. Because plants have a glyoxylate cycle and animals do not, plants synthesize one molecule of succinate and one molecule of malate from two molecules of acetyl-CoA and one of oxaloacetate. The malate is converted to oxaloacetate, which reacts with another molecule of acetyl-CoA and thereby continues the reactions of the glyoxylate cycle. The succinate is also converted to oxaloacetate via the enzymes of the citric acid cycle. Thus, one molecule of oxaloacetate is diverted to carbohydrate synthesis and, therefore, plants are able to achieve net synthesis of carbohydrate from acetyl-CoA. 

Reaction 4 is, again, a component of the citric acid cycle. This isomerization prepares the molecule for subsequent oxidation and decarboxylation by moving the hydroxyl group of citrate from a tertiary to a secondary position. 

Under aerobic conditions, the pyruvate generated from glucose is oxidatively decarboxylated to form acetyl CoA. In eukaryotes, the reactions of the citric acid cycle take place inside mitochondria, in contrast with those of glycolysis, which take place in the cytosol (Figure 171). 

Figure 17.1. Mitochondrion. The double membrane of the mitochondrion is evident in this electron micrograph. The numerous invaginations of the inner mitochondrial membrane are called cristae. The oxidative decarboxylation of pyruvate and the sequence of reactions in the citric acid cycle take place within the matrix. [(Left) Omikron/Photo Researchers.]...

This irreversible reaction is the link between glycolysis and the citric acid cycle. (Figure 17.4) Note that, in the preparation of the glucose derivative pyruvate for the citric acid cycle, an oxidative decarboxylation takes place and high-transfer-potential electrons in the form of NADH are captured. Thus, the pyruvate dehydrogenase reaction has many of the key features of the reactions of the citric acid cycle itself... 

We come now to the first of four oxidation-reduction reactions in the citric acid cycle. The oxidative decarboxylation of isocitrate is catalyzed by isocitrate dehydrogenase. 

Many bacteria and plants are able to subsist on acetate or other compounds that yield acetyl CoA. They make use of a metabolic pathway absent in most other organisms that converts two-carbon acetyl units into four-carbon units (succinate) for energy production and biosyntheses. This reaction sequence, called the glyoxylate cycle, bypasses the two decarboxylation steps of the citric acid cycle. Another key difference is that two molecules of acetyl CoA enter per turn of the glyoxylate cycle, compared with one in the citric acid cycle. 

The glyoxylate cycle enhances the metabolic versatility of many plants and bacteria. This cycle, which uses some of the reactions of the citric acid cycle, enables these organisms to subsist on acetate because it bypasses the two decarboxylation steps of the citric acid cycle. 

Recurring decarboxylations. Which reaction in the citric acid cycle is most analogous to the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate What kind of enzyme-bound intermediate is formed in both reactions ... 

The degradation of the hranched-chain amino acids employs reactions that we have encountered previously in the citric acid cycle and fatty acid oxidation. Leucine is transaminated to the corresponding a-ketoacid, a-ketoisocaproate. This a-ketoacid is oxidatively decarboxylated to isovaleryl CoA by the branched-chain a-ketoacid dehydrogenase complex. 

A fourth fate of pyruvate is its oxidative decarboxylation to acetyl CoA. This irreversible reaction inside mitochondria is a decisive reaction in metabolism it commits the carbon atoms of carbohydrates and amino acids to oxidation by the citric acid cycle or to the synthesis of lipids. The pyruvate dehydrogenase complex, which catalyzes this irreversible funneling, is stringently regulated by multiple allosteric interactions and covalent modifications. Pyruvate is rapidly converted into acetyl CoA only if ATP is needed or if two-carbon fragments are required for the synthesis of lipids. 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

Under aerobic conditions, pyruvate can be oxidatively decarboxylated via the pyruvate dehydrogenase multienzyme complex to yield acetyl-CoA, which can then be completely oxidised via the citric acid cycle (Fig. 2). In eubacteria growing anaerobically, pyruvate is metabolised fermentatively, thus serving as an electron sink for reducing equivalents generated in its formation from glucose. The diverse array of possible fermentative reactions from pyruvate is reviewed in [5]. 

Thiamine is required by the body as the pyrophosphate (TPP) in two general types of reaction, the oxidative decarboxylation of a keto acids catalyzed by dehydrogenase complexes and the formation of a-ketols (ketoses) as catalyzed by transketolase, and as the triphosphate (TTP) within the nervous system. TPP functions as the Mg -coordinated coenzyme for so-called active aldehyde transfers in mul-tienzyme dehydrogenase complexes that affect decarboxyia-tive conversion of a-keto (2 oxo) acids to acyl-coenzyme A (acyl-CoA) derivatives, such as pyruvate dehydrogenase and a-ketoglutarate dehydrogenase. These are often localized in the mitochondria, where efficient use in the Krebs tricarboxylic acid (citric acid) cycle follows. 

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