Citric acid cycle decarboxylation

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. 

Succinyl-CoA is converted to succinate by the enzyme succinate thiokinase (succinyl-CoA synthetase). This is the only example in the citric acid cycle of substrate-level phosphorylation. Tissues in which glu-coneogenesis occurs (the hver and kidney) contain two isoenzymes of succinate thiokinase, one specific for GDP and the other for ADP. The GTP formed is used for the decarboxylation of oxaloacetate to phos-phoenolpymvate in gluconeogenesis and provides a regulatory hnk between citric acid cycle activity and the withdrawal of oxaloacetate for gluconeogenesis. Nongluconeogenic tissues have only the isoenzyme that uses ADP. 

The citric acid cycle is the final pathway for the oxidation of carbohydrate, Upid, and protein whose common end-metabolite, acetyl-CoA, reacts with oxaloacetate to form citrate. By a series of dehydrogenations and decarboxylations, citrate is degraded, releasing reduced coenzymes and 2CO2 and regenerating oxaloacetate. 

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. 

Tin metabolic acidosis (p. 652) there is an increase in glutamine processing by the kidneys. Not all the excess NH4 thus produced is released into the bloodstream or converted to urea some is excreted directly into the urine. In the kidney, the NH% forms salts with metabolic acids, facilitating their removal in the urine. Bicarbonate produced by the decarboxylation of a-lcetoglutarate in the citric acid cycle can also serve as a buffer in blood plasma. Taken together, these effects of glutamine metabolism in the kidney tend to counteract acidosis. ... 

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. 

Individuals with either type of diabetes are unable to take up glucose efficiently from the blood recall that insulin triggers the movement of GLUT4 glucose transporters to the plasma membrane of muscle and adipose tissue (see Fig. 12-8). Another characteristic metabolic change in diabetes is excessive but incomplete oxidation of fatty acids in the liver. The acetyl-CoA produced by JS oxidation cannot be completely oxidized by the citric acid cycle, because the high [NADH]/[NAD+] ratio produced by JS oxidation inhibits the cycle (recall that three steps convert NAD+ to NADH). Accumulation of acetyl-CoA leads to overproduction of the ketone bodies acetoacetate and /3-hydroxybutyrate, which cannot be used by extrahepatic tissues as fast as they are made in the liver. In addition to /3-hydroxybutyrate and acetoacetate, the blood of diabetics also contains acetone, which results from the spontaneous decarboxylation of acetoacetate ... 

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) ... 

TPP also mediates the oxidative decarboxylation of a-ketoglutaric acid, another intermediate of carboxydrate metabolism in the citric acid cycle. The nutritional requirement for thiamine increases as dietary carbohydrate increases because of a greater demand for TPP. 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

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. 

---