Pyruvate dehydrogenase tricarboxylic cycle

The intermediary metabolism has multienzyme complexes which, in a complex reaction, catalyze the oxidative decarboxylation of 2-oxoacids and the transfer to coenzyme A of the acyl residue produced. NAD" acts as the electron acceptor. In addition, thiamine diphosphate, lipoamide, and FAD are also involved in the reaction. The oxoacid dehydrogenases include a) the pyruvate dehydrogenase complex (PDH, pyruvate acetyl CoA), b) the 2-oxoglutarate dehydrogenase complex of the tricarboxylic acid cycle (ODH, 2-oxoglutarate succinyl CoA), and c) the branched chain dehydrogenase complex, which is involved in the catabolism of valine, leucine, and isoleucine (see p. 414). 

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. 

Mitochondria are also described as being the cell s biochemical powerhouse, since—through oxidative phosphorylation (see p. 112)—they produce the majority of cellular ATP. Pyruvate dehydrogenase (PDH), the tricarboxylic acid cycle, p-oxidation of fatty acids, and parts of the urea cycle are located in the matrix. The respiratory chain, ATP synthesis, and enzymes involved in heme biosynthesis (see p. 192) are associated with the inner membrane. 

Oxidative decarboxylation of pyruvate by pyruvate dehydrogenase complex is an important pathway in tissues with a high oxidative capacity, such as cardiac muscle (Figure 8.24). Pyruvate dehydrogenase irreversibly converts pyruvate, the end product of glycolysis, into acetyl CoA, a major fuel for the tricarboxylic acid cycle (see p. 107) and the building block for fatty acid synthesis (see p. 181). 

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. 

Reactions of the TCA cycle Enzyme that oxidatively decarboxylates pyruvate, its coenzymes, activators, and inhibitors REACTIONS OF THE TRICARBOXYLIC ACID CYCLE (p. 107) Pyruvate is oxidatively decarboxylated by pyruvate dehydrogenase complex producing acetyl CoA, which is the major fuel for the tricarboxylic acid cycle (TCA cycle). The irreversible set of reactions catalyzed by this enzyme complex requires five coenzymes thiamine pyrophosphate, lipoic acid, coenzyme A (which contains the vitamin pantothenic acid), FAD, and NAD. The reaction is activated by NAD, coenzyme A, and pyruvate, and inhibited by ATP, acetyl CoA, and NADH. 

Figure 7-1. Pathways of fuel metabolism and oxidative phosphorylation. Pyruvate may be reduced to lactate in the cytoplasm or may be transported into the mitochondria for anabolic reactions, such as gluconeogenesis, or for oxidation to acetyl-CoA by the pyruvate dehydrogenase complex (PDC). Long-chain fatty acids are transported into mitochondria, where they undergo [ -oxidation to ketone bodies (liver) or to acetyl-CoA (liver and other tissues). Reducing equivalents (NADH, FADII2) are generated by reactions catalyzed by the PDC and the tricarboxylic acid (TCA) cycle and donate electrons (e ) that enter the respiratory chain at NADH ubiquinone oxidoreductase (Complex 0 or at succinate ubiquinone oxidoreductase (Complex ID- Cytochrome c oxidase (Complex IV) catalyzes the reduction of molecular oxygen to water, and ATP synthase (Complex V) generates ATP fromADP Reprinted with permission from Stacpoole et al. (1997).

TDP-dependent enzymes include transketolase, an enzyme component of the pentose shunt pathway, pyruvate dehydrogenase complex, and aKGDH a tricarboxylic acid cycle enzyme (Fig. 3). Branched-chain ketoacid dehydrogenases are also TDP-dependent. 

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. 

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... 

The mechanism of MCA toxicity seems to be via inhibition of the enzyme pyruvate dehydrogenase this inhibition blocks the Krebs (tricarboxylic acid) cycle and disrupts the cell s energy supply. Almost immediately, the cell finds itself without energy. Ketoglutarate dehydrogenase activity is also reduced, which causes lactic acidosis. The MCA also damages the blood-brain barrier, probably through the formation of vascular endothelial microlesions. 

Similarly, amino acids that produce tricarboxylic acid (TCA) cycle intermediates (Chapter 15) produce alanine by conversion to oxaloacetate. During starvation or intake of a carbohydrate-poor diet, conversion of pyruvate to alanine is preferred because pyruvate dehydrogenase is inactivated by oxidation of fatty acids and ketone bodies (Chapters 13 and 18). 

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. 

Both enzymes mentioned previously pertain to the tricarboxylic acid cycle proper, and are apparently vital to sustaining respiration. Accordingly, inhibitors can lead to undesirable and sometimes life-threatening consequences. Enzyme inhibitors for pyruvate dehydrogenase are more commonly known, those for a-ketoglutarate dehydrogenase less so. In the Jain reference respiration inhibitors are treated as a special category. 

Fig. 1 Oxidative metabolism and energy production by mitochondria. The oxidation of pyruvate and free fatty acids (FFA) inside mitochondria produces NADH and FADH2, which transfer their electrons to the mitochondrial respiratory chain. The flow of electrons in mitochondrial complexes I, III, and IV is coupled with the extrusion of protons from the mitochondrial matrix into the intermembrane space. When energy is needed, these protons reenter the matrix through ATP synthase, to generate ATP from ADP. The adenine nucleotide translocator (ANT) then exchanges the formed ATP for cytosolic ADP. G-6-P Glucose 6-phosphate, PDH pyruvate dehydrogenase, LCFA-CoA long-chain fatty acyl-CoA, CPTI carnitine palmitoyltransferase I, TCA cycle tricarboxylic acid cycle, c cytochrome c...

The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is the final oxidative pathway for carbohydrates, lipids, and amino acids. It is also a source of precursors for biosynthesis. The authors begin Chapter 17 with a detailed discussion of the reaction mechanisms of the pyruvate dehydrogenase complex, followed by a description of the reactions of the citric acid cycle. This description includes details of mechanism and stereospecificity of some of the reactions, and homologies of the enzymes to other proteins. In the following sections, they describe the stoichiometry of the pathway including the energy yield (ATP and GTP) and then describe control mechanisms. They conclude the chapter with a summary of the biosynthetic roles of the citric acid cycle and its relationship to the glyoxylate cycle found in bacteria and plants. 

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