Pyruvate to acetyl-CoA

Pyruvate produced by glycolysis is a significant source of acetyl-CoA for the TCA cycle. Because, in eukaryotic ceils, glycolysis occurs in the cytoplasm, whereas the TCA cycle reactions and ail subsequent steps of aerobic metabolism take place in the mitochondria, pyruvate must first enter the mitochondria to enter the TCA cycle. The oxidative decarboxylation of pyruvate to acetyl-CoA,... 

The pyruvate dehydrogenase complex (PDC) is a noncovalent assembly of three different enzymes operating in concert to catalyze successive steps in the conversion of pyruvate to acetyl-CoA. The active sites of ail three enzymes are not far removed from one another, and the product of the first enzyme is passed directly to the second enzyme and so on, without diffusion of substrates and products through the solution. The overall reaction (see A Deeper Look Reaction Mechanism of the Pyruvate Dehydrogenase Complex ) involves a total of five coenzymes thiamine pyrophosphate, coenzyme A, lipoic acid, NAD+, and FAD. 

Stepl of Figure 29.11 Addition of Thiamin Diphosphate The conversion of pyruvate to acetyl CoA begins by reaction of pyruvate with thiamin diphosphate, a derivative of vitamin B(. Formerly called thiamin pyrophosphate, thiamin diphosphate is usually abbreviated as TPP. The spelling thiamine is also correct and frequently used. 

Figure 29.11 MECHANISM Mechanism of the conversion of pyruvate to acetyl CoA through a multistep sequence of reactions that requires three different enzymes and four different coenzymes. The individual steps are explained in the text.

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of cr-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an -keto acid loses C02 and is oxidized to a thioester in a series of steps catalyzed by a multienzynie dehydrogenase complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate vlide, followed by decarboxylation, reaction with lipoamide, elimination of TPP vlide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

Glucose is metabolized to pyruvate by the pathway of glycolysis, which can occur anaerobically (in the absence of oxygen), when the end product is lactate. Aerobic tissues metabolize pyruvate to acetyl-CoA, which can enter the citric acid cycle for complete oxidation to CO2 and HjO, linked to the formation of ATP in the process of oxidative phosphorylation (Figure 16-2). Glucose is the major fuel of most tissues. 

THE OXIDATION OF PYRUVATE TO ACETYL-CoA IS THE IRREVERSIBLE ROUTE FROM GLYCOLYSIS TO THE CITRIC ACID CYCLE... 

The PDHC catalyzes the irreversible conversion of pyruvate to acetyl-CoA (Fig. 42-3) and is dependent on thiamine and lipoic acid as cofactors (see Ch. 35). The complex has five enzymes three subserving a catalytic function and two subserving a regulatory role. The catalytic components include PDH, El dihydrolipoyl trans-acetylase, E2 and dihydrolipoyl dehydrogenase, E3. The two regulatory enzymes include PDH-specific kinase and phospho-PDH-specific phosphatase. The multienzyme complex contains nine protein subunits, including... 

Figure 7-1. Conversion of pyruvate to acetyl CoA by the pyruvate dehydrogenase complex. The three enzymes, pyruvate dehydrogenase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase, exist in a complex associated with the mitochondrial matrix. Each enzyme requires at least one coenzyme that participates in the reaction. TPP, thiamine pyrophosphate Lip, lipoic acid CoA, coenzyme A.

Pyruvate dehydrogenase complex (type L) GYLRRASV Pyruvate to acetyl-CoA... 

The overall rate of the citric acid cycle is controlled by the rate of conversion of pyruvate to acetyl-CoA and by the flux through citrate synthase, isocitrate dehydrogenase, and a-lcetoglutarate dehydrogenase. These fluxes are largely determined by the concentrations of substrates and products the end products ATP and NADH are inhibitory, and the substrates NAD+ and ADP are stimulatory. 

Vertebrates cannot convert fatty acids, or the acetate derived from them, to carbohydrates. Conversion of phosphoenolpyruvate to pyruvate (p. 532) and of pyruvate to acetyl-CoA (Fig. 16-2) are so exergonic as to be essentially irreversible. If a cell cannot convert acetate into phosphoenolpyruvate, acetate cannot serve as the starting material for the gluconeogenic pathway, which leads from phosphoenolpyruvate to glucose (see Fig. 15-15). Without this capacity, then, a cell or organism is unable to convert fuels or metabolites that are degraded to acetate (fatty acids and certain amino acids) into carbohydrates. 

Regulation of the Pyruvate Dehydrogenase Complex In animal tissues, the rate of conversion of pyruvate to acetyl-CoA is regulated by the ratio of active, phosphory-lated to inactive, unphosphorylated PDH complex. Determine what happens to the rate of this reaction when a preparation of rabbit muscle mitochondria containing the PDH complex is treated with (a) pyruvate dehydrogenase kinase, ATP, and NADH (b) pyruvate dehydrogenase phosphatase and Ca2+ (c) malonate. 

Common Reaction Steps in the Fatty Acid Oxidation Cycle and Citric Acid Cycle Cells often use the same enzyme reaction pattern for analogous metabolic conversions. For example, the steps in the oxidation of pyruvate to acetyl-CoA and of a-ketoglutarate to succinyl-CoA, although catalyzed by different enzymes, are very similar. The first stage of fatty acid oxidation follows a reaction sequence closely resembling a sequence in the citric acid cycle. Use equations to show the analogous reaction sequences in the two pathways. 

Insulin also stimulates the storage of excess fuel as fat (Fig. 23-26). In the liver, insulin activates both the oxidation of glucose 6-phosphate to pyruvate via glycolysis and the oxidation of pyruvate to acetyl-CoA. If not oxidized further for energy production, this acetyl-CoA is used for fatty acid synthesis in the liver, and the fatty acids are exported as the TAGs of plasma lipoproteins (VLDLs) to the adipose tissue. Insulin stimulates TAG synthesis in adipocytes, from fatty acids released... 

The conversion of pyruvate to acetyl CoA and C02 A. is reversible. B. involves the participation of lipoic acid. C. is activated when pyruvate dehydrogenase complex is phosphorylated by a protein kinase in the pres ence of ATP. D. occurs in the cytosol. E. depends on the coenzyme biotin. Correct answer = B. Lipoic acid is an intermedi ate acceptor of the acetyl group formed in the reaction. Pyruvate dehydrogenase complex cat alyzes an irreversible reaction that is inhibited when the enzyme is phosphorylated. The enzyme is located in the mitochondrial matrix. 

Vitamin B1 (thiamine) has the active form, thiamine pyrophosphate. It is a cofactor of enzymes catalyzing the conversion of pyruvate to acetyl CoA, a-ketoglutarate to succinyl CoA, and the transketolase reactions in the pentose phosphate pathway. A deficiency of thiamine causes beriberi, with symptoms of tachycardia, vomiting, and convulsions. In Wernicke-Korsakoff syndrome (most common in alcoholics), individuals suffer from apa thy, loss of memory, and eye movements. There is no known toxicity for this vitamin. 

Oxidative decarboxylations of a-keto acids are mediated by either enzymes having more than one cofactor or complex multienzyme systems utilizing a number of cofactors. For example, pyruvate oxidase uses TPP and FAD as coenzymes, the function of the latter being to oxidize the intermediate (41). Conversion of pyruvate to acetyl-CoA requires a multienzyme complex with the involvement of no less than five coenzymes, TPP, CoA, dihydrolipoate, FAD and NAD+ (74ACR40). 

Pyruvate formate-lyase reaction. Anaerobic cleavage of pyruvate to acetyl-CoA and formate (Eq. 15-37) is essential to the energy economy of many cells, including those of E. coli. No external oxidant is needed, and the reaction does not require lipoic acid. 

The reductive carboxylation of acetyl-CoA to pyruvate (Eq. 17-47) occurs only in a few types of bacteria. For most species, from microorganisms to animals, the oxidative decarboxylation of pyruvate to acetyl-CoA is irreversible. This fact has many important consequences. For example, carbohydrate... 

The conversion of pyruvate to acetyl-CoA. The reactions are catalyzed by the enzymes of the pyruvate dehydrogenase complex. This complex has three enzymes pyruvate decarboxylase, dihydrolipoyl transacetylase, and dihydrolipoyl dehydrogenase. In addition, five coenzymes are required thiamine pyrophosphate, lipoic acid, CoASH, FAD, and NAD+. Lipoic acid is covalently attached to... 

At this point in the oxidation of glucose, four electrons per glucose molecule have been lost in the oxidation of glyceraldehyde-3-phosphate and four more in the conversion of pyruvate to acetyl-CoA. Thus, of the total of 24 electrons lost in the oxidation of glucose to C02, 16 remain to be transferred to oxidizing agents in the course of the oxidation of two molecules of acetyl-CoA. A major func-... 

The following sequence will do the task a-ketoglutarate, the tricarboxylic acid cycle to oxaloacetate, to phosphoenolpyruvate, to pyruvate, to acetyl-CoA, into the tricarboxylic acid cycle. 

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