Chapter 13

Which of the following is NOT a mechanism for controlling the pyruvate dehydrogenase complex oxidation of pyruvate? Acetyl-CoA competitively inhibits E2 of the complex. The E1 component of the complex is subject to covalent modification that alters its function. NADH competitively inhibits E3 of the complex. Acetyl-CoA competitively inhibits E1 of the complex.

Acetyl-CoA competitively inhibits E1 of the complex. E1 is not subject to feedback inhibition mechanisms but rather is subject to covalent modification

One substrate level phosphorylation occurs in the citric acid cycle in the reaction catalyzed by: succinyl CoA synthetase. α-ketoglutarate dehydrogenase. isocitrate dehydrogenase. succinate dehydrogenase. citrate synthase.

succinyl CoA synthetase.

Arrange the sequence of events for the conversion of succinyl-CoA to succinate and ATP (or GTP) in the correct order: 1) The phosphate group is transferred from a histidine residue of the phosphorylated enzyme intermediate to the nucleotide. 2) Inorganic phosphate reacts at the thioester of succinyl-CoA to yield a mixed phosphoanhydride. 3) A His residue from the protein is phosphorylated by succinyl phosphate. 3-1-2 1-2-3 2-3-1 3-2-1

2-3-1

Arrange the sequence of reactions that occur in the conversion of pyruvate to acetyl-CoA in the correct order: 1) The anion of the hydroxyethyl group attacks one sulfur of the disulfide in lipoic acid, and the resulting intermediate is oxidized to an acetyl thioester. 2) FAD is reduced to FADH2, while the two thiols of reduced lipoic acid are reoxidized back to a disulfide. 3) FADH2 is oxidized by NAD+. 4) Thiamine pyrophosphate decarboxylates pyruvate to yield hydroxyethyl-TPP. 5) The thioester derived from lipoic acid transfers its acetyl group to CoA. 4-1-3-2-5 5-3-1-2-4 2-4-1-5-3 4-1-5-2-3

4-1-5-2-3

Part A Part complete Which of the following coenzymes participate in the reactions of the pyruvate dehydrogenase complex? 1) thiamine pyrophosphate 2) lipoic acid 3) FAD 4) NAD+ 5) CoA Only 1, 2, and 3 participate. Only 1, 2, and 4 participate. Only 1, 3, and 5 participate. All of the listed coenzymes participate.

All of the listed coenzymes participate.

activation of pyruvate dehydrogenase phosphatase by Ca2+ Ca2+ mediates stimulation of PDH activity during muscle contraction, which can produce a huge amount of ATP molecules. Ca2+ is a critical signaling species for contraction in vertebrate muscles, which places a huge demand on ATP production. Ca2+ interacts with active sites of four proteins which participate in contraction of vertebrate muscles, which places a huge demand on ATP production. Ca2+ activates PDH through interaction with the active site of an enzyme, when a huge amount of ATP molecules is needed for protein synthesis activation.

Ca2+ is a critical signaling species for contraction in vertebrate muscles, which places a huge demand on ATP production. Ca2+ plays a significant signaling role during contraction of muscles in vertebrates. Pyruvate dehydrogenase phosphatase, on the other hand, catalyzes the hydrolytic removal of phosphate from the E1 component of the multienzyme complex pyruvate dehydrogenase (PDH), thereby reactivating the enzyme complex. The pyruvate dehydrogenase complex catalyzes the conversion of pyruvate to acetyl-CoA. Hence, activation of pyruvate dehydrogenase phosphatase by Ca2+ would lead to activation of PDH complex leading to increased formation of acetyl-CoA. This increase would be an indication to push the cell into the citric acid cycle to generate ATP. Thus, Ca2+ activation of pyruvate dehydrogenase phosphatase would be an indication for the cell to generate ATP for muscle contraction.

Explain why glucose consumption must increase in hypoxic tissues to provide the same amount of ATP that could be produced from glucose in normoxic (normal O2 levels) tissues. Oxygen is the limiting reactant in ATP producing, so ATP is produced in hypoxic tissues more slowly than in normoxic tissues. In hypoxic tissues, ATP is produced only by glycolysis, whereas in normoxic tissues, the citric acid cycle + oxidative phosphorylation can be used. In hypoxic tissues, only by the citric acid cycle is responsible for ATP production, whereas in normoxic tissues both glycolysis and oxidative phosphorylation are involved. In hypoxic tissues, in contrast to normoxic tissues, ATP is additionally consumed to transform lactate to pyruvate.

In hypoxic tissues, ATP is produced only by glycolysis, whereas in normoxic tissues, the citric acid cycle + oxidative phosphorylation can be used

Which of the following statements about the conversion of acetyl CoA to citrate are true? 1) The reaction is spontaneous. 2) The reaction involves the nucleophilic attack of the enol of acetyl-CoA onto the carbonyl carbon of oxaloacetate. 3) A serine residue hydrolyzes the thioester bond of citroyl-CoA to yield a protein ester intermediate that is subsequently hydrolyzed to citrate. 4) The final product, citrate, has two prochiral substituents. Only statements 1, 2, and 4 are correct. Only statements 1, 2, and 3 are correct. Only statements 1, 3, and 4 are correct. All of the listed statements are correct.

Only statements 1, 2, and 4 are correct. Citroyl-CoA is hydrolyzed directly by water and does not involve the formation of a temporary protein-bound intermediate

How would increasing PDK expression decrease the rate of mitochondrial respiration? PDK increases the conversion of pyruvate to lactate in the mitochondria, decreasing NADH production. Thus, the rate of mitochondrial respiration is decreased. PDK phosphorylates the E1 subunit of PDH complex, decreasing flux through citric acid cycle. Thus, NADH production and respiration is decreased. PDK activity requires a huge consumption of ATP molecules. ATP are formed easily by glycolysis in hypoxic tissues, so processes of mitochondrial respiration are deprived.

PDK phosphorylates the E1 subunit of PDH complex, decreasing flux through citric acid cycle. Thus, NADH production and respiration is decreased.

Which of the following citric acid intermediates is(are) not used in the preparation of the products shown? 1) citrate ⟶ fatty acids, steroids 2) alpha-ketoglutarate ⟶ steroids 3) oxaloacetate ⟶ purines, pyrimidines 4) alpha-ketoglutarate ⟶ heme Statements 1 and 4 are incorrect. Statements 2 and 4 are incorrect. Statements 1 and 3 are incorrect. All of the listed statements are correct.

Statements 2 and 4 are incorrect.

How would increasing LDH expression increase the rate of glycolysis? The LDH reaction ensures a continuous supply of oxidized NAD+ to allow glycolysis to operate at an increased rate. The LDH activity increases the level of lactate in tissues to allow glycolysis to operate at an increased rate. The LDH reaction produces supplementary amounts of reduced NADH to allow glycolysis to operate at an increased rate. The LDH activity increases the ratio NADH/NAD+ to allow glycolysis to operate at an increased rate.

The LDH reaction ensures a continuous supply of oxidized NAD+ to allow glycolysis to operate at an increased rate.

Fluoroacetate functions as a poison by what mechanism? The compound forms a covalent complex with the aconitase. Aconitase converts this compound to a reactive species that covalently modifies the next enzyme in the citric acid cycle. The compound binds very tightly to but does not form a covalent complex with aconitase. This compound is first converted to 2-fluorocitrate, which subsequently inhibits aconitase.

This compound is first converted to 2-fluorocitrate, which subsequently inhibits aconitase.

activation of pyruvate carboxylase by acetyl-CoA This is a signal that pyruvate can be shunted into gluconeogenesis instead of being oxidized in the citric acid cycle. In addition, it is a signal of unbalanced fat and carbohydrate metabolism. This is a signal that a part of available pyruvate can be metabolized into oxaloacetate, when the energy charge is low and production of additional ATP through the citric acid cycle is required. This is a signal that pyruvate can be oxidized in the citric acid cycle as well as shunted into gluconeogenesis. In addition, it is a signal of activation of fat metabolism. This is a signal that pyruvate can be oxidized in the citric acid cycle instead of being shunted into gluconeogenesis. In addition, it is a signal of activation of carbohydrate metabolism.

This is a signal that pyruvate can be shunted into gluconeogenesis instead of being oxidized in the citric acid cycle. In addition, it is a signal of unbalanced fat and carbohydrate metabolism. Pyruvate carboxylase catalyzes an anaplerotic reaction wherein three-carbon pyruvate is converted to four-carbon oxaloacetate (OAA) in presence of biotin and ATP. Therefore, when acetyl-CoA allosterically activates pyruvate carboxylase, it results in an increase in the conversion of pyruvate to OAA. Accumulation of OAA pushes it into the gluconeogenic pathway; as a consequence, OAA is not available for entry into the citric acid cycle, where it would have been oxidized. Further, acetyl-CoA is a molecule which serves as a connecting link between carbohydrate metabolism and fat metabolism. Hence, activation of pyruvate kinase by acetyl-CoA would also signal an imbalance between fat and carbohydrate metabolism.

inhibition of isocitrate dehydrogenase by NADH This is a signal to reduce flux through the citric acid cycle when an additional amount of acetyl-CoA is needed for lipid biosynthesis. This is a signal to reduce flux through the citric acid cycle when levels of reduced electron carriers are adequate for energy generation. This is a signal to reduce flux through the citric acid cycle when metabolism of acetyl-CoA through the glyoxylate pathway is more preferred. This is a signal to increase flux through the citric acid cycle when metabolism of acetyl-CoA through the glyoxylate pathway is less preferred.

This is a signal to reduce flux through the citric acid cycle when levels of reduced electron carriers are adequate for energy generation. Isocitrate dehydrogenase catalyzes the conversion of a tricarboxylic acid (isocitrate) to a dicarboxylic acid, α-ketoglutarate. This reaction is the third step in the citric acid cycle, where the first reducing equivalent is generated in the form of NADH, with associated release of CO2. Reducing equivalents play the role of electron carriers in energy generation. Therefore, inhibition of isocitrate dehydrogenase would be a signal suggesting that adequate reducing equivalents are present in the cell and that the flux through the citric acid cycle needs to be decreased.

inhibition of α-ketoglutarate dehydrogenase by succinyl-CoA This allows to turn back reversible stages of the citric acid cycle, to produce additional pyruvate which can be used in gluconeogenesis at the high glucose level in blood. This allows to increase α-ketoglutarate accumulation and thereby to increase the rate of amino acids transamination. This serves as a general indicator that when an energy-rich substrate (succinyl-CoA) is abundant, flux through the citric acid cycle can be reduced. This serves as a signal that there is an insufficient amount of NAD+, FAD, flux through the electron transport should be increased.

This serves as a general indicator that when an energy-rich substrate (succinyl-CoA) is abundant, flux through the citric acid cycle can be reduced. α-Ketoglutarate dehydrogenase catalyzes the conversion of α-ketoglutarate to succinyl-CoA. This reaction is the fourth step in the citric acid cycle, where the second reducing equivalent is generated in the form of NADH, with associated release of CO2. The reducing equivalents play the role of electron carriers in energy generation. Hence, inhibition of α-ketoglutarate dehydrogenase would be a signal suggesting that a high-energy substrate in the form of succinyl-CoA is present along with adequate reducing equivalents, and that the flux through the citric acid cycle needs to be decreased

activation of pyruvate dehydrogenase kinase by NADH This tends to inactivate pyruvate dehydrogenase when the level of NADH is sufficient for ATP production via the respiratory chain and, hence, to make pyruvate available for other purposes. This tends to activate pyruvate dehydrogenase when the level of NADH is sufficient for ATP production via the respiratory chain and, hence, to make pyruvate unavailable for other purposes. This tends to activate pyruvate dehydrogenase when the level of NADH is sufficient for ATP production via the citric acid cycle and, hence, to increase oxidation of lipids and carbohydrates. This tends to inactivate pyruvate dehydrogenase and to activate pyruvate carboxylase and to increase oxaloacetate production and, hence, to activate gluconeogenesis.

This tends to inactivate pyruvate dehydrogenase when the level of NADH is sufficient for ATP production via the respiratory chain and, hence, to make pyruvate available for other purposes. Pyruvate dehydrogenase kinase (PDK) inhibits the action of PDH by phosphorylating a component of the PDH complex (E1). The pyruvate dehydrogenase (PDH) complex is a multienzyme complex which catalyzes the conversion of pyruvate to acetyl-CoA. The acetyl-CoA enters the citric acid cycle to yield energy in the form of ATP. Therefore, activation of pyruvate dehydrogenase kinase by NADH would result in inhibition of PDH, and subsequently no conversion of pyruvate to acetyl-CoA for entry into the citric acid cycle, suggesting that there is an abundance of ATP in the cell produced via the citric acid cycle and mitochondrial respiration (through the respiratory chain). Consequently, the pyruvate would become available to participate in other cellular reactions.

All of the enzymes of the citric acid cycle are located in the mitochondrion. True False

True

NAD+, coenzyme A, thiamine pyrophosphate, lipoic acid and FAD are all cofactors used in the reaction catalyzed by pyruvate dehydrogenase. True False

True

activation of isocitrate dehydrogenase by ADP Accumulation of ADP provides a signal to activate the isocitrate dehydrogenase and thereby increase the level of α-ketoglutarate in an organism preventing ATP consumption in the process of glutamine desamination. When the energy charge is high, accumulation of ADP provides a signal to activate the citric acid cycle and thereby increase oxidation of nutrients for production of proteins. When the energy charge is low, the accumulation of ADP provides a signal to activate the citric acid cycle and thereby increase oxidation of nutrients for ATP production. When the energy charge is high, accumulation of ADP provides a signal to activate the citric acid cycle and thereby increase succinate production for the electron transport chain

When the energy charge is low, the accumulation of ADP provides a signal to activate the citric acid cycle and thereby increase oxidation of nutrients for ATP production. As determined in Part C, reducing equivalents play the role of electron carriers in energy generation. Therefore, activation of isocitrate dehydrogenase would indicate that more reducing equivalents need to be generated in order to generate energy when the energy charge (ATP/ADP ratio) in the cell is low. The signal indicates that the cell needs to increase the flux in the citric acid cycle and in the process also produce ATP via mitochondrial respiration.

Which of these enters the citric acid cycle? acetyl CoA pyruvate glucose G3P NADH + H+

acetyl CoA

Which of these is NOT a product of the citric acid cycle? CO2 NADH + H+ ATP acetyl CoA FADH2

acetyl CoA Acetyl CoA enters the citric acid cycle.

The mechanism for the conversion of α-ketoglutarate to succinyl-CoA resembles the mechanism of which of the following enzymes? pyruvate dehydrogenase complex phosphoenolpyruvate carboxykinase aconitase succinate dehydrogenase

pyruvate dehydrogenase complex The enzyme complex that catalyzes this reaction, α-ketoglutarate dehydrogenase complex, converts its substrate to product by essentially the same mechanism as PDC

In the citric acid cycle, ATP molecules are produced by _____. photosynthesis substrate-level phosphorylation photophosphorylation cellular respiration oxidative phosphorylation

substrate-level phosphorylation A phosphate group is transferred from GTP to ADP.

Which of the following is NOT one of the three stages of respiration? the synthesis of pyruvate the conversion of pyruvate to acetyl-CoA electron transport and oxidative phosphorylation the oxidation of acetyl-CoA to two molecules of CO2

the synthesis of pyruvate This is part of glycolysis, which need not run under aerobic conditions.