Chapter 16 - Citric Acid Cycle

What enzymes compose the pyruvate dehydrogenase complex? (PDC)

(E1) pyruvate dehydrogenase (E2) dihydrolipoyl transacetylase (E3) dihydrolipoyl dehydrogenase

Step one of Pyruvate Processing (in general)

1) Decarboxylation of pyruvate which produces a 2 carbon aldehyde

What are the advantages of multi enzyme complexes?

1) short distance between catalytic sites allows channeling of substrates from one catalytic site to another 2) Substrate channeling minimizes side reactions 3)Regulation of activity of one subunit affects the entire complex

Co-substrate

A secondary substrate that is needed for a reaction to take place on a multi-substrate enzyme

The oxidative decarboxylation of a-ketoglutarate proceeds by means of multistep reactions in which all but one of the following cofactors are required. Which one is not required? A) ATP B) Coenzyme A C) Lipoic acid D) NAD+ E) Thiamine pyrophosphate

A) ATP

Which of the below is not required for the oxidative decarboxylation of pyruvate to form acetyl-CoA? A) ATP B) CoA-SH C) FAD D) Lipoic acid E) NAD+

A) ATP

Which of the following is not an intermediate of the citric acid cycle? A) Acetyl-coA B) Citrate C) Oxaloacetate D) Succinyl-coA E) a-Ketoglutarate

A) Acetyl-coA

Which of the following is not true of the CAC? A) All enzymes of the cycle are located in the cytoplasm, except succinate dehydrogenase, which is bound to the inner mitochondrial membrane. B) In the presence of malonate, one would expect succinate to accumulate. C) Oxaloacetate is used as a substrate but is not consumed in the cycle. D) Succinate dehydrogenase channels electrons directly into the electron transfer chain. E) The condensing enzyme is subject to allosteric regulation by ATP and NADH.

A) All enzymes of the cycle are located in the cytoplasm, except succinate dehydrogenase, which is bound to the inner mitochondrial membrane.

Which of the following is not true of the reaction catalyzed by the pyruvate dehydrogenase complex? A) Biotin participates in the decarboxylation. B) Both NAD+ and a flavin nucleotide act as electron carriers. C) The reaction occurs in the mitochondrial matrix. D) The substrate is held by the lipoyl-lysine "swinging arm." E) Two different cofactors containing —SH groups participate.

A) Biotin participates in the decarboxylation.

Which of the following intermediates of the citric acid cycle is prochiral? A) Citrate B) Isocitrate C) Malate D) Oxaloacetate E) Succinate

A) Citrate

Which of the following statements about the oxidative decarboxylation of pyruvate in aerobic conditions in animal cells is correct? A) One product of the PDC is a thioester of acetate. B) The methyl (—CH3) group is eliminated as CO2. C) Process occurs in cell's cytosolic compartment. D) PDC uses NAD+, lipoic acid, pyridoxal phosphate (PLP), and FAD. as cofactors. E) The reaction is so important to energy production that pyruvate dehydrogenase operates at full speed under all conditions.

A) One product of the PDC is a thioester of acetate

Oxaloacetate and a crude tissue preparation capable of carrying out the reactions of the citric acid cycle. After one turn of the cycle, oxaloacetate would have 14C in: A) all four carbon atoms. B) no pattern that is predictable from the information provided. C) none of its carbon atoms. D) the keto carbon and one of the carboxyl carbons. E) the two carboxyl carbons

A) all four carbon atoms.

Anaplerotic reactions . A) produce oxaloacetate and malate to maintain constant levels of citric acid cycle intermediates B) produce biotin needed by pyruvate carboxylase C) recycle pantothenate used to make CoA D) produce pyruvate and citrate to maintain constant levels of citric acid cycle intermediates E) all of the above

A) produce oxaloacetate and malate to maintain constant levels of citric acid cycle intermediates

Conversion of 1 mol of acetyl-CoA to 2 mol of CO2 and CoA via the citric acid cycle results in the net production of: A) 1 mol of citrate. B) 1 mol of FADH2. C) 1 mol of NADH. D) 1 mol of oxaloacetate. E) 7 mol of ATP.

B) 1 mol of FADH2.

Oxaloacetate uniformly labeled with 14C (i.e., with equal amounts of 14C in each of its carbon atoms) is condensed with unlabeled acetyl-CoA. After a single pass through the citric acid cycle back to oxaloacetate, what fraction of the original radioactivity will be found in the oxaloacetate? A) all B) 1/2 C) 1/3 D) 1/4 E) 3/4

B) 1/2

Malonate is a competitive inhibitor of succinate dehydrogenase. If malonate is added to a mitochondrial preparation that is oxidizing pyruvate as a substrate, which of the following compounds would you expect to decrease in concentration? A) Citrate B) Fumarate C) Isocitrate D) Pyruvate E) Succinate

B) Fumarate

Fumarate + 2H+ + 2e- --> succinate E'° = +0.031 V FAD + 2H+ + 2e- --> FADH2 E'° = -0.219 V If succinate, fumarate, FAD, and FADH2were mixed together w/ succinate dehydrogenase, initially: A) Fumarate/succinate: oxidized; FAD and FADH2 would become reduced. B) Fumarate: reduced; FADH2: oxidized. C) No rxn b/c all are at standard concentrations. D) Succinate: oxidized; FAD: reduced. E) Succinate: oxidized; FADH2 would be unchanged because it is a cofactor, not a substrate.

B) Fumarate: reduced; FADH2: oxidized.

Which of the following cofactors is required for the conversion of succinate to fumarate in the citric acid cycle? A) ATP B) Biotin C) FAD D) NAD+ E) NADP+

C) FAD

Give and Example of a carrier molecule:

CoA is a carrier molecule, it "carries" the acetyl group from glycolysis to the citric acid cycle

The conversion of 1 mol of pyruvate to 3 mol of CO2 via pyruvate dehydrogenase and the citric acid cycle also yields _____ mol of NADH, _____ mol of FADH2, and _____ mol of ATP (or GTP). A) 2; 2; 2 B) 3; 1; 1 C) 3; 2; 0 D) 4; 1; 1 E) 4; 2; 1

D) 4; 1; 1

For the following reaction, DG'° = 29.7 kJ/mol. L-Malate + NAD+ ® oxaloacetate + NADH + H+ The reaction as written: A) can never occur in a cell. B) can only occur in a cell if it is coupled to another reaction for which DG'° is positive. C) can only occur in a cell in which NADH is converted to NAD+ by electron transport. D) may occur in cells at certain concentrations of substrate and product. E) would always proceed at a very slow rate

D) may occur in cells at certain concentrations of substrate and product.

In mammals, each of the following occurs during the citric acid cycle except: A) formation of a-ketoglutarate. B) generation of NADH and FADH2. C) metabolism of acetate to carbon dioxide and water. D) net synthesis of oxaloacetate from acetyl-CoA. E) oxidation of acetyl-CoA.

D) net synthesis of oxaloacetate from acetyl-CoA.

All of the oxidative steps of the citric acid cycle are linked to the reduction of NAD+ except the reaction catalyzed by: A) isocitrate dehydrogenase. B) malate dehydrogenase. C) pyruvate dehydrogenase D) succinate dehydrogenase. E) the a-ketoglutarate dehydrogenase complex

D) succinate dehydrogenase.

Entry of acetyl-CoA into the citric acid cycle is decreased when: A) [AMP] is high. B) NADH is rapidly oxidized through the respiratory chain. C) the ratio of [ATP]/[ADP is low D) the ratio of [ATP]/[ADP] is high. E) the ratio of [NAD+]/[NADH] is high.

D) the ratio of [ATP]/[ADP] is high.

Glucose labeled with 14C in C-3 and C-4 is completely converted to acetyl-CoA via glycolysis and the PDC. What % of the acetyl-CoA molecules formed will be labeled with 14C, and where will the acetyl moiety w/ the 14C label be found? A) 100% of the acetyl-CoA will be labeled at C-1 (carboxyl). B) 100% of the acetyl-CoA will be labeled at C-2. C) 50% of the acetyl-CoA will be labeled, all at C-2 (methyl). D)No label will be found in the molecules. E) Not enough information is given.

D)No label will be found in the molecules.

Citrate synthase and the NAD+-specific isocitrate dehydrogenase are two key regulatory enzymes of the citric acid cycle. These enzymes are inhibited by: A) acetyl-CoA and fructose 6-phosphate. B) AMP and/or NAD+. C) AMP and/or NADH. D) ATP and/or NAD+. E) ATP and/or NADH

E) ATP and/or NADH

Which one of the following is not associated with the oxidation of substrates by the citric acid cycle? A) All of the below are involved. B) CO2 production C) Flavin reduction D) Lipoic acid present in some of the enzyme systems E) Pyridine nucleotide oxidation

E) Pyridine nucleotide oxidation

Which combination of cofactors is involved in the conversion of pyruvate to acetyl-CoA? A) Biotin, FAD, and TPP B) Biotin, NAD+, and FAD C) NAD+, biotin, and TPP D) Pyridoxal phosphate, FAD, and lipoic acid E) TPP, lipoic acid, and NAD+

E) TPP, lipoic acid, and NAD+

Which one of the following enzymatic activities would be decreased by thiamine deficiency? A) Fumarase B) Isocitrate dehydrogenase C) Malate dehydrogenase D) Succinate dehydrogenase E) a-Ketoglutarate dehydrogenase complex

E) a-Ketoglutarate dehydrogenase complex

The reaction of the citric acid cycle that is most similar to the pyruvate dehydrogenase complex-catalyzed conversion of pyruvate to acetyl-CoA is the conversion of: A) citrate to isocitrate. B) fumarate to malate. C) malate to oxaloacetate. D) succinyl-CoA to succinate. E) a-ketoglutarate to succinyl-CoA.

E) a-ketoglutarate to succinyl-CoA.

Intermediates in the citric acid cycle are used as precursors in the biosynthesis of: A) amino acids B) nucleotides C) fatty acids D) sterols E) all of the above

E) all of the above - amino acids - nucleotides - fatty acids - sterols

In the citric acid cycle, a flavin coenzyme is required for: A) condensation of acetyl-CoA and oxaloacetate. B) oxidation of fumarate. C) oxidation of isocitrate. D) oxidation of malate. E) oxidation of succinate

E) oxidation of succinate

The reaction of the citric acid cycle that produces an ATP equivalent (in the form of GTP) by substrate level phosphorylation is the conversion of: A) citrate to isocitrate. B) fumarate to malate. C) malate to oxaloacetate. D) succinate to fumarate. E) succinyl-CoA to succinate.

E) succinyl-CoA to succinate.

The two moles of CO2 produced in the first turn of the citric acid cycle have their origin in the: A) carboxyl and methylene carbons of oxaloacetate B) carboxyl group of acetate and a carboxyl group of oxaloacetate. C) carboxyl group of acetate and the keto group of oxaloacetate. D) two carbon atoms of acetate. E) two carboxyl groups derived from oxaloacetate

E) two carboxyl groups derived from oxaloacetate.

Overall reaction of the oxidative decarboxylation of Pyruvate:

Enzyme 1: 1) Decarboxylation of pyruvate to an aldehyde 2) Oxidation of aldehyde to a carboxylic acid Enzyme 2: 3) Formation of acetyl-CoA (Product 1) Enzyme 3: 4) Reoxidatiion of the lipoamide cofactor 5) Regeneration of the oxidized FAD cofactor (Forming NADH = product 2)

What composes the structure of Lipoyllysine?

Is composed of lipoic acid which is covalently linked to the second enzyme (dihydrolipoyl transacetylase) via a lysine residue

In what form is the lipoic acid active?

It is not active in its oxidized form but is active in its reduced form

Where does glycolysis occur?

It occurs in the cytosol of cells

What are the products of pyruvate processing?

NADH and acetyl-CoA

Are the 5 coenzymes in the pyruvate dehydrogenase complex a permanent part of the enzymes structures?

No the coenzymes are not permanent, they associate, fulfill a function and then dissociate

Mitochondrial Pyruvate Carriers

Protein carriers that transport pyruvate from cytosol into the mitochondrial inner membrane and finally into the mitochondrial matrix where pyruvate processing occurs

What do cancerous tumour cells lack that make them so inefficient at pyruvate oxidation?(Eukaryotic cancer cells)

Some cancerous tumour cells lack mitochondrial pyruvate carriers so may not be able to oxidize pyruvate at all and therefore rely on glycolysis for their ATP production

What 5 coenzyme are needed in the pyruvate dehydrogenase complex?

TPP, lipoyllysine and FAD are prosthetic groups and NAD+ and CoA-SH are co-substrates

Where do the Krebs cycle and Pyruvate processing take place?

Takes place in the mitochondrion

Decarboxylation

The removal of a carboxyl group from an organic molecule leading to its release as a carbon-dioxide

What is the function of Coenzyme A (CoA)?

Tje function of CoA is to accept and carry an acetyl group. The Thiol group (SH) at one end is very reactive and will therefore attach to acetyl readily

Prosthetic Group

are non-protein structures that are tightly bound to an enzyme and are needed for enzyme activity

What happens to most of pyruvate produced from Glycolysis?

oxidized in TCA/Krebs/citric acid cycle to generate ATP since while glycolysis releases some potential energy, it is far from all of it. The majority of the energy is still contained within the 2 pyruvate molecules produced from each glucose molecule after its been oxidized during glycolysis.