Unit 11 Question Bank

The eight steps · 10 ATP per turn · amphibolic · anaplerotic

25 MCQ · five options4 Definitions2 Written answersHarper's verified

Format note: the TMU Biochemistry paper gives five suggested answers (A–E), not four — these MCQs match that. Items tagged TMU 2019 or TMU 2020/21 come from the real papers. Answers are verified against Harper's Illustrated Biochemistry; the "marking schemes" in the source folder are other students' answer sheets, not official, so they are never used as the authority.

0 / 25 answered

1The citric acid cycle is best defined as ( ).

A. the cytosolic pathway converting glucose into pyruvate

B. a mitochondrial series of reactions oxidising acetyl residues to CO₂

C. the pathway converting non-carbohydrate precursors into glucose

D. the series of membrane carriers that transfer electrons to oxygen

E. the pathway generating NADPH and ribose 5-phosphate

Answer: B

This is the TMU slide's wording verbatim, and the answer to their study question. Note what it does not say: the cycle does not chiefly make ATP — it reduces coenzymes which are then linked to ATP formation. Options A, C, D and E are glycolysis, gluconeogenesis, the respiratory chain and the pentose phosphate pathway.TMU Lecture 10 Slide 3 · Harper's ch.16, p.161

2The enzymes of the citric acid cycle are located in ( ).

A. the cytosol, close to the glycolytic enzymes

B. the membranes of the endoplasmic reticulum

C. the mitochondrial matrix and the inner membrane

D. the outer mitochondrial membrane only

E. the mitochondrial intermembrane space

Answer: C

They are in the matrix, either free or attached to the inner mitochondrial membrane and the crista membrane, where the enzymes and coenzymes of the respiratory chain are also found. That co-location matters: the coenzymes the cycle reduces are handed straight to the chain.Harper's ch.16, p.162

3The citric acid cycle is described as the “final common pathway” because ( ).

A. it is the only pathway that consumes oxygen directly

B. all its intermediates are excreted unchanged

C. it was the last metabolic pathway to evolve

D. glucose, fatty acids and amino acids all converge on acetyl-CoA

E. it occurs in every organelle of the cell

Answer: D

Three fuels, three different dismantling routes, one destination: carbohydrate via glycolysis and PDH, lipid via β-oxidation, protein via transamination and deamination. That convergence makes the phrase precise rather than rhetorical.Harper's ch.16, p.161 · TMU Lecture 10 Slide 4

4Which enzyme catalyses the first reaction of the cycle?

A. Aconitase

B. Isocitrate dehydrogenase

C. Malate dehydrogenase

D. Succinate thiokinase

E. Citrate synthase

Answer: E

Citrate synthase forms a carbon–carbon bond between the methyl carbon of acetyl-CoA and the carbonyl carbon of oxaloacetate. The resultant citryl-CoA thioester is then hydrolysed, releasing citrate and CoASH — an exothermic reaction.Harper's ch.16, p.162

5Oxaloacetate is described as playing a catalytic role in the cycle because ( ).

A. it is regenerated each turn, so a little oxidises a great deal

B. it is the only intermediate that leaves the mitochondrion

C. it lowers the activation energy of citrate synthase

D. it is not consumed by any reaction of the cycle

E. it can substitute for coenzyme A when scarce

Answer: A

That is the meaning of the word cycle. But note the vulnerability it creates: if oxaloacetate is drained away the cycle stops, however much acetyl-CoA is available. That is the basis of anaplerotic reactions — and of starvation ketosis.Harper's ch.16, p.162

6Which enzyme of the citric acid cycle is ALSO a complex of the respiratory chain?

A. Malate dehydrogenase — Complex I

B. Succinate dehydrogenase — Complex II

C. Isocitrate dehydrogenase — Complex III

D. Citrate synthase — Complex IV

E. Fumarase — Complex II

Answer: B

Succinate dehydrogenase IS Complex II. Every other enzyme of the cycle is in the matrix; this one is embedded in the inner membrane, so its FADH₂ hands electrons straight to Q. And because Complex II pumps no protons, that FADH₂ yields only 1.5 ATP against NADH's 2.5.Harper's ch.16, p.163 · Harper's ch.13, p.128

7The only substrate-level phosphorylation in the citric acid cycle occurs at the step catalysed by ( ).

A. the α-ketoglutarate dehydrogenase complex

B. malate dehydrogenase, the last step

C. succinate thiokinase (succinyl-CoA synthetase)

D. citrate synthase, the first enzyme

E. aconitase, which isomerises citrate

Answer: C

Succinyl-CoA → succinate, generating one ATP directly. This is the single ~℗ that Unit 8 attributed to the citric acid cycle — the other nine ATP per turn come from the respiratory chain reoxidising the reduced coenzymes.Harper's ch.16, p.164 · TMU Lecture 10 Slide 11

8How many NADH and FADH₂ are produced per turn of the cycle?

A. Two NADH and two FADH₂

B. Four NADH and one FADH₂

C. Three NADH and two FADH₂

D. Three NADH and one FADH₂

E. One NADH and three FADH₂

Answer: D

NADH at isocitrate dehydrogenase, α-ketoglutarate dehydrogenase and malate dehydrogenase; FADH₂ at succinate dehydrogenase. With 2.5 ATP per NADH and 1.5 per FADH₂, plus the one substrate-level ATP: 7.5 + 1.5 + 1 = 10 ATP per turn.TMU Lecture 10 Slide 13 · Harper's ch.16

9How many molecules of ATP are produced in one complete turn of the citric acid cycle?

A. 12

B. 2

C. 24

D. 4

E. 10

Answer: E

3 NADH × 2.5 = 7.5, plus 1 FADH₂ × 1.5 = 1.5, plus 1 substrate-level = 10. This is the second half of the TMU study question, and your Lecture 10 slide 13 states the 2.5 and 1.5 values explicitly — which is also the evidence that settles the P:O controversy raised in Unit 9.TMU Lecture 10 Slide 13

10Two molecules of CO₂ are released per turn, at the steps catalysed by ( ).

A. isocitrate and α-ketoglutarate dehydrogenases

B. malate dehydrogenase and citrate synthase

C. succinate dehydrogenase and fumarase

D. succinate thiokinase and fumarase

E. citrate synthase and aconitase

Answer: A

Both are oxidative decarboxylations. Note that in isocitrate dehydrogenase the intermediate oxalosuccinate remains enzyme-bound, and the decarboxylation requires Mg²⁺ or Mn²⁺.Harper's ch.16, p.163

11The α-ketoglutarate dehydrogenase complex requires the same cofactors as ( ).

A. succinate dehydrogenase (complex II)

B. the pyruvate dehydrogenase complex

C. aconitase of the citric acid cycle

D. citrate synthase, the first enzyme

E. fumarase, which hydrates fumarate

Answer: B

It is a multienzyme complex similar to PDH, requiring thiamin diphosphate, lipoate, NAD⁺, FAD and CoA. Two identical machines for two analogous jobs — both perform oxidative decarboxylation of an α-keto acid. Its equilibrium so favours succinyl-CoA that the reaction is physiologically unidirectional.Harper's ch.16, p.163

12Although citrate is a symmetrical molecule, the two carbons lost as CO₂ in one turn are NOT those just added from acetyl-CoA. This is because ( ).

A. isocitrate dehydrogenase selects only the labelled carbons

B. the CO₂ is derived from bicarbonate, not from citrate

C. aconitase channels citrate directly and so treats it asymmetrically

D. the acetyl-CoA carbons are always the first lost

E. citrate is racemised before aconitase acts on it

Answer: C

Channelling — transfer without entering free solution. The carbons lost come from the portion of citrate derived from oxaloacetate. This also has a regulatory consequence: citrate is only available in free solution for export to the cytosol when aconitase is inhibited by accumulation of its product, isocitrate.Harper's ch.16, p.162

13Fluoroacetate is toxic because ( ).

A. it chelates the Mg²⁺ required by isocitrate dehydrogenase

B. it competitively inhibits succinate dehydrogenase

C. it reacts with the –SH groups of lipoic acid

D. it forms fluorocitrate, which inhibits aconitase

E. it uncouples oxidation from phosphorylation

Answer: D

A lethal synthesis — the cell makes the poison out of it, and Unit 6's suicide-inhibitor concept occurring in nature. Citrate accumulates. Found in some plants and fatal to grazing animals; some fluorinated anticancer agents and pesticides are metabolised to fluoroacetate.Harper's ch.16, p.162

14Which poison inhibits succinate dehydrogenase competitively?

A. Fluoroacetate

B. Arsenite

C. Cyanide

D. Oligomycin

E. Malonate

Answer: E

Malonate — Unit 6's worked example of competitive inhibition, now appearing in its native pathway. Arsenite inhibits α-ketoglutarate dehydrogenase via the –SH groups of lipoic acid; fluoroacetate inhibits aconitase.Harper's ch.16, p.163

15Which four B vitamins are essential to the citric acid cycle?

A. Riboflavin, niacin, thiamin and pantothenic acid

B. Thiamin, pyridoxine, biotin and folate

C. Niacin, biotin, cobalamin and riboflavin

D. Ascorbic acid, thiamin, niacin and biotin

E. Riboflavin, pyridoxine, folate and pantothenic acid

Answer: A

Riboflavin → FAD, niacin → NAD, thiamin → thiamin diphosphate, pantothenic acid → coenzyme A. The same four the PDH complex requires — which is why thiamin deficiency cripples two α-keto acid dehydrogenases at once, and why the presentation is neurological.TMU Lecture 10 Slide 14 · Harper's ch.16

16The citric acid cycle is described as AMPHIBOLIC because ( ).

A. it can run in either direction

B. it functions in both oxidative and synthetic processes

C. it occurs in both mitochondria and cytosol

D. it uses both NAD⁺ and NADP⁺

E. it is regulated by both hormones and allosteric effectors

Answer: B

It is not only a pathway for oxidation of two-carbon units but also a major pathway for interconversion of metabolites arising from transamination and deamination, and it provides substrates for amino acid synthesis, gluconeogenesis and fatty acid synthesis. Catabolic and anabolic at once.Harper's ch.16, p.164 · TMU Lecture 10 Slide 15

17The key enzyme catalysing net transfer of carbon OUT of the cycle into gluconeogenesis is ( ).

A. pyruvate carboxylase, an anaplerotic enzyme

B. malate dehydrogenase, the last cycle step

C. phosphoenolpyruvate carboxykinase

D. citrate synthase, the first cycle enzyme

E. ATP-citrate lyase, in the cytosol

Answer: C

It decarboxylates oxaloacetate to phosphoenolpyruvate, with GTP acting as the phosphate donor. All the intermediates of the cycle are potentially glucogenic, since they can give rise to oxaloacetate — in liver and kidney, the organs that carry out gluconeogenesis.Harper's ch.16, p.164 · TMU Lecture 10 Slide 17

18Which amino acids give rise to succinyl-CoA?

A. Alanine, cysteine and glycine

B. Arginine, histidine and proline

C. Tyrosine and phenylalanine

D. Isoleucine, methionine and valine

E. Aspartate and asparagine

Answer: D

Learn the four groups: alanine, cysteine, glycine, hydroxyproline, serine, threonine, tryptophan → pyruvate; arginine, histidine, glutamine, proline → α-ketoglutarate; isoleucine, methionine, valine → succinyl-CoA; tyrosine, phenylalanine → fumarate.Harper's ch.16, p.164 · TMU Lecture 10 Slide 19

19An anaplerotic reaction is one that ( ).

A. removes intermediates from the cycle for biosynthesis

B. transfers electrons from the cycle to the respiratory chain

C. converts cycle intermediates to ketone bodies

D. oxidises acetyl-CoA to two molecules of CO₂

E. transfers metabolites INTO the cycle, replenishing it

Answer: E

The most important is the formation of oxaloacetate by carboxylation of pyruvate, catalysed by pyruvate carboxylase — important in maintaining an adequate concentration of oxaloacetate for the condensation reaction with acetyl-CoA.Harper's ch.16, pp.164–165

20If acetyl-CoA accumulates, it ensures its own supply of oxaloacetate by ( ).

A. activating pyruvate carboxylase and inhibiting pyruvate dehydrogenase

B. activating pyruvate dehydrogenase and inhibiting pyruvate carboxylase

C. inhibiting citrate synthase

D. activating PEP carboxykinase

E. stimulating the transamination of aspartate

Answer: A

One metabolite, two opposite effects on two enzymes. Acetyl-CoA is piling up because there is too little oxaloacetate to condense with — so it diverts pyruvate away from making more acetyl-CoA and towards making oxaloacetate instead. This is the mechanism behind “fat burns in the flame of carbohydrate”, and behind starvation ketosis.Harper's ch.16, pp.164–165 · TMU Lecture 10 Slide 18

21Acetyl-CoA for cytosolic fatty acid synthesis is made available by ( ).

A. direct diffusion of acetyl-CoA across the inner membrane

B. citrate is exported and cleaved by ATP-citrate lyase

C. conversion of acetyl-CoA to acetate and back

D. the malate-aspartate shuttle

E. the carnitine shuttle

Answer: B

The mitochondrial membrane is impermeable to acetyl-CoA. So the first intermediate of the cycle doubles as a carrier smuggling two carbons out. Note the elegant control: citrate is only free to leave when aconitase is inhibited by isocitrate — i.e. when the cycle is backed up and the cell has energy to spare, exactly when making fat is appropriate.Harper's ch.16, p.166 · TMU Lecture 10 Slide 21

22Which three enzymes are the principal sites of regulation of the citric acid cycle?

A. Aconitase, fumarase and malate dehydrogenase

B. Succinate thiokinase, succinate dehydrogenase and fumarase

C. Citrate synthase, isocitrate dehydrogenase and α-ketoglutarate dehydrogenase

D. Citrate synthase, aconitase and fumarase

E. Pyruvate carboxylase, PEP carboxykinase and ATP-citrate lyase

Answer: C

Regulated by the NAD⁺/NADH, ADP/ATP and acetyl-CoA/CoA ratios, plus Ca²⁺ and Mg²⁺. Every one of those signals says the same thing — the cell needs energy and has the means to make it. And Ca²⁺ is the signal for muscle contraction: demand switching on supply.TMU Lecture 10 Slide 23

23Hyperammonaemia impairs the citric acid cycle by ( ).

A. inhibiting citrate synthase directly and completely

B. chelating the Fe²⁺ that aconitase requires

C. uncoupling oxidation from phosphorylation

D. withdrawing α-ketoglutarate to form glutamate and glutamine

E. displacing coenzyme A from succinyl-CoA

Answer: D

Both mechanisms operate. The result is reduced ATP formation, and clinically loss of consciousness, coma and convulsions — which is why advanced liver disease presents neurologically. The same logic explains why the few genetic defects of cycle enzymes cause severe neurological damage.Harper's ch.16, p.161

24In ruminants, the major glucogenic product of rumen fermentation enters the cycle as ( ).

A. α-ketoglutarate, transaminated from glutamate

B. oxaloacetate, carboxylated from pyruvate

C. acetyl-CoA, formed directly from acetate

D. fumarate, from phenylalanine catabolism

E. succinyl-CoA, from propionate via methylmalonyl-CoA

Answer: E

Propionate is the principal glucogenic fatty acid of rumen fermentation, converted to succinyl-CoA via the methylmalonyl-CoA pathway. (Acetate is indeed the direct source of acetyl-CoA in ruminants, but for lipogenesis, not gluconeogenesis.)Harper's ch.16, p.165 · TMU Lecture 10 Slide 20

25Which tissue carries out ALL the functions of the citric acid cycle to a significant extent?

A. Liver

B. Skeletal muscle

C. Brain

D. Erythrocyte

E. Adipose tissue

Answer: A

Many of these processes occur in most tissues, but liver is the only tissue in which all occur to a significant extent. Hence the profound repercussions when hepatic cells are damaged, as in acute hepatitis or replaced by connective tissue in cirrhosis.Harper's ch.16, p.161

1 The citric acid cycle — 3′ · TMU study question+

A series of reactions in mitochondria that oxidise acetyl residues (as acetyl-CoA) and reduce coenzymes that, upon reoxidation, are linked to the formation of ATP.

It is the final common pathway for the aerobic oxidation of carbohydrate, lipid and protein, because glucose, fatty acids and most amino acids are metabolised to acetyl-CoA or to intermediates of the cycle. The enzymes are located in the mitochondrial matrix, alongside the respiratory chain. Acetyl-CoA condenses with oxaloacetate to form citrate; two molecules of CO₂ are released and oxaloacetate is regenerated, so that it plays a catalytic role. One turn yields 10 ATP.TMU Lecture 10 Slide 3 · Harper's ch.16, p.161

2 Amphibolic — 2′+

Functioning in both oxidative (catabolic) and synthetic (anabolic) processes.

The citric acid cycle is not only a pathway for the oxidation of two-carbon units; it is also a major pathway for the interconversion of metabolites arising from transamination and deamination of amino acids, and it provides the substrates for amino acid synthesis by transamination, for gluconeogenesis, and for fatty acid synthesis. Because it functions in both, it is described as amphibolic.Harper's ch.16, p.164

3 Anaplerotic reaction — 2′+

A reaction producing net transfer of metabolites INTO the citric acid cycle, replenishing its intermediates.

The most important is the formation of oxaloacetate by the carboxylation of pyruvate, catalysed by pyruvate carboxylase, which maintains an adequate concentration of oxaloacetate for condensation with acetyl-CoA. Notably, if acetyl-CoA accumulates it acts both as an allosteric activator of pyruvate carboxylase and as an inhibitor of pyruvate dehydrogenase, thereby ensuring its own supply of oxaloacetate.Harper's ch.16, pp.164–165

4 Substrate-level phosphorylation in the cycle — 2′+

The formation of ATP by direct transfer of a phosphate group from a substrate, independent of the respiratory chain.

In the citric acid cycle there is a single example: the conversion of succinyl-CoA to succinate by succinate thiokinase (succinyl-CoA synthetase), yielding one ATP. The remaining nine of the ten ATP per turn arise from reoxidation of the reduced coenzymes in the respiratory chain.Harper's ch.16, p.164 · TMU Lecture 10 Slide 11

1 Illustrate the significance of the citric acid cycle in the metabolism of carbohydrates, triacylglycerol and amino acids. 8′ — ⭐ ASKED IN BOTH THE 2019 AND 2020/21 PAPERS

The unifying principle

The citric acid cycle is the final common pathway for the aerobic oxidation of carbohydrate, lipid and protein, because glucose, fatty acids and most amino acids are metabolised to acetyl-CoA or to intermediates of the cycle. It is also amphibolic — functioning in both oxidative and synthetic processes — and therefore has a central role in gluconeogenesis, lipogenesis and the interconversion of amino acids.

1 · Carbohydrate

Catabolic. Glucose is metabolised by glycolysis to pyruvate, which undergoes irreversible oxidative decarboxylation by the pyruvate dehydrogenase complex to acetyl-CoA, entering the cycle by condensation with oxaloacetate.

Anabolic. All the intermediates of the cycle are potentially glucogenic, since they can give rise to oxaloacetate and thus to net production of glucose in liver and kidney. The key enzyme catalysing net transfer out of the cycle into gluconeogenesis is phosphoenolpyruvate carboxykinase, which decarboxylates oxaloacetate to phosphoenolpyruvate using GTP.

2 · Triacylglycerol (lipid)

Catabolic. Fatty acids are oxidised by β-oxidation to acetyl-CoA, which enters the cycle.

Anabolic. Acetyl-CoA, formed from pyruvate by pyruvate dehydrogenase, is the major building block for long-chain fatty acid synthesis. But pyruvate dehydrogenase is mitochondrial and fatty acid synthesis is cytosolic, and the mitochondrial membrane is impermeable to acetyl-CoA. Acetyl-CoA is therefore made available in the cytosol from citrate, synthesised in the mitochondrion, transported out, and cleaved by ATP-citrate lyase.

3 · Amino acids

Catabolic. The cycle is a major pathway for interconversion of metabolites arising from transamination and deamination. Aminotransferase reactions form pyruvate from alanine, oxaloacetate from aspartate, and α-ketoglutarate from glutamate. Other amino acids contribute their carbon skeletons:

Amino acids	Enter as
Alanine, cysteine, glycine, hydroxyproline, serine, threonine, tryptophan	Pyruvate
Arginine, histidine, glutamine, proline	α-Ketoglutarate
Isoleucine, methionine, valine	Succinyl-CoA
Tyrosine, phenylalanine	Fumarate

Anabolic. Because the aminotransferase reactions are reversible, the cycle also serves as a source of carbon skeletons for the synthesis of these amino acids.

Maintaining the cycle — anaplerotic reactions

Since carbon is continually withdrawn for biosynthesis, intermediates must be replenished by anaplerotic reactions. The most important is the carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase. Elegantly, if acetyl-CoA accumulates it both activates pyruvate carboxylase and inhibits pyruvate dehydrogenase, ensuring a supply of oxaloacetate. Lactate enters by oxidation to pyruvate and carboxylation to oxaloacetate; in ruminants, propionate enters as succinyl-CoA via the methylmalonyl-CoA pathway.

Conclusion

Because it functions in both oxidative and synthetic processes the cycle is amphibolic. Liver is the only tissue in which all these processes occur to a significant extent, which is why hepatic damage — acute hepatitis, cirrhosis — has such profound metabolic repercussions.

Marking guide: final common pathway stated with the reason 1 · carbohydrate: entry via PDH and exit via PEP carboxykinase 1.5 · lipid: β-oxidation in, and the citrate/ATP-citrate lyase route out 2 · amino acids: transamination both ways, with at least two groups of amino acids named 2 · the term amphibolic used correctly 0.5 · anaplerotic replenishment with pyruvate carboxylase 1.

2 What is the citric acid cycle? How many ATP are produced in one turn of the cycle? 5′ — TMU study question

Definition

The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) is a series of reactions in mitochondria that oxidise acetyl residues (as acetyl-CoA) and reduce coenzymes that, upon reoxidation, are linked to the formation of ATP.

Its enzymes are located in the mitochondrial matrix, free or attached to the inner and crista membranes where the respiratory chain is also found. The process is aerobic, requiring oxygen as the final oxidant of the reduced coenzymes.

The reactions

Acetyl-CoA condenses with the four-carbon oxaloacetate to form the six-carbon citrate. By a series of dehydrogenations and decarboxylations, citrate is degraded, releasing reduced coenzymes and 2 CO₂, and regenerating oxaloacetate — which therefore plays a catalytic role.

Enzyme	Product	Yield
Citrate synthase	Citrate
Aconitase	Isocitrate
Isocitrate dehydrogenase	α-Ketoglutarate	NADH + CO₂
α-Ketoglutarate dehydrogenase	Succinyl-CoA	NADH + CO₂
Succinate thiokinase	Succinate	1 ATP — substrate level
Succinate dehydrogenase (= Complex II)	Fumarate	FADH₂
Fumarase	L-Malate
Malate dehydrogenase	Oxaloacetate	NADH

The ATP yield

Three molecules of NADH and one of FADH₂ are produced for each molecule of acetyl-CoA catabolised in one turn. Reoxidation of each NADH results in the formation of 2.5 ATP and of FADH₂ in 1.5 ATP. In addition, 1 ATP is formed by substrate-level phosphorylation at the succinate thiokinase step.

(3 × 2.5) + (1 × 1.5) + 1 = 10 ATP per turn

Note that only one of these ten is made by the cycle itself; the other nine come from the respiratory chain reoxidising the coenzymes the cycle reduced.

Marking guide: definition in the standard wording 1.5 · location in the mitochondrial matrix 0.5 · entry by condensation with oxaloacetate and regeneration of oxaloacetate 1 · 3 NADH + 1 FADH₂ + 1 substrate-level ATP identified 1 · arithmetic shown giving 10 ATP 1.