Krebs Cycle

Krebs Cycle

Q. 1

Among the many molecules of high-energy phosphate compounds formed as a result of the functioning of the citric acid cycle, one molecule is synthesized at the substrate level. In which of the following reactions
does this occur?
 A Citrate → α-ketoglutarate
 B α-ketoglutarate → succinate 
 C Succinate → fumarate
 D Fumarate → malate
Q. 1

Among the many molecules of high-energy phosphate compounds formed as a result of the functioning of the citric acid cycle, one molecule is synthesized at the substrate level. In which of the following reactions
does this occur?
 A Citrate → α-ketoglutarate
 B α-ketoglutarate → succinate 
 C Succinate → fumarate
 D Fumarate → malate
Ans. B

Explanation:

A molecule of guanosine triphosphate is synthesized from guanosine diphosphate and phosphate at the cost of

hydrolyzing succinyl CoA to succinate and CoA. This constitutes substrate-level phosphorylation, and, in contrast to oxidative phosphorylation, this is the only reaction in the citric acid cycle that directly yields a high-energy phosphate bond. The sequence of reactions from alpha-ketoglutarate to succinate is catalyzed by the α -ketoglutarate dehydrogenase complex and succinyl-CoA synthetase, respectively.


Q. 2

Substrate level phosphorylation in TCA cycle is in step?

 A Isocitrate dehydrogenase
 B

Malate dehydrogenase

 C Aconitase
 D

Succinate thiokinase

Q. 2

Substrate level phosphorylation in TCA cycle is in step?

 A Isocitrate dehydrogenase
 B

Malate dehydrogenase

 C Aconitase
 D

Succinate thiokinase

Ans. D

Explanation:

Succinate thiokinase REF: Murray pp 130-135, Scriver, pp 1521-1552, http://en.wikipedia.org/wiki/Substratelevel

Substrate-level phosphorylation is a type of metabolism that results in the formation and creation of adenosine triphosphate (ATP) or guanosine triphosphate (GTP) by the direct transfer and donation of a phosphoryl (P03) group to adenosine diphosphate (ADP) or guanosine diphosphate (GDP) from a phosphorylated reactive intermediate. Note that the phosphate group does not have to directly come from the substrate. By convention, the phosphoryl group that is transferred is referred to as a phosphate group.

Once the pyruvate product of glycolysis is moved to the mitochondrial matrix, the pyruvate is converted to acetate and binds Coenzyme A to form Acetyl CoA to enter the Krebs cycle. While the Krebs cycle is oxidative respiration, one more instance of substrate-level phosphorylation occurs as Guanosine triphosphate (GTP) is created from GDP by transfer of a phosphate group during the conversion of Succinyl CoA to Succinate. This phosphate is transferred to ADP in another substrate-level phosphorylation event. The reaction is catalyzed by the enzyme Succinyl­CoA synthetase.

A molecule of guanosine triphosphate is synthesized from guanosine diphosphate and phosphate at the cost of hydrolyzing succinyl-CoA to succinate and CoA. This constitutes substrate-level phosphorylation, and, in contrast to oxidative phosphorylation, this is the only reaction in the citric acid cycle that directly yields a high-energy phosphate bond. The sequence of reactions from a ketoglutarate to succinate is catalyzed by the a ketoglutarate dehydrogenase complex and succinyl-CoA synthetase, respectively. Succinate thiokinase is the only enzyme that generates ATP directly by substrate-level phosphorylation.

a ketoglutarate + NAD’ + acetyl-CoA            succinyl-CoA + CO2 + NADH

succinyl-CoA + Pi + GDP           succinate + GTP + acetyl-CoA


Q. 3

Thiamine deficiency in a chronic alcoholic can reduce energy production as a result of which of the following reasons?

 A

It is required for the process of transamination

 B

It is a co-factor for oxidative reduction

 C

It is a co-enzyme for transketolase in pentose phosphate pathway

 D

It is a co-enzyme for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in TCA pathway

Q. 3

Thiamine deficiency in a chronic alcoholic can reduce energy production as a result of which of the following reasons?

 A

It is required for the process of transamination

 B

It is a co-factor for oxidative reduction

 C

It is a co-enzyme for transketolase in pentose phosphate pathway

 D

It is a co-enzyme for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in TCA pathway

Ans. D

Explanation:

Thiamine is a co-enzyme for pyruvate dehydrogenase and α-ketoglutarate dehydrogenase in the TCA pathway. Both the steps of the TCA cycle have thiamine requiring enzymes are required for the aerobic generation of ATP through krebs cycle. So thiamine deficiency can result in decreased energy production.

Ref: Textbook of biochemistry by DM Vasudevan, 3rd Edition, Page 268

Quiz In Between


Q. 4

Number of vitamins required in citric acid cycle is:

 A

2

 B

3

 C

4

 D

5

Q. 4

Number of vitamins required in citric acid cycle is:

 A

2

 B

3

 C

4

 D

5

Ans. C

Explanation:

Four of the B vitamins are essential in the citric acid cycle and hence energy-yielding metabolism:
 
(1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor for succinate dehydrogenase;
 
(2) niacin, in the form of nicotinamide adenine dinucleotide (NAD), the electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase;
 
(3) thiamin (vitamin B1), as thiamin diphosphate, the coenzyme for decarboxylation in the α-ketoglutarate dehydrogenase reaction; and
 
(4) pantothenic acid, as part of coenzyme A, the cofactor attached to “active” carboxylic acid residues such as acetyl-CoA and succinyl-CoA
Ref: Harper 28th edition, chapter 17.

 


Q. 5

All of the following vitamins are required in citric acid cycle, EXCEPT:

 A

Riboflavin

 B

Niacin

 C

Thiamin

 D

Ascorbic acid

Q. 5

All of the following vitamins are required in citric acid cycle, EXCEPT:

 A

Riboflavin

 B

Niacin

 C

Thiamin

 D

Ascorbic acid

Ans. D

Explanation:

Four of the B vitamins are essential in the citric acid cycle and hence energy-yielding metabolism: (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor for succinate dehydrogenase; (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD), the electron acceptor for isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase; (3) thiamin (vitamin B1), as thiamin diphosphate, the coenzyme for decarboxylation in the α-ketoglutarate dehydrogenase reaction; and (4) pantothenic acid, as part of coenzyme A, the cofactor attached to “active” carboxylic acid residues such as acetyl-CoA and succinyl-CoA
 
Ref: Harper 28th edition, chapter 17

Q. 6

Heart muscle is critically dependent on the citric acid cycle for the provision of energy during aerobic metabolism. Which of the following statements about the citric acid cycle are correct?

 A

The citric acid cycle is acting at its fastest rate when energy-rich intermediates like ATP and NADH are at their highest concentrations

 B

The rate limiting step is catalyzed by pyruvate carboxylase

 C

The formation of citrate from acetyl CoA and oxaloacetate is irreversible

 D

Carbon dioxide is formed at the steps catalyzed by malate dehydrogenase and by succinate dehydrogenase

Q. 6

Heart muscle is critically dependent on the citric acid cycle for the provision of energy during aerobic metabolism. Which of the following statements about the citric acid cycle are correct?

 A

The citric acid cycle is acting at its fastest rate when energy-rich intermediates like ATP and NADH are at their highest concentrations

 B

The rate limiting step is catalyzed by pyruvate carboxylase

 C

The formation of citrate from acetyl CoA and oxaloacetate is irreversible

 D

Carbon dioxide is formed at the steps catalyzed by malate dehydrogenase and by succinate dehydrogenase

Ans. C

Explanation:

The equilibrium for the condensation of acetyl CoA and oxaloacetate lies strongly in the direction of citrate.

 
NADH is a feedback control regulator of both isocitrate dehydrogenase and alpha ketoglutarate dehydrogenase. Pyruvate carboxylase is not a citric acid cycle enzyme, although it produces oxaloacetate, a TCA cycle component.  
 
There are three rate limiting enzymes in the TCA cycle: citrate synthase, isocitrate dehydrogenase, and alpha ketoglutarate dehydrogenase. Both malate and succinate are four carbon intermediates, indicating that the two carbons introduced by acetyl CoA have already been lost as CO2. Carbon dioxide is formed at the steps catalyzed by isocitrate dehydrogenase and alpha ketoglutarate dehydrogenase.

Quiz In Between


Q. 7

The citric acid cycle is a hub of energy transformation. Which citric acid cycle enzyme produces ATP by substrate level phosphorylation?

 A

Aconitase

 B

Succinate dehydrogenase

 C

Isocitrate dehydrogenase

 D

Succinate thiokinase

Q. 7

The citric acid cycle is a hub of energy transformation. Which citric acid cycle enzyme produces ATP by substrate level phosphorylation?

 A

Aconitase

 B

Succinate dehydrogenase

 C

Isocitrate dehydrogenase

 D

Succinate thiokinase

Ans. D

Explanation:

Succinate thiokinase uses the energy from removal of CoA from succinyl CoA to phosphorylate GDP to GTP.

Aconitase isomerizes citrate to isocitrate. Succinate dehydrogenase produces FADH2, but not ATP, by oxidizing succinate to fumarate.
 
Isocitrate dehydrogenase generates NADH, but not ATP, by oxidizing isocitrate to -ketoglutarate.

 


Q. 8

Substrate level phosphorylation occur in step catalysed by which of the following enzyme in TCA cycle?

 A

Isocitrate dehydrogenase

 B

Malate dehydrogenase

 C

Aconitase

 D

Succinate thiokinase

Q. 8

Substrate level phosphorylation occur in step catalysed by which of the following enzyme in TCA cycle?

 A

Isocitrate dehydrogenase

 B

Malate dehydrogenase

 C

Aconitase

 D

Succinate thiokinase

Ans. D

Explanation:

Succinate thiokinase is the enzyme that generates ATP directly by substrate-level phosphorylation. In Krebs cycle succinate thiokinase catalyze the conversion of Succinyl CoA into succinate. In this step GDP is phosphorylated to GTP. GTP can then be converted to ATP by reacting with an ADP molecule. 
 
Substrate-level phosphorylation is a type of metabolism that results in the formation and creation of adenosine triphosphate (ATP) or guanosine triphosphate (GTP) by the direct transfer and donation of a phosphoryl (P03) group to adenosine diphosphate (ADP) or guanosine diphosphate (GDP) from a phosphorylated reactive intermediate.
 
In glycolysis substrate level phosphorylation occur in two steps:
Conversion of 1,3 BPG to 3 Phosphoglycerate catalyzed by Phosphoglycerate kinase
Conversion of Phosphoenolpyruvate to pyruvate catalyzed by Pyruvate kinase
 
Ref: Textbook of Biochemistry By D M Vasudevan, 3rd Edition, Page 195

 


Q. 9

Which one of the following TCA cycle intermediate may not be added or removed by other metabolic pathways?

 A

Oxaloacetate

 B

Isocitrate

 C

Citrate

 D

Fumarate

Q. 9

Which one of the following TCA cycle intermediate may not be added or removed by other metabolic pathways?

 A

Oxaloacetate

 B

Isocitrate

 C

Citrate

 D

Fumarate

Ans. B

Explanation:

Oxaloacetate may be  formed from pyruvate during gluconeogenesis.

Degradation of tyrosine and phenylalanine may produce fumarate.

Citrate is transported out of mitochondria to be used as a source of cytoplasmic acetyl CoA. Isocitrate is formed and removed in TCA cycle.

Ref: Textbook of Biochemistry for Medical Students by D.M. Vasudevan, 6th Edition, Page: 218

Quiz In Between


Q. 10

The citric acid cycle is inhibited by which of the following?

 A

Fluoroacetate

 B

Fluorouracil

 C

Arsenic

 D

Aerobic conditions

Q. 10

The citric acid cycle is inhibited by which of the following?

 A

Fluoroacetate

 B

Fluorouracil

 C

Arsenic

 D

Aerobic conditions

Ans. A

Explanation:

Fluoroacetate can be converted to fluorocitrate, which is an inhibitor of aconitase. Arsenic is not a direct inhibitor, but arsenite is an inhibitor of lipoic acid–containing enzymes such as α-ketoglutarate dehydrogenase. Malonate, not malic acid, is an inhibitor of succinate dehydrogenase. The citric acid cycle requires oxygen and would be inhibited by anaerobic, not aerobic, conditions. Fluorouracil is a suicide inhibitor of thymidylate synthase and blocks deoxythymidylate synthesis. 

Ref: Bender D.A., Mayes P.A. (2011). Chapter 17. The Citric Acid Cycle: The Catabolism of Acetyl-CoA. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

Q. 11

Citric acid cycle is an example of:

 A

Anabolic pathway

 B

Catabolic pathway

 C

Amphibolic pathway

 D

None of the above

Q. 11

Citric acid cycle is an example of:

 A

Anabolic pathway

 B

Catabolic pathway

 C

Amphibolic pathway

 D

None of the above

Ans. C

Explanation:

Amphibolic pathways, which occur at the “crossroads” of metabolism, acting as links between the anabolic and catabolic pathways, eg, the citric acid cycle.
 
Ref: Harper 28th edition, chapter 16.

Q. 12

Which one of the following statements concerning the citric acid cycle is correct?

 A

The cycle produces three moles of NADH and one mole of FADH2 per mole of acetyl CoA oxidized to CO2 and H2O

 B

The cycle is inhibited by malate

 C

Elevated levels of NADH stimulate oxidation of acetyl CoA by the cycle

 D

The oxidation of acetate leads to a net consumption (loss) of oxaloacetate

Q. 12

Which one of the following statements concerning the citric acid cycle is correct?

 A

The cycle produces three moles of NADH and one mole of FADH2 per mole of acetyl CoA oxidized to CO2 and H2O

 B

The cycle is inhibited by malate

 C

Elevated levels of NADH stimulate oxidation of acetyl CoA by the cycle

 D

The oxidation of acetate leads to a net consumption (loss) of oxaloacetate

Ans. A

Explanation:

The cycle is inhibited by malonate, not malate, which is an intermediate of the cycle.

Elevated levels of NADH inhibit the oxidation of acetyl CoA by the cycle. The oxidation of acetate neither consumes or produces a change in oxaloacetate. 

Ref: Bender D.A., Mayes P.A. (2011). Chapter 17. The Citric Acid Cycle: The Catabolism of Acetyl-CoA. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

 

Quiz In Between


Q. 13

Which of the following reaction is an example of substrate level phosphorylation occurring in TCA cycle?

 A

Isocitrate to oxalosuccinate

 B

Alpha keto glutarate to succinyl CoA

 C

Succinyl CoA to succinate

 D

Succinate to fumarate

Q. 13

Which of the following reaction is an example of substrate level phosphorylation occurring in TCA cycle?

 A

Isocitrate to oxalosuccinate

 B

Alpha keto glutarate to succinyl CoA

 C

Succinyl CoA to succinate

 D

Succinate to fumarate

Ans. C

Explanation:

In Krebs cycle, succinyl CoA is converted to succinate by the enzyme succinate thiokinase. This is the only example of substrate level phosphorylation in the citric acid cycle. 
 
Substrate level phosphorylation is the biochemical process whereby ATP is produced from ADP through direct transfer of a high energy phosphoryl group from a reaction substrate to ADP. 
 
In glycolysis, substrate level phosphorylation occur during two reactions:
  • Conversion of 1,3 BPG to 3 phosphoglycerate by phosphoglyerate kinase
  • Conversion of phosphoenol pyruvate to pyruvate by pyruvate kinase.
Ref: Bender D.A., Mayes P.A. (2011). Chapter 17. The Citric Acid Cycle: The Catabolism of Acetyl-CoA. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

 


Q. 14

In all of these pathways ATP is produced, EXCEPT:

 A

Urea cycle

 B

Electron transport chain

 C

TCA cycle

 D

Anaerobic glycolysis

Q. 14

In all of these pathways ATP is produced, EXCEPT:

 A

Urea cycle

 B

Electron transport chain

 C

TCA cycle

 D

Anaerobic glycolysis

Ans. A

Explanation:

The urea cycle utilises 4 ATPs. 2 ATPs are needed to make carbamoyl phosphate, one is needed to make argininosuccinate and one is needed to restore AMP to ATP.  Thus urea cycle does not produce ATP it only utilises ATP.
 
The net yield of ATP from anaerobic glycolysis is 2. TCA cycle generate 3 NADH2, 2 FADH2 and 1 GTP. Hence, each TCA cycle yields 12 ATP molecules. As two acetyl CoA molecules are generated from each glucose this cycle occur twice yielding 24 ATP molecules.  
 
Complete oxidation of glucose via glycolysis, pyruvate dehydrogenase, Krebs cycle, and the oxidative phosphorylation pathway yields 38 ATP. Since two ATPs were used during stage I reactions of glycolysis the net yield is 36 ATP.
 
Ref: Textbook of Medical Biochemistry,3e By Dinesh Puri PAGE 175. Essentials of Biochemistry By Pankaja Naik Page 233.

Q. 15

In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. The key intermediate enzymic step involves:

 A

The decarboxylation of a hydroxy acid

 B

The decarboxylation of an alpha-ketoacid

 C

The decarboxylation of a beta-ketoacid

 D

The oxidation of a keto group to a carboxylic acid

Q. 15

In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. The key intermediate enzymic step involves:

 A

The decarboxylation of a hydroxy acid

 B

The decarboxylation of an alpha-ketoacid

 C

The decarboxylation of a beta-ketoacid

 D

The oxidation of a keto group to a carboxylic acid

Ans. B

Explanation:

The enzyme is a-ketoglutarate dehydrogenase, an a-keto acid, which is decarboxylated. Therefore the second choice is correct. The only confusion might be with the last choice, since in effect it involves the oxidation of a keto group to a carboxylic acid. However, the product is the thio-ester of a carboxylic acid.

Quiz In Between


Q. 16

TCA cycle does not take place in:

 A

Hepatocyte

 B

Osteocyte

 C

Neuron

 D

Erythrocyte

Q. 16

TCA cycle does not take place in:

 A

Hepatocyte

 B

Osteocyte

 C

Neuron

 D

Erythrocyte

Ans. D

Explanation:

D i.e. Erythrocytes

Erythrocytes (RBC) lack mitochondria (so TCA cycle enzymes) and completely rely on glucose as their metabolic fuel. They metabolize it by anaerobic glycolysisQ.


Q. 17

Mature RBC contains all except –

 A

Enzyme of HMP shunt pathway

 B

Enzyme of TCA cycle

 C

Glycoytic enzymes

 D

Pyridine nubleotides

Q. 17

Mature RBC contains all except –

 A

Enzyme of HMP shunt pathway

 B

Enzyme of TCA cycle

 C

Glycoytic enzymes

 D

Pyridine nubleotides

Ans. B

Explanation:

Ans:B .) Enzymes of TCA cycle

Erythrocytes (RBC) lack mitochondria (so TCA cycle enzymes) and completely rely on glucose as their metabolic fuel. They metabolize it by anaerobic glycolysis.


Q. 18

In TCA cycle of tricarboxylic acid, which is first formed

 A

Isocitrate

 B

Citrate

 C

Succinate

 D

Fumarate

Q. 18

In TCA cycle of tricarboxylic acid, which is first formed

 A

Isocitrate

 B

Citrate

 C

Succinate

 D

Fumarate

Ans. B

Explanation:

B i.e. Citrate

Quiz In Between


Q. 19

In TCA cycle, citrate is converted into, after losing a molecul of H20 –

 A

Isocitrate

 B

Cis-aconitate

 C

Oxaloacelate

 D

Glutarate

Q. 19

In TCA cycle, citrate is converted into, after losing a molecul of H20 –

 A

Isocitrate

 B

Cis-aconitate

 C

Oxaloacelate

 D

Glutarate

Ans. B

Explanation:

B i.e. Cis aconitate


Q. 20

Which of following inhibitor in TCA cycle acts by blocking citrate –

 A

Fluroacetate

 B

Arsenite

 C

Malonate

 D

None of the above

Q. 20

Which of following inhibitor in TCA cycle acts by blocking citrate –

 A

Fluroacetate

 B

Arsenite

 C

Malonate

 D

None of the above

Ans. A

Explanation:

A i.e. Fluroacetate


Q. 21

Intermediate metabolite in TCA cycle are A/E:

 A

Pvruvate

 B

Malonate

 C

Nitric oxide

 D

All of the above

Q. 21

Intermediate metabolite in TCA cycle are A/E:

 A

Pvruvate

 B

Malonate

 C

Nitric oxide

 D

All of the above

Ans. D

Explanation:

All of the above

Quiz In Between


Q. 22

Which of the following substance binds to acetyl CoA and condenses OAA, inhibiting the TCA cycle:

 A

Arsenite

 B

Fluoroacetate

 C

Malonate

 D

Fumararte

Q. 22

Which of the following substance binds to acetyl CoA and condenses OAA, inhibiting the TCA cycle:

 A

Arsenite

 B

Fluoroacetate

 C

Malonate

 D

Fumararte

Ans. B

Explanation:

B i.e. Fluoroacetate


Q. 23

In TCA cycle, CO2 is released from:

 A

Thiokinase

 B

Isocitrate dehydrogenase

 C

Citrate dehydrogenase

 D

Alpha ketoglutrate

Q. 23

In TCA cycle, CO2 is released from:

 A

Thiokinase

 B

Isocitrate dehydrogenase

 C

Citrate dehydrogenase

 D

Alpha ketoglutrate

Ans. B

Explanation:

B i.e. Isocitrate dehydrogenase

2 molecules of carbon dioxide are released in TCA cycle. One before and one after a – ketoglatarate. Enzymes used are – isocitrate dehydrogenase and a ketoglutarate dehydrogenaseQ respectively.


Q. 24

. Proper functioning of Kreb cycle does not occur with deficiency of:

 A

Thiamane

 B

Riboflavin

 C

Vitamin E

 D

Vit K

Q. 24

. Proper functioning of Kreb cycle does not occur with deficiency of:

 A

Thiamane

 B

Riboflavin

 C

Vitamin E

 D

Vit K

Ans. A

Explanation:

A i.e. Thiamine; B i.e. Riboflavin

Quiz In Between


Q. 25

Which among the following controls is an allosteric inhibitor of TCA cycle:

 A

Pyruvate dehydrogenase

 B

Keto glutarate dehydrogenase

 C

Isocitrate dehydrogenase

 D

Malate dehydrogenase

Q. 25

Which among the following controls is an allosteric inhibitor of TCA cycle:

 A

Pyruvate dehydrogenase

 B

Keto glutarate dehydrogenase

 C

Isocitrate dehydrogenase

 D

Malate dehydrogenase

Ans. C

Explanation:

C i.e. Isocitrate dehydrogenase

The enzyme of TCA cycle having allosteric inhibitor is isocitrate dehydrogenaseQ.

The allosteric activator is ADP and allosteric inhibitor is ATP and long chain aceyl COA


Q. 26

In vivo control of citric acid cycle is effected by:

 A

Acetyl CoA

 B

Coenzyme A

 C

ATP

 D

Citrate

Q. 26

In vivo control of citric acid cycle is effected by:

 A

Acetyl CoA

 B

Coenzyme A

 C

ATP

 D

Citrate

Ans. C

Explanation:

C i.e. ATP, E i.e. NADH


Q. 27

Of the following groups of amino acids produce common component of the TCA cycle is:

 A

Alaine, isoleucine, leucine, lysine

 B

Serine, asparagines, glycine, glutamate

 C

Isoleucine, valine, methionine

 D

Prolone, leucine, tryptophan

Q. 27

Of the following groups of amino acids produce common component of the TCA cycle is:

 A

Alaine, isoleucine, leucine, lysine

 B

Serine, asparagines, glycine, glutamate

 C

Isoleucine, valine, methionine

 D

Prolone, leucine, tryptophan

Ans. C

Explanation:

C i.e. Isoleucine, Valine, methionine

Valine, Isoleucine and MethionineQ produce succinyl COA which is an important component of TCA cycle.

Quiz In Between


Q. 28

Products formed from alcohol and not intermediates of TCA cycle/glycolysis:

 A

Acetaldehyde

 B

Pyruvate

 C

Lactate

 D

Oxalate

Q. 28

Products formed from alcohol and not intermediates of TCA cycle/glycolysis:

 A

Acetaldehyde

 B

Pyruvate

 C

Lactate

 D

Oxalate

Ans. A

Explanation:

A i.e. Acetyldehyde


Q. 29

Which end product of citric acid cycle is used in detoxification of ammonia in brain?

 A

Oxaloacetate

 B

Alphaketoglutarate

 C

Succinate

 D

Citrate

Q. 29

Which end product of citric acid cycle is used in detoxification of ammonia in brain?

 A

Oxaloacetate

 B

Alphaketoglutarate

 C

Succinate

 D

Citrate

Ans. B

Explanation:

Q. 30

Hyperammonaemia inhibit TCA cycle by depleting:

 A

Oxaloacetate

 B

a-ketoglutarate

 C

Citrate

 D

Succinyl Co-A

Q. 30

Hyperammonaemia inhibit TCA cycle by depleting:

 A

Oxaloacetate

 B

a-ketoglutarate

 C

Citrate

 D

Succinyl Co-A

Ans. B

Explanation:

 

Hyper ammonaemia inhibit TCA cycle by depleting a-keto glutarate

Quiz In Between


Q. 31

Aminoacid that enters the TCA cycle for gluconeogenesis & Ketogenin in nature –

 A

Pheylalanine

 B

Alanine

 C

Glycine

 D

Serine

Q. 31

Aminoacid that enters the TCA cycle for gluconeogenesis & Ketogenin in nature –

 A

Pheylalanine

 B

Alanine

 C

Glycine

 D

Serine

Ans. A

Explanation:

Q. 32

All are components of Citric acid cycle EXCEPT:

 A

Fumarase

 B

Malonate

 C

Succinate dehydrogenase

 D

Alpha-ketoglutarate dehydrogenase

Q. 32

All are components of Citric acid cycle EXCEPT:

 A

Fumarase

 B

Malonate

 C

Succinate dehydrogenase

 D

Alpha-ketoglutarate dehydrogenase

Ans. B

Explanation:

 

Malonate competitively inhibits succinate dehydrogenase, which converts succinate to fumarate


Q. 33

Substrate level phosphorylation in TCA cycle involves:          

 A

lsocitrate dehydrogenase

 B

Fumarase

 C

Malate dehydrogenase

 D

Succinate thiokinase

Q. 33

Substrate level phosphorylation in TCA cycle involves:          

 A

lsocitrate dehydrogenase

 B

Fumarase

 C

Malate dehydrogenase

 D

Succinate thiokinase

Ans. D

Explanation:

Quiz In Between


Q. 34

Substrate level phosphorylation in citric acid cycle is catalysed by ‑

 A

Pyruvate kinase

 B

Phosphoglycerate kinase

 C

Malate dehydrogenase

 D

Succinate thiokinase

Q. 34

Substrate level phosphorylation in citric acid cycle is catalysed by ‑

 A

Pyruvate kinase

 B

Phosphoglycerate kinase

 C

Malate dehydrogenase

 D

Succinate thiokinase

Ans. D

Explanation:

 

There are 2 types of phosphorylation of ADP to ATP.

1) Substrate level phosphorylation

2) Oxidative phosphorylation at respiratory chain level

Substrate level phosphorylation is the production of ATP at the substrate level without participation of ETC.

It occurs in TCA cycle and glycolysis


Q. 35

In citric acid cycle, NADH is produced by

 A

Succinate thiokinase

 B

 Succinate dehydrogenase

 C

Isocitrate dehydrogenase

 D

Fumarase

Q. 35

In citric acid cycle, NADH is produced by

 A

Succinate thiokinase

 B

 Succinate dehydrogenase

 C

Isocitrate dehydrogenase

 D

Fumarase

Ans. C

Explanation:

 

NADH is produced and CO2 is liberated at 3 steps :

i) Conversion of isocitrate to a-ketoglutarate by isocitrate dehydrogenase

ii) Conversion of a-ketoglutarate to succinyl CoA by oc-ketoglutarate dehydrogenase

iii) Conversion of L-malate to oxaloacetate by malate dehydrogenase.


Q. 36

Which of the following glucogenic amino acid enters the citric acid cycle at succinyl CoA

 A

Phenylalanine

 B

Tyrosine

 C

Isoleucine

 D

Tryptophan

Q. 36

Which of the following glucogenic amino acid enters the citric acid cycle at succinyl CoA

 A

Phenylalanine

 B

Tyrosine

 C

Isoleucine

 D

Tryptophan

Ans. C

Explanation:

Ans. is ‘c’ i.e., isoleucine

Quiz In Between


Q. 37

Number of ATP generated in one TCA cycle ‑

 A

2

 B

8

 C

10

 D

11

Q. 37

Number of ATP generated in one TCA cycle ‑

 A

2

 B

8

 C

10

 D

11

Ans. C

Explanation:

Ans. is ‘c’ i.e., 10

  • In a single TCA cycle 10 molecules of ATP are produced (12 molecules according to older calculations).
  • One turn of the TCA cycle, starting with acetyl CoA produces 10 ATPs. When the starting molecule is pyruvate, the oxidative decarboxylation of pyruvate, the oxidative decarboxylation of pyruvate yields 2.5 ATPs and therefore, 12.5 ATPs are produced when starting compound is pyruvate. Since, two molecules of pyruvate enter the TCA cycle when glucose is metabolized (glycolysis produces 2 molecules of pyruvate), the number of ATPs is doubled. Therefore, 25 ATP molecules, per glucose molecule, are produced when pyruvate enters the TCA cycle.
  • Note : Previously calculations were made assuming that NADH produces 3 ATPs and FADH generates 2 ATPs. This will amount a net generation of 30 ATP molecules in TCA per molecule glucose and total 38 molecules from starting. Recent experiments show that these values are overestimates and NADH produces 2.5 ATPs and FADH produces 1.5 ATPs. Therefore, net generation during TCA is 25 ATPs and complete oxidation of glucose through glycolysis plus citric acid cycle yield a net 32 ATPs.
  • Energy yield (number of ATP generated) per molecule of glucose when it is completely oxidized through glycoly­sis plus citric acid cycle, under aerobic conditions, is as follows :-

Q. 38

Two carbon atoms leave in the form of CO2 in TCA cycle are derived from ‑

 A

Oxaloacetate

 B

Acetyl CoA

 C

Succinyl CoA

 D

fumerate

Q. 38

Two carbon atoms leave in the form of CO2 in TCA cycle are derived from ‑

 A

Oxaloacetate

 B

Acetyl CoA

 C

Succinyl CoA

 D

fumerate

Ans. B

Explanation:

Ans. is ‘b’ i.e., Acetyl CoA

TCA cycle consists of sequential reactions. It begins with condensation of four carbon oxaloacetate molecule with 2 carbon acetyl CoA molecule to form six carbon citrate molecule. In subsequent reaction two carbon atoms are lost in the form of CO2 at two different reactions. A series of modifications occur in the remaining four carbon atoms to ultimately form oxaloacetate. Thus the last substrate (oxaloacetate) of one cycle is ready for use as a substrate in the next cycle. In this way, there is no net generation of OAA, or of any of the cycle intermediates.

So, two carbon atoms enter the cycle as acetyl CoA (and condense with oxaloacetate) and two carbons leave in the form of two molecules of CO2. Thus, TCA cycle basically involves oxidation of acetyl CoA to CO2 and as such there is no net consumption or regeneration of oxaloacetate or any of the other cycle intermediates.


Q. 39

Allosteric inhibitors of TCA cycle are all except‑

 A

ADP

 B

ATP

 C

NADH

 D

Succinly CoA

Q. 39

Allosteric inhibitors of TCA cycle are all except‑

 A

ADP

 B

ATP

 C

NADH

 D

Succinly CoA

Ans. A

Explanation:

 

Regulation of TCA cycle

As the primary function of TCA cycle is to provide energy, respiratory control via the respiratory chain (ETC) and oxidative phosphorylation regulates citric acid cycle activity. Excess of ATP, NADH and succinyl-CoA which signals high energy status of the cell, inhibit TCA cycle. ADP and NAD+ stimulate the cycle as these signal low energy status. Regulation occurs at three allosteric enzyms by allosteric modulation :-

i) Citrate synthase :- It is inhibited by ATP, long-chain acyl-CoA, and succinyl CoA.

ii) Isocitrate dehydrogenase :- It is activated by ADP and is inhibited by ATP and NADH.

iii) α-ketoglutarate dehydrogenase complex :- It is inhibits by succinyl-CoA and NADH.

Quiz In Between


Q. 40

Unaltered final product of TCA cycle ‑

 A

Oxaloacetate

 B

Acetyl CoA

 C

CO2

 D

Pyrovate

Q. 40

Unaltered final product of TCA cycle ‑

 A

Oxaloacetate

 B

Acetyl CoA

 C

CO2

 D

Pyrovate

Ans. A

Explanation:

 

TCA cycle consists of sequential reactions. It begins with condensation of four carbon oxaloacetate molecule with 2 carbon acetyl CoA molecule to form six carbon citrate molecule.

In subsequent reaction two carbon atoms are lost in the form of CO2 at two different reactions.

A series of modifications occur in the remaining four carbon atoms to ultimately form oxaloacetate.

Thus the last substrate (oxaloacetate) of one cycle is ready for use as a substrate in the next cycle.

In this way, there is no net generation of OAA, or of any of the cycle intermediates.


Q. 41

Not an intermediate product of citric acid cycle is:

 A

Acyl Co-A

 B

Succinyl Co-A

 C

Citrate

 D

a-ketoglutarate

Q. 41

Not an intermediate product of citric acid cycle is:

 A

Acyl Co-A

 B

Succinyl Co-A

 C

Citrate

 D

a-ketoglutarate

Ans. A

Explanation:

Ans. a. Acetyl Co-A


Q. 42

Substrate level phosphorylation is seen in reaction catalyzed by which enzyme of citric acid cycle‑

 A

Pyruvate kinase

 B

Succinate thiokinase

 C

Phosphoglycerate kinase

 D

All of the above

Q. 42

Substrate level phosphorylation is seen in reaction catalyzed by which enzyme of citric acid cycle‑

 A

Pyruvate kinase

 B

Succinate thiokinase

 C

Phosphoglycerate kinase

 D

All of the above

Ans. B

Explanation:

Ans. is ‘b’ i.e., Succinate thiokinase 


Q. 43

Vitamin not required in TCA cycle ‑

 A

Niacin

 B

Riboflavin

 C

Thiamine

 D

Folic acid

Q. 43

Vitamin not required in TCA cycle ‑

 A

Niacin

 B

Riboflavin

 C

Thiamine

 D

Folic acid

Ans. D

Explanation:

Ans. is ‘d’ i.e., Folic acid

  • Four of the B vitamins are essential in the citric acid cycle :
  1. Riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor for succinate dehydrogenase.
  2. Niacin, in the form of nicotinamide adenine dinucleotide (NAD) the electron acceptor for isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, and malate dehydrogenase.
  3. Thiamine (vitamin B1), as thiamine diphosphate, the coenzyme for decarboxylation in a-ketoglutarate dehydrogenase reaction.
  4. Pantothenic acid, as part of coenzyme A, the cofactor attached to “active” carboxylic acid residues such as acetyl-CoA and succinyl CoA.

Quiz In Between



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