Citric Acid Cycle Quiz

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Questions and Answers

What is the primary role of pyruvate in cellular respiration?

It is a substrate for oxidative phosphorylation.

It is the end product of the citric acid cycle.

It acts as an energy-rich molecule.

It links glycolysis and the citric acid cycle. (correct)

Which stage of cellular respiration involves the oxidation of acetyl groups to carbon dioxide?

Electron transfer chain

Glycolysis

Citric acid cycle (correct)

Lactic acid fermentation

What does the breakdown of acetyl-CoA in the citric acid cycle primarily produce?

GTP and CO2 only

NADH, FADH2, and one GTP (correct)

NADH and ATP only

Only high-energy intermediates

Why is the citric acid cycle considered a 'hub' of metabolism?

It links catabolic and anabolic pathways. Signup and view all the answers

How does the body regulate the citric acid cycle?

By means of allosteric and covalent mechanisms Signup and view all the answers

Which of the following pathways is NOT an output of the citric acid cycle?

Glycolysis Signup and view all the answers

What is the significance of forming enzyme complexes in metabolism?

To accomplish reactions without intermediate liberation Signup and view all the answers

What could be a consequence of mutations affecting the citric acid cycle?

Tumor formation Signup and view all the answers

What is the main purpose of converting the methyl group of acetyl-CoA to a methylene group in the citric acid cycle?

To enhance the reactivity for further enzymatic reactions Signup and view all the answers

Which of the following statements about the citric acid cycle is incorrect?

Its sole function is energy conservation. Signup and view all the answers

What enzyme catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate?

Citrate synthase Signup and view all the answers

What characterizes the citrate synthase reaction in the citric acid cycle?

It involves the formation of citroyl-CoA as an intermediate. Signup and view all the answers

How does the binding of oxaloacetate affect citrate synthase?

It triggers a change that facilitates acetyl-CoA binding. Signup and view all the answers

Which of the following processes is NOT involved in the citrate synthase step of the cycle?

Formation of ATP Signup and view all the answers

In the context of the citric acid cycle, what does the term 'highly exergonic' refer to?

A reaction that releases energy Signup and view all the answers

Which compound is NOT produced during the first complete turn of the citric acid cycle?

Citroyl-CoA Signup and view all the answers

What is the primary function of the PDH complex in cellular metabolism?

To catalyze the oxidation of pyruvate through decarboxylation Signup and view all the answers

Which enzyme in the PDH complex is responsible for the decarboxylation of pyruvate?

Pyruvate dehydrogenase Signup and view all the answers

Which coenzyme is associated with lipoate in the PDH complex?

Coenzyme A (CoA-SH) Signup and view all the answers

Which of the following statements about the PDH complex is accurate?

It facilitates a series of chemical transformations without releasing intermediates. Signup and view all the answers

What role does lipoate play in the PDH complex?

It is a coenzyme that can be reversibly oxidized. Signup and view all the answers

Which vitamin is NOT a coenzyme involved in the PDH complex?

Vitamin C Signup and view all the answers

Why have enzymes evolved to form complexes like the PDH complex?

To perform reactions efficiently without releasing intermediates Signup and view all the answers

How many different coenzymes does the PDH complex utilize?

Five Signup and view all the answers

Which enzyme in the citric acid cycle is involved in substrate-level phosphorylation?

succinyl-CoA synthetase Signup and view all the answers

Which of the following reactions in the citric acid cycle produces FADH2?

Oxidation of succinate to fumarate Signup and view all the answers

Which statement is true regarding succinate dehydrogenase?

It is an integral membrane protein. Signup and view all the answers

Which competitive inhibitor is known to inhibit succinate dehydrogenase?

Malonate Signup and view all the answers

How does fumarase exhibit stereospecificity?

It exclusively catalyzes the trans double bond of fumarate. Signup and view all the answers

What is the primary function of L-malate dehydrogenase in the citric acid cycle?

To regenerate oxaloacetate from malate Signup and view all the answers

Which of the following statements regarding the PDH complex is incorrect?

It converts acetyl-CoA to citrate. Signup and view all the answers

What is the approximate ATP yield from the complete oxidation of glucose to CO2 through the citric acid cycle?

32 ATP Signup and view all the answers

What does the prefix amph- mean?

Both kinds Signup and view all the answers

Which key molecule is conserved in the glyoxylate cycle but not in the citric acid cycle?

Carbon dioxide Signup and view all the answers

How does the glyoxylate cycle differ from the citric acid cycle?

It does not involve decarboxylation steps. Signup and view all the answers

What is the function of anaplerotic reactions?

To replenish citric acid cycle intermediates. Signup and view all the answers

Which enzyme is NOT associated with anaplerotic reactions?

Acetyl-CoA carboxylase Signup and view all the answers

In what metabolic roles do acetate groups from acetyl-CoA participate?

In both catabolic and anabolic pathways Signup and view all the answers

What does the term 'cataplerotic reactions' refer to?

Reactions that deplete cycle intermediates Signup and view all the answers

What is a defining characteristic of strains of aerobic organisms regarding the citric acid cycle?

They use the citric acid cycle for both metabolic processes. Signup and view all the answers

Which enzymes are associated with tumors due to mutations?

Succinate dehydrogenase and fumarase Signup and view all the answers

What ability is gained by mutant NADPH-dependent isocitrate dehydrogenase?

Converting α-ketoglutarate to 2-hydroxyglutarate Signup and view all the answers

What are metabolons described as?

Integrated multienzyme complexes held together by noncovalent interactions Signup and view all the answers

Which intermediates play a role in the function of metabolons?

Metabolic intermediates that enhance enzyme activity Signup and view all the answers

What occurs within metabolons to ensure metabolic efficiency?

Efficient passage of the product of one enzyme to the next enzyme in the pathway Signup and view all the answers

Study Notes

Cellular Respiration

Cellular respiration is the process where pyruvate, produced by glycolysis, is further oxidized to form water (H₂O) and carbon dioxide (CO₂).

Stage 1 of Cellular Respiration

Stage 1 involves the oxidation of fuels to acetyl-CoA.
This stage generates ATP, NADH, and FADH₂.
Fuels include amino acids, fatty acids, and glucose.

Stage 2 of Cellular Respiration

Stage 2 involves the oxidation of acetyl groups to CO₂ in the citric acid cycle (tricarboxylic acid (TCA) cycle, Krebs cycle).
This pathway is nearly universal in cells.
It generates NADH, FADH₂, and GTP.

Stage 3 of Cellular Respiration

Stage 3 involves electron transfer chain and oxidative phosphorylation.
This stage generates the majority of ATP from catabolism.

Principle 1

Pyruvate links glycolysis and the citric acid cycle.
Pyruvate's rate of partitioning affects catabolic activity.

Principle 2

Citric acid cycle reactions follow a chemical logic.
The cycle oxidizes acetyl-CoA to CO₂ and H₂O.
The cycle's energy drives ATP synthesis.
Strategies for activating groups for oxidation and conserving energy are used in other metabolic pathways.

Principle 3

The citric acid cycle is a hub of metabolism.
Catabolic pathways lead into it, while anabolic pathways lead out.
Acetate groups are used in synthesizing amino acids, fatty acids, and sterols.
Nucleotides and amino acid breakdown products are intermediates in the cycle.

Principle 4

The citric acid cycle's central role in metabolism requires regulation.
Regulation occurs through allosteric and covalent mechanisms.
These mechanisms maintain homeostasis.
Some mutations in cycle reactions can cause tumor formation.

Principle 5

Enzymes form complexes to efficiently transform intermediates without releasing them into the bulk solvent.
This strategy is ubiquitous in other metabolic pathways, respiration, and informational macromolecules.

Production of Acetyl-CoA

Coenzyme A (CoA-SH) is a critical acyl carrier.
The -SH group forms a thioester with acetate in acetyl-CoA.

Pyruvate Oxidation

Mitochondrial pyruvate carrier (MPC) facilitates pyruvate transport into the mitochondrial membrane.
Pyruvate dehydrogenase (PDH) complex oxidizes pyruvate to acetyl-CoA and CO₂ in the mitochondrial matrix.
The chemical intermediates remain bound to the enzyme subunits.
Regulation results in precisely controlled flux.

PDH Complex

The PDH complex employs three enzymes (E₁, E₂, E₃) and five coenzymes.
These are thiamine pyrophosphate (TPP), lipoate, coenzyme A (CoA), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide (NAD).
Lipoate acts as an electron (hydrogen) carrier and acyl carrier.
The complex is a highly organized cluster in the mitochondrial matrix.

PDH Complex Mechanisms

Oxidative decarboxylation is an irreversible process where the carboxyl group is removed, resulting in the formation of CO₂.
This process involves steps 1 and 2.
Steps 3—5 involve dihydrolipoyl transacetylase and dehydrogenase.

Citric Acid Cycle Reactions

One oxaloacetate molecule can theoretically oxidize an infinite number of acetyl groups.
Energy from the four oxidations is conserved as NADH and FADH₂.

Formation of Citrate

Citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to form citrate.
Citroyl-CoA is an intermediate.
A large, negative ∆G'° is needed due to the typically low concentration of oxaloacetate.

Citrate Synthase Structure

Oxaloacetate binding creates a binding site for acetyl-CoA.
Induced fit decreases the likelihood of premature cleavage of the thioester bond of acetyl-CoA.

Mechanism of Citrate Synthase

His 274 and Asp375 are acid-base catalysts in this step.

Oxidation of Isocitrate

Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate.

Oxidation of α-Ketoglutarate

α-ketoglutarate dehydrogenase complex catalyzes the oxidative decarboxylation of α-ketoglutarate to succinyl-CoA.

Conversion of Succinyl-CoA

Succinyl-CoA synthetase (succinic thiokinase) catalyzes the breakage of the thioester bond of succinyl-CoA to form succinate.
Energy release drives GTP or ATP synthesis (substrate-level phosphorylation).

Succinyl-CoA Synthetase Reaction

The enzyme molecule becomes phosphorylated at a His residue in the active site.
The phosphoryl group is transferred to ADP or GDP to form ATP or GTP.
Animal cells contain different isozymes.

Nucleoside Diphosphate Kinase

Nucleoside diphosphate kinase catalyzes reversible conversion of GTP and ATP.

Succinate Oxidation

Succinate dehydrogenase, a flavoprotein, catalyzes the reversible oxidation of succinate to fumarate.
It is an integral protein of the mitochondrial inner membrane and contains iron-sulfur clusters and FAD.

Malate Oxidation

L-malate dehydrogenase catalyzes the oxidation of L-malate to oxaloacetate, coupled to the reduction of NAD+.

Citric Acid Cycle Regulation

Regulation balances the supply of key intermediates with energy production and biosynthetic processes.
Regulation occurs at the PDH complex, citrate synthase, isocitrate dehydrogenase complex, and α-ketoglutarate dehydrogenase complex.

Citric Acid Cycle and Tumours

Mutations in the PDH complex or citric acid cycle enzymes can be oncogenic.
Oncometabolites stimulate tumor growth.

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Description

Test your knowledge on the citric acid cycle, a crucial process in cellular respiration. This quiz covers the roles of pyruvate, enzymatic reactions, metabolic regulation, and the significance of enzyme complexes. Assess your understanding of this metabolic hub and its various functions.