Comprehensive Guide to the Citric Acid Cycle and Aerobic Metabolism

What is aerobic metabolism?

The process by which nutrients are oxidized to carbon dioxide and water, yielding more energy than anaerobic oxidation.

How many ATP molecules can be produced from one glucose molecule during complete aerobic oxidation?

30 to 32 ATP molecules.

What are the three main processes involved in aerobic metabolism?

The citric acid cycle, electron transport, and oxidative phosphorylation.

What is the citric acid cycle also known as?

The Krebs cycle or the tricarboxylic acid (TCA) cycle.

Where does glycolysis occur in eukaryotic cells?

In the cytosol.

Where does the citric acid cycle take place in eukaryotic cells?

In the mitochondrial matrix.

What is the role of the citric acid cycle in metabolism?

It is amphibolic, meaning it plays a role in both catabolism and anabolism.

What is oxidative phosphorylation?

A process for generating ATP that depends on the creation of a pH gradient within the mitochondrion as a result of electron transport.

What is the function of the electron transport chain?

A series of oxidation-reduction reactions by which electrons derived from nutrient oxidation are passed to oxygen.

What is the significance of the mitochondrial matrix?

It is the part of the mitochondrion where the citric acid cycle reactions occur.

What is the intermembrane space in mitochondria?

The region between the inner and outer mitochondrial membranes.

What happens to pyruvate under aerobic conditions?

It is oxidized to one carbon dioxide molecule and one acetyl group, which enters the citric acid cycle.

What is produced for each molecule of acetyl-CoA that enters the citric acid cycle?

Two molecules of carbon dioxide and electrons are transferred.

What is the immediate electron acceptor in the citric acid cycle?

NAD+, which is reduced to NADH.

What is the main advantage of aerobic oxidation over anaerobic oxidation?

Aerobic oxidation yields far more energy from nutrients.

What is the role of coenzyme A (CoA) in the citric acid cycle?

It links the acetyl group to the cycle, allowing it to enter and participate in the reactions.

What is the first step in the citric acid cycle?

The conversion of acetyl-CoA to citrate.

How does the citric acid cycle contribute to anabolism?

Some molecules in the cycle serve as starting points for biosynthetic pathways.

What is the importance of the pH gradient created during electron transport?

It is essential for ATP synthesis during oxidative phosphorylation.

What is the relationship between catabolism and anabolism in metabolism?

They operate simultaneously, with catabolism breaking down nutrients and anabolism building biomolecules.

What is the significance of the citric acid cycle in aerobic metabolism?

It is a central metabolic pathway that facilitates the complete oxidation of nutrients.

What type of reactions occur in the electron transport chain?

Oxidation-reduction reactions.

What is the final product of aerobic metabolism?

Carbon dioxide and water.

What is a key feature of the enzymes involved in the citric acid cycle?

Most are located in the mitochondrial matrix.

What is the role of NADH and FADH2 in aerobic metabolism?

They act as electron carriers that transport electrons to the electron transport chain.

How does the citric acid cycle relate to glycolysis?

Glycolysis produces pyruvate, which is then oxidized to acetyl-CoA for entry into the citric acid cycle.

What is the overall purpose of aerobic metabolism?

To efficiently convert nutrients into usable energy in the form of ATP.

What is the role of FAD in the electron transport chain?

FAD takes up two electrons and two hydrogen ions to produce FADH2.

What is the final electron acceptor in the electron transport chain?

Oxygen, which produces water as a byproduct.

What is produced from pyruvate during its conversion to acetyl-CoA?

Carbon dioxide is lost, and acetyl-CoA is produced.

What high-energy compound is produced directly in the citric acid cycle?

GTP (guanosine triphosphate).

What is oxidative decarboxylation?

The loss of carbon dioxide accompanied by oxidation.

What two-carbon compound condenses with oxaloacetate in the citric acid cycle?

Acetyl group.

What is the first product formed when acetyl-CoA combines with oxaloacetate?

Citrate (six-carbon citrate ion).

How many steps are in the citric acid cycle?

Eight steps, each catalyzed by a different enzyme.

Which steps of the citric acid cycle are oxidation reactions?

Steps 3, 4, 6, and 8.

What is the function of the pyruvate dehydrogenase complex?

It catalyzes the conversion of pyruvate to acetyl-CoA and carbon dioxide.

What are the five enzymes that make up the pyruvate dehydrogenase complex?

Pyruvate dehydrogenase, dihydrolipoyl transacetylase, dihydrolipoyl dehydrogenase, pyruvate dehydrogenase kinase, and pyruvate dehydrogenase phosphatase.

What coenzyme is required for the first step of the pyruvate dehydrogenase reaction?

Thiamine pyrophosphate (TPP).

What is the significance of lipoic acid in the pyruvate dehydrogenase complex?

It acts as a coenzyme and is involved in the transfer of the acetyl group.

What type of compound is acetyl-CoA classified as?

A thioester.

What is the overall reaction of the conversion of pyruvate to acetyl-CoA?

Pyruvate + CoA-SH + NAD+ → Acetyl-CoA + CO2 + NADH + H+.

What happens to the carbon skeletons during the citric acid cycle?

They are effectively lost as CO2 is produced.

What is the energy yield of the reaction converting pyruvate to acetyl-CoA?

Exergonic with a ΔG° of -33.4 kJ/mol.

What is the role of NAD+ in the citric acid cycle?

It acts as an oxidizing agent in several steps of the cycle.

What is produced during the oxidative decarboxylation of isocitrate?

The five-carbon compound α-ketoglutarate and CO2.

What is the final product of the citric acid cycle?

Oxaloacetate, which is regenerated to continue the cycle.

What type of reactions occur in Steps 5 and 6 of the citric acid cycle?

Step 5 involves phosphorylation to produce GTP, and Step 6 is an oxidation reaction.

What is the significance of thioesters in biochemistry?

They are high-energy compounds that release energy upon hydrolysis.

What is the role of GDP in the citric acid cycle?

GDP is phosphorylated to produce GTP, which can be converted to ATP.

What happens to the hydroxyl group during the transfer of the acetyl group in the pyruvate dehydrogenase complex?

It is oxidized to produce an acetyl group.

What is the importance of the citric acid cycle in metabolism?

It produces reduced electron equivalents (NADH and FADH2) for ATP generation.

What is the function of the pyruvate dehydrogenase kinase and phosphatase?

They are involved in the regulation of the pyruvate dehydrogenase complex.

What is the first step in the conversion of pyruvate to acetyl-CoA?

Pyruvate loses CO2 and hydroxyethyl-TPP (HETPP) is formed.

What enzyme catalyzes the transfer of the acetyl group to CoA?

Dihydrolipoyl transacetylase.

What is produced when the acetyl group is transferred to CoA?

Acetyl-CoA.

What is the role of lipoic acid in the pyruvate dehydrogenase complex?

Lipoic acid acts as a swinging arm to facilitate the transfer of the acetyl group.

What happens to the reduced form of lipoic acid after the transfer of the acetyl group?

It remains covalently bound to dihydrolipoyl transacetylase.

What is the function of dihydrolipoyl dehydrogenase in the reaction?

It reoxidizes the reduced lipoic acid back to its disulfide form.

What coenzyme is associated with dihydrolipoyl dehydrogenase?

FAD (flavin adenine dinucleotide).

What is the product of the reduction of FAD in the reaction?

FADH2.

What is the overall result of the conversion of pyruvate to acetyl-CoA?

The production of carbon dioxide and acetyl-CoA.

How many ATP can be generated from the conversion of one glucose molecule through pyruvate to acetyl-CoA?

Five ATP (two pyruvate produced per glucose).

What is the first step of the citric acid cycle?

The reaction of acetyl-CoA and oxaloacetate to form citrate.

What enzyme catalyzes the first step of the citric acid cycle?

Citrate synthase.

What type of reaction occurs when acetyl-CoA and oxaloacetate combine?

A condensation reaction.

What is the second step of the citric acid cycle?

The isomerization of citrate to isocitrate.

Which enzyme catalyzes the isomerization of citrate to isocitrate?

Aconitase.

What metal ion is required by aconitase for its activity?

Fe2+ (iron).

What distinguishes isocitrate from citrate?

Isocitrate is a chiral compound, while citrate is achiral.

What is the overall reaction of the citric acid cycle?

Acetyl-CoA combines with oxaloacetate to produce citrate and CoA-SH.

What is the significance of thioesters in the citric acid cycle?

Thioesters are high-energy compounds that release energy upon hydrolysis.

What happens to NAD+ during the citric acid cycle?

It is reduced to NADH.

What is the role of the multienzyme complex in the conversion of pyruvate to acetyl-CoA?

It allows for efficient catalysis and regulatory control of the reaction steps.

What is the final product of the reaction catalyzed by pyruvate dehydrogenase?

Acetyl-CoA and carbon dioxide.

How does the spatial arrangement of enzymes in the pyruvate dehydrogenase complex benefit the reaction?

It allows for more efficient reactions due to proximity of reactants and enzymes.

What is the importance of the swinging arm action of lipoic acid?

It facilitates the movement of the acetyl group between different enzyme active sites.

What are the products of the reaction catalyzed by isocitrate dehydrogenase?

α-Ketoglutarate, NADH, and CO2.

What is the significance of the citric acid cycle in cellular respiration?

It is a key metabolic pathway that generates energy through the oxidation of acetyl-CoA.

What is the role of GTP in the citric acid cycle?

GTP is produced during the conversion of succinyl-CoA to succinate.

What enzyme catalyzes the conversion of citrate to isocitrate?

Aconitase

What is the role of fluoride ion in metabolism?

It inhibits glycolytic enzymes such as phosphoglyceromutase, enolase, and pyruvate kinase.

What are fluoroacetate and fluorocitrate known for?

They inhibit the citric acid cycle.

What is fluoroacetyl-CoA a precursor to?

Fluorocitrate, which inhibits citrate synthase.

Why are plants that produce fluoroacetate considered poisonous?

They produce a potent inhibitor of life processes.

What is Compound 1080?

Sodium fluoroacetate, used as a poison for pest control.

What is the mechanism of poisoning by Compound 1080?

It inhibits aconitase, similar to plant poisons.

What is the significance of the enzyme isocitrate dehydrogenase?

It catalyzes the first oxidative decarboxylation in the citric acid cycle.

What are the products of the oxidative decarboxylation of isocitrate?

B-ketoglutarate and carbon dioxide.

What is produced from NAD+ during the oxidative decarboxylation of isocitrate?

NADH

How many ATP can be produced from one molecule of NADH?

2.5 ATP

What is the role of the α-ketoglutarate dehydrogenase complex?

It catalyzes the conversion of B-ketoglutarate to succinyl-CoA.

What coenzymes are required for the α-ketoglutarate dehydrogenase complex?

Thiamine pyrophosphate (TPP), FAD, lipoic acid, and Mg2+.

What is the overall reaction of the α-ketoglutarate dehydrogenase complex?

Conversion of B-ketoglutarate to succinyl-CoA, producing NADH and CO2.

What happens to the citric acid cycle when CO2 is removed?

It becomes irreversible in vivo.

What is the first step in the citric acid cycle involving citrate?

Citrate is converted to cis-aconitate and then to isocitrate.

What intermediate remains bound to the enzyme during the reaction from citrate to isocitrate?

Cis-aconitate.

What is the significance of the 'ferrous wheel' analogy in enzyme binding?

It describes how citrate curls back on itself in the active site of aconitase.

What is the effect of sodium fluoroacetate on animal metabolism?

It leads to the formation of fluoroacetyl-CoA, inhibiting the citric acid cycle.

What is the role of aconitase in the citric acid cycle?

It catalyzes the conversion of citrate to isocitrate.