Biochem Unit 11 - Citric Acid Cycle

What is the primary function of the Citric Acid Cycle?

To oxidize acetyl CoA to CO2, releasing energy in the form of NADH, FADH2, and GTP.

What are the three major stages of cellular respiration?

1. Oxidation of small fuel molecules to acetyl CoA

2. Acetyl CoA enters the Citric Acid Cycle

3. Oxidation of reduced coenzymes (NADH and FADH2) to produce ATP.

What is the net reaction of the Citric Acid Cycle?

Acetyl CoA + 2 CO2 + 1 GTP + 3 NADH + 1 FADH2.

What is the role of thiamine pyrophosphate (TPP) in the conversion of pyruvate to acetyl CoA?

TPP decarboxylates pyruvate in the first step of the reaction sequence.

What is the function of coenzyme A (CoA) in the pyruvate dehydrogenase complex?

CoA acts as an acyl carrier and is essential for forming acetyl CoA.

What is the significance of lipoic acid in the conversion of pyruvate to acetyl CoA?

Lipoic acid acts as a prosthetic group and participates in oxidation/reduction reactions.

What is the difference between stoichiometric cofactors and regenerated cofactors?

Stoichiometric cofactors are used up in the reaction, while regenerated cofactors are restored to their original state.

How many ATP molecules can one molecule of glucose produce under aerobic conditions?

32 ATP molecules.

What is the total ATP yield from glycolysis?

7 ATP molecules (2 direct and 5 through NADH and oxidative phosphorylation).

What is the purpose of anaplerotic reactions in the Citric Acid Cycle?

To replenish cycle intermediates that are depleted during the cycle.

What is the role of biotin in metabolic reactions?

Biotin serves as a cofactor for carboxylation reactions.

What are the control sites for the Citric Acid Cycle?

Key control sites include citrate synthase, isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase.

What type of reaction occurs in step 1 of the pyruvate dehydrogenase complex?

Decarboxylation of pyruvate.

What is produced in step 5 of the pyruvate dehydrogenase complex?

NADH + H+ and regenerates FAD.

What is the significance of the disulfide bond in lipoic acid?

It undergoes oxidation/reduction reactions and can serve as an acyl carrier.

How does the pyruvate dehydrogenase complex control its activity?

Through product feedback ratios and reversible phosphorylation.

What is the overall ATP yield from one molecule of pyruvate in the Citric Acid Cycle?

12.5 ATP molecules.

What is the main product of the Citric Acid Cycle that is used in oxidative phosphorylation?

NADH and FADH2.

What happens to the electrons from NADH and FADH2 in cellular respiration?

They are transferred to the electron transport chain, where ATP is produced.

What is the role of GTP in the Citric Acid Cycle?

GTP can be converted to ATP, serving as an energy currency.

What is the significance of the Krebs cycle's name?

It is named after Hans Krebs, who elucidated the cycle's steps.

What is the first substrate formed in the Citric Acid Cycle?

Citrate, formed from acetyl CoA and oxaloacetate.

How does the Citric Acid Cycle contribute to the metabolic pathway of glucose?

It oxidizes acetyl CoA derived from glucose to produce energy carriers (NADH, FADH2) for ATP synthesis.

What is the function of the Pyruvate Dehydrogenase (PDH) Complex?

It converts pyruvate into acetyl CoA, linking glycolysis to the citric acid cycle.

Name the three enzymes that make up the Pyruvate Dehydrogenase Complex.

Pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).

What is a key advantage of the Pyruvate Dehydrogenase Complex being a multienzyme complex?

It allows for channeling of substrates and prevents loss of intermediates.

What is the significance of the tethered lipoyllysine group in the PDH complex?

It facilitates the transfer of acyl groups during the conversion of pyruvate to acetyl CoA.

What are the three major fates of acetyl CoA?

Oxidation by the citric acid cycle, biosynthesis of lipids, and acetylation reactions.

How is the Pyruvate Dehydrogenase Complex controlled?

Through allosteric binding, reversible phosphorylation, and substrate availability.

What role do hormones like insulin and epinephrine play in the function of the PDH complex?

They stimulate the PDH complex to enhance the conversion of pyruvate to acetyl CoA.

What is the first step of the Citric Acid Cycle?

Formation of citrate from acetyl CoA and oxaloacetate.

Which step of the Citric Acid Cycle directly produces GTP?

The conversion of succinyl-CoA to succinate.

What is the role of NADH and FADH2 in the Citric Acid Cycle?

They are produced during oxidation reactions and are used in the electron transport chain to generate ATP.

What is the significance of the irreversible steps in the first half of the Citric Acid Cycle?

They ensure that the cycle operates in one direction and are key control points.

What happens during the isomerization of citrate to isocitrate?

A hydroxyl group is moved from one carbon to an adjacent carbon, preparing for further oxidation.

What is produced when isocitrate is oxidized to α-ketoglutarate?

One carbon is released as CO2 and NADH is produced.

What is the mechanism of the enzyme that converts α-ketoglutarate to succinyl-CoA?

It is identical to that of the PDH complex and requires similar cofactors.

What is the role of biotin in the Citric Acid Cycle?

Biotin acts as a carrier of CO2 in the carboxylation of pyruvate to oxaloacetate.

What are anaplerotic reactions?

Reactions that replenish citric acid cycle intermediates that have been depleted.

How does the equilibrium of the reaction converting malate to oxaloacetate favor the forward reaction?

The continuous consumption of oxaloacetate drives the reaction forward.

What factors govern the control of the Citric Acid Cycle?

Substrate availability, product inhibition, and allosteric feedback ratios.

What is the relationship between the Citric Acid Cycle and amino acids?

Transamination reactions link the cycle with amino acid metabolism.

What is the significance of the high energy thioester bond in succinyl-CoA?

It is used to generate GTP during the conversion to succinate.

What is the final product of the Citric Acid Cycle?

Oxaloacetate, which is regenerated to continue the cycle.

What is the role of Ca++ in the control of the Citric Acid Cycle?

Calcium ions can stimulate certain enzymes in the cycle, enhancing metabolic activity