Citric Acid Cycle: Practice Questions and Key Concepts

CITRIC ACID CYCLE

Overview of the Citric Acid Cycle

The Citric Acid Cycle (also known as the Krebs Cycle or TCA Cycle) is a central metabolic pathway that completes the oxidation of organic molecules, producing energy in the form of ATP, NADH, and FADH2. It occurs in the mitochondrial matrix and is essential for aerobic respiration.

• Main function: Oxidation of acetyl-CoA to CO2 and generation of high-energy electron carriers.

• Location: Mitochondrial matrix in eukaryotes.

• Products per acetyl-CoA: 3 NADH, 1 FADH2, 1 GTP (or ATP), and 2 CO2.

Key Reactions and Enzymes

• Pyruvate Dehydrogenase Complex: Converts pyruvate to acetyl-CoA, linking glycolysis to the Citric Acid Cycle.

• Citrate Synthase: Catalyzes the condensation of acetyl-CoA and oxaloacetate to form citrate.

• Isocitrate Dehydrogenase: Produces NADH and CO2 from isocitrate.

• α-Ketoglutarate Dehydrogenase: Similar to pyruvate dehydrogenase, produces NADH and CO2 from α-ketoglutarate.

• Succinyl-CoA Synthetase: Converts succinyl-CoA to succinate, generating GTP (or ATP).

Energy Yield and Stoichiometry

The complete oxidation of one mole of acetyl-CoA in the Citric Acid Cycle yields:

• 3 moles of NADH

• 1 mole of FADH2

• 1 mole of GTP (or ATP)

• 2 moles of CO2

Overall reaction:

$\text{acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{GDP} + \text{Pi} + 2\text{H}_2\text{O} \rightarrow 2\text{CO}_2 + 3\text{NADH} + 3\text{H}^+ + \text{FADH}_2 + \text{GTP} + \text{CoA}$

GTP Production in the Cycle

GTP is produced by the enzyme succinyl-CoA synthetase during the conversion of succinyl-CoA to succinate. This is an example of substrate-level phosphorylation.

• Substrate-level phosphorylation: Direct synthesis of ATP or GTP from ADP or GDP and a high-energy substrate.

Comparison of Dehydrogenase Enzymes

Pyruvate dehydrogenase and α-ketoglutarate dehydrogenase are similar in their mechanism and cofactors:

• Both catalyze oxidative decarboxylation reactions.

• Both require cofactors: NAD+, CoA, thiamine pyrophosphate (TPP), lipoic acid, and FAD.

Citric Acid Cycle Intermediates

• Succinate: Formed from succinyl-CoA, later converted to fumarate.

• Fumarate: Formed from succinate, converted to malate.

• Malate: Converted to oxaloacetate, completing the cycle.

Summary Table: Citric Acid Cycle Products per Acetyl-CoA

Product	Moles Produced
NADH	3
FADH2	1
GTP (or ATP)	1
CO2	2

Glycolysis and Citric Acid Cycle Linkage

• Glycolysis: Occurs in the cytoplasm, converts glucose to pyruvate.

• Pyruvate: Transported into mitochondria, converted to acetyl-CoA by pyruvate dehydrogenase.

• Acetyl-CoA: Enters the Citric Acid Cycle for further oxidation.

Additional info:

• The Citric Acid Cycle is amphibolic, serving both catabolic and anabolic functions.

• Intermediates of the cycle are used in biosynthetic pathways (e.g., amino acid synthesis).