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, carbon dioxide, and reducing equivalents. It is a key component of cellular respiration in mitochondria.

Pyruvate Dehydrogenase Complex

• Function: Converts pyruvate (from glycolysis) into acetyl-CoA, linking glycolysis to the citric acid cycle.

• Key Products: NADH, FADH2, acetyl-CoA, and CO2.

• Reaction: $\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + \text{CO}_2$

• Key Steps:

  • Decarboxylation of pyruvate

  • Reduction of NAD+ to NADH

  • Formation of acetyl-CoA

Labeling of Carbons in the Citric Acid Cycle

• When glucose labeled at the third and fourth carbons with 14C is converted to pyruvate and then to acetyl-CoA, the fate of the labeled carbon can be traced through the cycle.

• Key Point: The labeled carbon is released as CO2 during the cycle, but not immediately in the first turn.

• Example: If 100% of the label is in the third and fourth carbons, after one turn of the cycle, 50% of the label remains in the cycle.

Inhibition of Succinate Dehydrogenase by Malonate

• Malonate: A competitive inhibitor of succinate dehydrogenase, an enzyme in the citric acid cycle.

• Effect: Inhibits the conversion of succinate to fumarate, causing accumulation of succinate.

• Application: Used experimentally to study the mechanism of the citric acid cycle.

Intermediates of the Citric Acid Cycle

• Key Intermediates: Citrate, isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, oxaloacetate.

• Not an Intermediate: Acetyl-CoA is not an intermediate; it is a substrate that enters the cycle.

Radioactive Labeling and Turnover in the Citric Acid Cycle

• When acetyl-CoA is labeled with 14C at all carbons and added to mitochondria, the fate of the label can be tracked through the cycle.

• Key Point: After one turn of the cycle, 50% of the label remains in the cycle.

• Equation: $12\text{C} + 14\text{C-labeled acetyl-CoA} \rightarrow \text{Citric Acid Cycle}$

Summary Table: Key Intermediates and Inhibitors

Intermediate	Role in Cycle	Inhibitor
Succinate	Oxidized to fumarate	Malonate (competitive)
Fumarate	Hydrated to malate	None specific
Acetyl-CoA	Entry substrate	Not an intermediate

Additional info: The citric acid cycle is amphibolic, serving both catabolic and anabolic roles in metabolism. It is tightly regulated by energy status (ATP/ADP ratio), substrate availability, and feedback inhibition by cycle intermediates.