Which tricarboxylic acid (TCA) cycle intermediate acts as a precursor for fatty acid synthesis?

AIIMS CRE SNO - 2023

Biochemistry & Nutrition

Medium

Appeared in: AIIMS CRE SNO - 2023

Explanation

Fatty acid synthesis occurs in the cytosol, but its primary building block, acetyl-CoA, is produced in the mitochondria.
Acetyl-CoA cannot cross the inner mitochondrial membrane directly.
To be transported to the cytosol, acetyl-CoA first condenses with oxaloacetate to form citrate.
Citrate is then transported out of the mitochondria via the citrate shuttle.
In the cytosol, the enzyme ATP-citrate lyase cleaves citrate, releasing acetyl-CoA, which then enters the fatty acid synthesis pathway.

Option A: Succinate is an intermediate within the TCA cycle and is involved in heme synthesis, but it does not serve as the carbon-carrying precursor for fatty acid synthesis.
Option C: While malate plays a role, it is not the direct carbon precursor. Cytosolic malate can be converted to pyruvate by the malic enzyme, a reaction that produces NADPH. This NADPH is essential for fatty acid synthesis, but the carbon atoms come from acetyl-CoA, not malate.
Option D: Fumarate is an intermediate in both the TCA cycle and the urea cycle. It does not have a direct role in providing carbon atoms for fatty acid synthesis.

Visual 1: Diagram - The Citrate Shuttle. A diagram showing the transport of citrate from the mitochondrial matrix to the cytosol and its subsequent cleavage into acetyl-CoA and oxaloacetate, clearly linking the TCA cycle to the fatty acid synthesis pathway.
Visual 2: Flowchart - Overview of Fatty Acid Synthesis. A flowchart starting from glucose/pyruvate, showing its conversion to mitochondrial acetyl-CoA, the citrate shuttle, and the final steps of fatty acid chain elongation in the cytosol.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Role of TCA cycle intermediates in biosynthetic pathways as background academic context rather than a clinical decision trigger.
This pathway is highly active in states of high energy and carbohydrate intake (e.g., after a large meal). Excess glucose is converted to acetyl-CoA, which is then shuttled out as citrate to be stored as fat.
Insulin promotes this process by activating key enzymes like acetyl-CoA carboxylase, the rate-limiting step of fatty acid synthesis.
Understanding this link is crucial in metabolic disorders like obesity and type 2 diabetes, where dysregulation of fat storage is a key feature.

How to Approach the Question

First, identify the key metabolic pathways in the question: the Tricarboxylic Acid (TCA) cycle and fatty acid synthesis.
Recall the cellular location of each pathway. The TCA cycle occurs in the mitochondrial matrix, while fatty acid synthesis occurs in the cytosol.
Recognize that acetyl-CoA is the starting molecule for fatty acid synthesis but is produced inside the mitochondria.
Consider how acetyl-CoA can be moved from the mitochondria to the cytosol, as it cannot pass through the membrane directly.
Recall the 'citrate shuttle' mechanism, where acetyl-CoA is converted to citrate to be transported out.
Based on this shuttle system, identify citrate as the TCA cycle intermediate that crosses into the cytosol to provide the acetyl-CoA precursor.

Concept Tested & Keywords

Concept Tested: Role of TCA cycle intermediates in biosynthetic pathways
Stem keywords: TCA cycle, tricarboxylic acid cycle, intermediate, precursor, fatty acid synthesis
Lead-in keywords: Which
Negative lead-in flag: false

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