AIIMS CRE, SNO-2024: Which of the following is a purely ketogenic amino acid?

AIIMS CRE, SNO-2024

Biochemistry & Nutrition

Easy

Appeared in: AIIMS CRE, SNO-2024

Explanation

Leucine is a purely ketogenic amino acid, meaning its carbon skeleton is degraded into acetyl-CoA and acetoacetate.
These products are direct precursors for the synthesis of ketone bodies (e.g., acetoacetate, β-hydroxybutyrate) and cannot be used to synthesize glucose.
Lysine is the only other amino acid that is exclusively ketogenic.
A helpful mnemonic to remember the purely ketogenic amino acids is that they both start with the letter 'L' (Leucine, Lysine).

Option A: Alanine is incorrect because it is a purely glucogenic amino acid. Its carbon skeleton is converted to pyruvate, a key substrate for gluconeogenesis (glucose synthesis).
Option B: Valine is incorrect because it is a purely glucogenic amino acid. Its breakdown yields succinyl-CoA, an intermediate of the citric acid cycle, which can be used for gluconeogenesis.
Option D: Glycine is incorrect because it is the simplest amino acid and is purely glucogenic. It can be converted to serine and then to pyruvate, entering the gluconeogenesis pathway.

Visual 1: Diagram: A flowchart illustrating the metabolic fates of amino acids, clearly showing the entry points of glucogenic amino acids (into pyruvate and TCA cycle intermediates) and ketogenic amino acids (into acetyl-CoA and acetoacetate).

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of amino acids based on their metabolic fate (ketogenic vs. glucogenic) as background academic context rather than a clinical decision trigger.
Understanding this classification is crucial for managing patients with inborn errors of metabolism, such as Maple Syrup Urine Disease (MSUD), which involves impaired breakdown of branched-chain amino acids including the ketogenic Leucine and the glucogenic Valine.
This knowledge is fundamental for designing therapeutic diets. For example, in a ketogenic diet used for refractory epilepsy, the intake of ketogenic amino acids contributes to the state of ketosis.
What if? If a patient with classic MSUD consumes a meal high in protein (containing leucine, isoleucine, and valine), toxic branched-chain ketoacids would accumulate, leading to severe neurological symptoms. Dietary management requires strict lifelong limitation of these three amino acids.

How to Approach the Question

First, identify the key phrase in the question: 'purely ketogenic amino acid'.
Recall the definition: a ketogenic amino acid is one whose carbon skeleton breaks down into acetyl-CoA or acetoacetate, which can form ketone bodies but cannot be used to make new glucose.
Remember the two amino acids that are exclusively ketogenic: Leucine and Lysine. A simple mnemonic is that both start with 'L'.
Scan the options provided: Alanine, Valine, Leucine, Glycine.
Identify 'Leucine' as the correct match from your recall.
To confirm, quickly classify the other options: Alanine, Valine, and Glycine are all known to be glucogenic, reinforcing that Leucine is the only ketogenic option listed.

Concept Tested & Keywords

Concept Tested: Classification of amino acids based on their metabolic fate (ketogenic vs. glucogenic).
Stem keywords: purely ketogenic, amino acid
Lead-in keywords: Which of the following
Negative lead-in flag: false

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Textbook of Biochemistry for Medical Students 6th Edition — DM Vasudevan p. 123-145