Amino acid metabolism (classification): Which of the following is the best example of a glucogenic amino acid in human metabolism?

Introduction / Context:
In biochemistry, amino acids are categorized as glucogenic, ketogenic, or both, based on the fate of their carbon skeletons after deamination. A glucogenic amino acid can yield intermediates that become glucose via gluconeogenesis. This classification is crucial for understanding fasting metabolism, inborn errors, and dietary planning in metabolic disorders.

Given Data / Assumptions:

• Lysine and leucine are considered purely ketogenic in standard textbooks.
• Valine is a branched-chain amino acid that is purely glucogenic.
• Tryptophan is both glucogenic and ketogenic (dual fate).

Concept / Approach:
Identify which option yields carbon skeletons that become pyruvate or tricarboxylic acid (TCA) cycle intermediates (such as oxaloacetate, fumarate, succinyl-CoA, or alpha-ketoglutarate) that feed gluconeogenesis. Valine is catabolized to propionyl-CoA and then to succinyl-CoA, a TCA intermediate, making it classically glucogenic. By contrast, lysine and leucine produce acetyl-CoA or acetoacetyl-CoA and cannot yield net glucose in humans, so they are ketogenic. Tryptophan produces both glucogenic and ketogenic products, so it is not the best single example of purely glucogenic behavior.

Step-by-Step Solution:

Verification / Alternative check:
Clinical nutrition texts list valine, isoleucine (partial), and methionine as contributors to succinyl-CoA; lysine and leucine are explicitly labeled ketogenic. This scheme aligns with standard metabolic maps used in medical biochemistry courses.

Why Other Options Are Wrong:

• Lysine: strictly ketogenic; does not yield net glucose.
• Tryptophan: both glucogenic and ketogenic; not the best exclusively glucogenic example.
• Leucine: strictly ketogenic.
• None of these: incorrect because valine is a clear glucogenic amino acid.

Common Pitfalls:
Confusing the dual-fate amino acids (e.g., isoleucine, phenylalanine, tryptophan, tyrosine) with purely glucogenic or purely ketogenic ones; also assuming all branched-chain amino acids behave identically.

Final Answer:
Valine