Amino Acid Classification—Fate of Histidine Carbon Skeleton Histidine is catabolized to α-ketoglutarate. Based on this metabolic fate, how is histidine classified?

Introduction / Context:
Amino acids are labeled glucogenic if their carbon skeletons can be converted into glucose precursors (e.g., pyruvate or TCA intermediates), and ketogenic if they form acetyl-CoA or acetoacetyl-CoA, fueling ketone body synthesis. Correctly classifying amino acids is fundamental to understanding fasting metabolism and dietary management in metabolic disorders.

Given Data / Assumptions:

• Histidine degrades to α-ketoglutarate.
• α-Ketoglutarate is a TCA cycle intermediate.
• TCA intermediates can generate oxaloacetate and thereby contribute to gluconeogenesis.

Concept / Approach:
Because α-ketoglutarate can be converted to oxaloacetate and then to phosphoenolpyruvate and glucose, any amino acid whose catabolism yields α-ketoglutarate is glucogenic. Only amino acids yielding acetyl-CoA or acetoacetyl-CoA without producing gluconeogenic intermediates are purely ketogenic (e.g., leucine and lysine). Some amino acids are both, but histidine is classically glucogenic via α-ketoglutarate.

Step-by-Step Solution:

Verification / Alternative check:
Standard biochemical tables list histidine among glucogenic amino acids, consistent with its entry into the TCA cycle at α-ketoglutarate.

Why Other Options Are Wrong:

• Ketogenic: would require degradation to acetyl-CoA/acetoacetyl-CoA.
• Both: histidine does not significativamente yield acetyl-CoA as an exclusive fate.
• “Gluco amino acid”: non-standard terminology; correct term is glucogenic.
• Neither: contradicted by its clear glucogenic pathway.

Common Pitfalls:
Assuming all amino acids can become both glucose and ketone bodies; only a subset is mixed. Terminology should be precise.

Final Answer:
Glucogenic amino acid