Amino Acid Catabolism—Definition of a Ketogenic Amino Acid A “ketogenic” amino acid is defined as one that degrades to which end-product(s)?

Introduction / Context:Amino acids are classified as glucogenic, ketogenic, or both based on the metabolic fate of their carbon skeletons. This classification predicts whether they can contribute to net glucose synthesis or ketone body formation, which is central to understanding fasting metabolism and certain inborn errors.

Given Data / Assumptions:

• Glucogenic amino acids yield pyruvate or TCA intermediates that can form oxaloacetate and, ultimately, glucose.
• Ketogenic amino acids yield acetyl-CoA or acetoacetyl-CoA, precursors for ketone bodies and fatty acid synthesis.
• Some amino acids are both glucogenic and ketogenic.

Concept / Approach:“Ketogenic” specifically denotes degradation to acetyl-CoA or acetoacetyl-CoA, which cannot produce net glucose because the two-carbon acetyl unit is lost as CO2 in the TCA cycle. Classic purely ketogenic amino acids are leucine and lysine; several others are mixed (e.g., isoleucine, phenylalanine, tryptophan, tyrosine).

Step-by-Step Solution:

Verification / Alternative check:Biochemistry texts list leucine/lysine as exclusively ketogenic, confirming the definition anchored in acetyl-CoA/acetoacetyl-CoA outcomes.

Why Other Options Are Wrong:

• Keto-sugars: a carbohydrate term, not a metabolic fate of amino acids.
• Pyruvate/TCA intermediates: describe glucogenic fates.
• “Multiple intermediates…”: describes “both” category, not strictly ketogenic.
• Only succinyl-CoA: one TCA intermediate (glucogenic via OAA), not ketogenic.

Common Pitfalls:Assuming acetyl-CoA can become glucose; net glucose cannot be formed from pure acetyl units in animals.

Final Answer:Either acetyl-CoA or acetoacetyl-CoA