Definition checkpoint: A glucogenic amino acid is one whose carbon skeleton is ultimately degraded to which class of metabolic products?

Introduction / Context:
Glucogenic amino acids are those that can serve as precursors for gluconeogenesis. After transamination or deamination, their carbon skeletons enter central metabolism as compounds that the liver can use to synthesize glucose, especially during fasting or prolonged exercise. Understanding this definition is foundational for physiology and clinical biochemistry.

Given Data / Assumptions:

• Gluconeogenesis requires substrates that can be converted to oxaloacetate and then to phosphoenolpyruvate.
• Pyruvate and TCA cycle intermediates (e.g., oxaloacetate, fumarate, succinyl-CoA, alpha-ketoglutarate) are direct entry points to glucose formation.
• Acetyl-CoA and acetoacetyl-CoA carbons cannot produce net glucose in humans.

Concept / Approach:
Relate the endpoint of amino acid carbon skeletons to gluconeogenic capacity. If the endpoint is pyruvate or a TCA intermediate that can yield oxaloacetate, the amino acid is glucogenic. If the endpoint is acetyl-CoA or acetoacetyl-CoA (which lose carbons as CO2 in the TCA cycle), the amino acid is ketogenic. Lipid-related endpoints do not directly re-enter gluconeogenesis in humans for net glucose production.

Step-by-Step Solution:

Verification / Alternative check:
Standard metabolic charts group amino acids by their ability to produce pyruvate, oxaloacetate, fumarate, succinyl-CoA, or alpha-ketoglutarate (glucogenic) versus those that produce acetyl-CoA or acetoacetyl-CoA (ketogenic).

Why Other Options Are Wrong:

• Keto-sugars: not a standard endpoint of amino acid catabolism.
• Acetyl-CoA/acetoacetyl-CoA: defines ketogenic, not glucogenic, fate.
• Long-chain fatty acyl-CoA: belongs to fatty acid metabolism, not direct gluconeogenesis.
• None of the above: incorrect because the correct definition is provided.

Common Pitfalls:
Assuming acetyl-CoA can be converted back to glucose in animals; the pyruvate dehydrogenase step is irreversible, preventing net glucose production from even-chain fatty acids or purely ketogenic amino acids.

Final Answer:
Pyruvate or tricarboxylic acid (TCA) cycle intermediates