Amino acid metabolism — identifying amino acids that are both ketogenic and glucogenic Which of the following amino acids is classified as both ketogenic (yielding acetoacetyl CoA/acetyl CoA) and glucogenic (yielding TCA intermediates for gluconeogenesis) in human metabolism?

Introduction / Context:
Amino acids are categorized as glucogenic, ketogenic, or both based on the metabolic fate of their carbon skeletons after deamination. This classification is important in biochemistry and clinical nutrition, especially for understanding fasting metabolism, inborn errors, and dietary planning for certain metabolic diseases. The question tests recognition of amino acids that can contribute carbon to glucose synthesis and also to ketone body or fatty acid synthesis.

Given Data / Assumptions:

• Glucogenic amino acids yield pyruvate or TCA intermediates (e.g., oxaloacetate, alpha-ketoglutarate) that can form glucose.
• Ketogenic amino acids yield acetyl CoA or acetoacetyl CoA, precursors of ketone bodies and fatty acids.
• Some amino acids can be both, producing a mix of these products.

Concept / Approach:

To answer correctly, recall the standard classification: leucine and lysine are strictly ketogenic; valine, histidine, methionine are strictly glucogenic; isoleucine, phenylalanine, tyrosine, tryptophan, and threonine are both ketogenic and glucogenic. Therefore, among the options, tryptophan is the correct dual-class amino acid.

Step-by-Step Solution:

Verification / Alternative check:

Pathway maps show tryptophan degradation produces alanine/pyruvate pathway intermediates (glucogenic) and acetoacetyl CoA (ketogenic), confirming its dual status.

Why Other Options Are Wrong:

Valine is glucogenic only (→ succinyl CoA). Lysine is ketogenic only (→ acetoacetyl CoA). Leucine (added distractor) is also strictly ketogenic. “None of these” is incorrect because tryptophan fits.

Common Pitfalls:

Memorizing only leucine and lysine as ketogenic and forgetting the broader “both” category, or misplacing valine into the dual list.

Final Answer:

Tryptophan