Catabolic fate of tyrosine — classification by products Tyrosine catabolism yields acetoacetyl CoA (ketogenic) and fumarate (glucogenic). How is tyrosine therefore classified?

Introduction / Context:
Amino acid classification by end products clarifies how their carbon contributes to energy metabolism. Tyrosine, derived from phenylalanine, degrades to both acetoacetyl CoA (a ketone body precursor) and fumarate (an anaplerotic TCA intermediate). This dual fate defines its category and has clinical implications in metabolic disorders like tyrosinemia and PKU.

Given Data / Assumptions:

• Acetoacetyl CoA contributes to ketogenesis and lipid synthesis (ketogenic).
• Fumarate enters the TCA cycle and supports gluconeogenesis via oxaloacetate (glucogenic).
• Classification reflects carbon skeleton outcomes after deamination and side-chain breakdown.

Concept / Approach:

If an amino acid yields both acetyl/acetoacetyl CoA and TCA cycle intermediates, it is both ketogenic and glucogenic. Tyrosine meets this criterion based on its pathway through homogentisate and subsequent cleavage steps.

Step-by-Step Solution:

Verification / Alternative check:

Biochemical maps consistently place tyrosine (and phenylalanine) in the “both” list along with tryptophan, isoleucine, and threonine.

Why Other Options Are Wrong:

Options A and B consider only one product class. Option D is informal and not a standard category. Option E contradicts known metabolism.

Common Pitfalls:

Confusing tyrosine with lysine or leucine (strictly ketogenic) or with valine (strictly glucogenic).

Final Answer:

Ketogenic and glucogenic amino acid