In the figure below,

  • Purple letters represents both glycogenic and ketogenic amino acids
  • Brown letters represents glycogenic amino acids, only
  • Green letters represents ketogenic amino acids, only

enter image description here

So, the following glycogenic amino acids are also ketogenic;

  • Threonine, Tryptophan
  • Isoleucine
  • Tyrosine
  • Phenylalanine

However, as can be seen in the figure below, at least the following of these can be converted to pyruvate, which can eventually become acetyl CoA, and in this sense, their carbon skeletons can also be used in the synthesis of ketone bodies.

  • Alanine, cysteine, glycine, and serine

My question:

Why are "alanine, cysteine, glycine, serine" not ketogenic amino acids, even though they can be converted to pyruvate and thus their carbon skeleton can be used to generate acetyl CoA?

As I understand it, whether it is ketogenic or not should depend on "whether it can be used to synthesize ketone bodies or fatty acids". While pyruvate can be used for glycogenesis directly back through the glycolytic system, some metabolic systems go through acetyl CoA; of course, if acetyl CoA is produced, the carbon skeleton part cannot be used for glycogenesis.

Amino acids that can be converted directly to pyruvate should be able to be converted to acetyl CoA via pyruvate and thus be used in the reaction to produce ketones from acetyl CoA.

Those that must once go through pyruvate (alanine, cysteine, glycine, serine) are not traditionally included in the ketogenic amino acids?

Wikipedia has the following description; https://en.wikipedia.org/wiki/Ketogenic_amino_acid

A ketogenic amino acid is an amino acid that can be degraded directly into acetyl-CoA, which is the precursor of ketone bodies and myelin, particularly during early childhood, when the developing brain requires high rates of myelin synthesis.1 This is in contrast to the glucogenic amino acids, which are converted into glucose. Ketogenic amino acids are unable to be converted to glucose as both carbon atoms in the ketone body are ultimately degraded to carbon dioxide in the citric acid cycle.

The answer to this question would depend on how one concretizes "degraded directly" in the quoted above.

  • 2
    $\begingroup$ It always helps to define your terms — so we know what the poster is talking about, and the poster himself does. Please do that — it will probably lead you to the answer to your question. $\endgroup$
    – David
    Dec 14, 2023 at 19:26
  • $\begingroup$ @David I added a definition, as far as I know. If there were an additional clause such as "those that cannot become acetyl CoA without going through pyruvate are not included in ketogenic amino acids", then my question would be answered, but I have yet to see such direct language. $\endgroup$ Dec 14, 2023 at 19:33
  • 1
    $\begingroup$ I think your definition is incorrect, and I suspect you are not aware of the physiological/pathological context — starvation or diabetes. And what effect does ketogenesis have on acid-base balance? Will expand later if necessary. $\endgroup$
    – David
    Dec 14, 2023 at 20:30
  • 3
    $\begingroup$ Does this answer your question? About the definition of ketogenic amino acid $\endgroup$
    – David
    Dec 14, 2023 at 21:43
  • $\begingroup$ I now find that your question has been asked before, and that I have already provided an answer from the context of starvation and provision of glucose for the brain. I think it is a much more pertinent answer than the other one, even though it is losing 0-6 in the scoring stakes. Perhaps you and other readers might wish to reassess it. $\endgroup$
    – David
    Dec 14, 2023 at 21:47


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