glucogenic vs ketogenic amino acid

[akimhaneul]

What makes some amino acids glucogenic vs ketogenic?

I noticed that leucine and lysine are ketogenic but not glucogenic.

Is there a special reason why amino acids are divided like this?

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[aldol16]

It's a chemical reason that's a bit beyond the scope of the MCAT. It's because C-C bonds are quite difficult to break and re-make unless they're already activated (e.g. retro-aldol) so the amino acids with five connected carbons are glucogenic along with some others. Leucine has a branch at the end of a long aliphatic chain and would have to be de-branched while lysine has a six-carbon chain. These simply cannot be biologically made into one of the TCA cycle intermediates or pyruvate.

 

[wizzed101]

Those two can only enter the Krebs cycle as acetyl-CoA, which is 2C. And human don't have the enzymes needed for the conversion. It has nothing to do with whether it's easy or difficult. "We" just decided that it would not be advantageous to possess those enzymes.