ketogenic amino acids

[justforgotfrag]

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Hi, I found this online:

Ketogenic amino acids: Their carbon skeletons are degraded to acetyl-CoA or acetoacetate. Acetyl CoA, and its precursor acetoacetate, cannot yield net production of oxaloacetate, the precursor for the gluconeogenesis pathway. For every 2-C acetyl residue entering Krebs Cycle, two carbon atoms leave as CO2.

For example: lysine and leucine are both ketogenic. What I dont really get is how losing 2 carbons cannot yield the net product of oxaloacetate in the citric acid cycle? Can someone explain why?

 

[GorgeousBorges]

You don't need to know this much about biochemistry for the MCAT.

 

[Trayshawn]

Yeah, you don't need to know this much detail for the test. But you will for med school and its good information to have in general.

Yes, funneling the 2 carbons from the amino acid derived- acetyl CoA into the TCA cycle WILL lead to the production of oxaloacetate. But later in the cycle, oxaloacetate is also used up and thus there is no NET production of oxaloacetate. Remember, its a cycle.

Acetyl CoA condenses with oxaloacetate to form citrate. After some rearrangements, citrate is decarboxylated twice and oxidized to later reform oxaloacetate. So whatever OAA was formed is used up and whatever OAA was used up is reformed. Got it?