[Aymeric]

Hello,

Would anyone have ever seen a diagram of the Krebs cycle clearly identifying the two carbon atoms coming from acetylCoA and then released in the form of CO2?
I’ve been writing the developed formulas of all intermediaries to better understand what goes on in between each reaction, and I’m confused. My textbook says that the CO2 eliminated by isocitrate dH and alpha-cetoglutarate dH come from the acetylCoA added to oxaloacetate at the start of the cycle and that from succinyl-CoA onwards, it can be said that the acetylCoA has already been fully eliminated.

However, if I properly understood the reaction catalysed by citrate synthase, then the two carbons from acetylCoA simply latch onto the C1 of oxaloacetate, whose carboxylic and ketone functions becomes citrate’s OH-C-COOH in C3. Then, when moving from oxalosuccinate to alpha-cetoglutarate, it looks like the released CO2 comes from the COOH in C3, i.e. the original carboxylic function in oxaloacetate, and not from acetylCoA whose two carbons are still there(C1 and C2).

It’s hard to describe without drawing it, especially with my very lacking command of nomenclature (an area barely touched upon in my curriculum), but I hope someone can understand what my problem is.

Thank you!

[Aymeric]

I think I found a way to include illustrations.

So, first step in Krebs, where we see that the acetylCoA is just stacked on top of oxaloacetate, whose oxalo bit becomes OH-C-COOH in C3.


But then the upper part of the molecule (i.e. the acetylCoA) remains unchanged even after the first CO2 is released:

Here the illustration suggests that the upper two carbons (COOH-CH2, supposedly originating from acetylCoA according to the first image) remain untouched, and that the CO2 that goes away is actually the COO- borne by C3 (which originally was one of oxaloacetate’s carboxylic function), which would mean that at least this CO2 does not come from acetylCoA.

Am I missing something?

Thanks!

[Aymeric]

I feel really stupid, I spent hours looking for an answer and it’s only after posting this that I found it here, sorry for the trouble!

[Babcock_Hall]

Something else you should think about is that citrate is a symmetric molecule, but it reacts in a seemingly asymmetric way.  This paradox was resolved in 1948.

[AWK]

Check the Youtube video from 2.00 min.
https://www.youtube.com/watch?v=FmEm0CgHGdA