Question

How many ‘turns’ of the TCA cycle does it take to completely oxidize the carbons from one citrate molecule? Why?

Answer 1

Ans. Oxaloacetate (4C) + Acetyl-CoA (2C) -----> Citrate (6C).

In each TCA cycle, a citrate molecule loses two carbon atoms in forms of CO₂. To lose all the 6C, the citrate molecule must re-enter TCA for at least 3 turns or cycles.

Note that it is only the –COO^- groups being eliminated as CO₂ (Oxidation). The two carbon atoms forming the backbone (shaded in pink rectangle) are conserved. So, these two C-atoms would never be lost from citrate molecule.

So –

I. If complete oxidation means “losing all the 6 C-atoms of a citrate molecule”, the answer would be “NEVER”. Because as mentioned above, the two C-toms (shaded in pink rectangle) are CONSERVED.

II. If complete oxidation means “releasing just 6 CO₂ on stoichiometry basis”, then 3 turns would be required to yield 6 CO₂ or to yield equivalent oxidation results. Because each turn of TCA removes 2 C-atoms as CO₂.