How many ‘turns’ of the TCA cycle does it take to completely oxidize the carbons from...

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

Expert Solution

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.

queen_honey_blossom answered 2 years ago

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