Question

If the Citric Acid cycle was altered such that no NADH molecules were generated from the oxidation of acetyl CoA but instead FADH2 molecules were generated, how would this affect the number of ATP molecules generated from a molecule of glucose? Briefly explain your answer.

Answer 1

Answer is 14 ATP in this case and 20 ATP in normal condition

Normally citric acid cycle is started with acetyl coA and ends with oxaloacetate to again citrate using acetyl coA and keep going. So if we see steps where NADPh was produced we can say how much impact will be on total ATP count when FADH2 are produced instead of NADH.

So starts with normal cycle Acetyl coA converted to Citrate, Citrate to isocitrate, Isocitrate to alpha ketogluatarate releasing NADH, alpha ketoglutarate to succcinyl coA giving NADH, succinyl coA to succinate giving GTP, Succinate to fumarate giving FADH2,Fumarate to malate , Malate to oxaloacetate giving NADH and finally oxaloacetate and acetyl coa gives citrate.

Hence there are three steps where NADH is produced

1.Isocitrate to alpha ketogluatarate

2.alpha ketoglutarate to succcinyl coA

3.Malate to oxaloacetate

One step gives FADH2 and one step gives GTP

So in normal condition there are total 20 Atp produced by one glucose molecule because two acetyl coA are produced by one glucose molecule.

3NADH gives 3*2.5 ATP=7.5 ATP

1FADH2 gives 1.5 ATP

And one GTP

So total 10 ATP but two cycles are performed by two acetylcoA so 10*2=20 ATP

Now comes to the case where FADH2 is produced intead of NADH

so there will be total 4 FADH2 which gives 1.5*4=6 ATP

and one GTP

So total 7 ATP by one acetylcoA hence 14 ATP by one glucose molecule.