oxaloacetate question

[jmyhre]

Ok the question is:

"Oxaloacetate is formed in the last step of the citric acid cycle by the NAD+-dependent oxidation of L-malate. Can a net synthesis of oxaloacetate take place from acetyl-CoA using only the enzymes and cofactors of the citric acid cycle, without depleting the intermediates of the cycle? Explain your answer. How is oxaloacetate replenished?"

I"m not sure I even understand the question BUT I think that oxaloacetate is regenerated by the 2 molecules of CO2 that are released during the steps catalyzed by isocitrate dehydrogenase and a-ketoglutarate dehydrogenase complex. If I’m reading this right you can’t have a net synthesis because oxaloacetate reacts with acetyl-CoA to start the chain and is replenished at the completion of the cycle so there is no net gain or loss?? I don’t really understand the part about depleting the intermediates of the cycle, if someone could dumb it down for me I would really appreciate it. Thank you.

J

[jonmoulton]

I agree, there will be no net synthesis from a cycle where an acetyl is converted to two CO2 molecules and you are not allowed to scavenge intermediates to make an extra few molecules of oxaloacetate. And the whole thing grinds to a halt without the electron transport chain regenerating NAD+, so even running the cycle without net synthesis requires other input (e.g. regenerated NAD+).

[jmyhre]

I just wanted to say thank you for your help. Sorry it took awhile.

J

[jonmoulton]

Wow – you’re welcome. I grinned, so thanks for that too.

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