How is Kreb's cycle regulated by ADP?

[scilearner]

My textbook says,

 

Increase in the concentration of ADP accelerates the rate of reactions that use ADP to generate ATP. If ADP is present in limiting concentration, the formation of ATP by oxidative phosphorylation decreases owing to lack of phosphate acceptor or inorganic phosphate. As NADH and FADH2 accumulate, their oxidized forms become depleted, causing the oxidation of acetyl CoA by TCA cycle to be inhibited, owing to lack of oxidized coenzymes.

 

I checked the whole citric acid cycle, and only the conversion of succinyl coA requires ADP or GDP. I don't understand how a lack of this would make NADH accumulate. Could anyone explain this. Thanks a lot

[CharonY]

You have to look broader. More specifically check oxidative phosphorylation. While I do not want to tell you everything in detail, think about the following: what role does oxidative phosphorylation has and what happens to NADPH in that pathway. Then think about what would happen to NADPH if oxidative phosphorylation was inhibited due to lack of ADP.

 

Only after that make the link to the TCA cycle.

[scilearner]

You have to look broader. More specifically check oxidative phosphorylation. While I do not want to tell you everything in detail, think about the following: what role does oxidative phosphorylation has and what happens to NADPH in that pathway. Then think about what would happen to NADPH if oxidative phosphorylation was inhibited due to lack of ADP.

 

Only after that make the link to the TCA cycle.

 

Hello thanks a lot for your response So in the electron transport chain lack of ADP inhibits oxidative phosphorylation and caused NADH to accumulate. Are NADs from electron transport chain used in the citric acid cycle? So now there are less NADS and citric acid cycle is affected. Is that right? Thanks again

[CharonY]

Precisely. This is a bit easier to see in fermentation reactions in which energy generation and NAD+ regeneration is sometimes decoupled. An example is in the pathway from acetyl-CoA to ethanol, in which only NAD+ is regenerated, but no ATP is produced.