ATP Synthase Regulation

ADP is a regulator of ATP Synthase. In fact, ADP is the substrate that undergoes the transformation via ATP synthase, so if this is not present, there is no ATP that can be produced. Therefore, the system does not spin. 

What happens to the ETC if ATP synthase is no longer spinning or producing ATP? 

The ETC keeps pumping protons until there is such a high proton gradient on the outside that it is thermodynamically unfavorable to send another one through. 

Essentially, it would need WAY more energy than just an NADH or FADH2 could supply to pump that H+ across.

Lets look into this a bit further: 

1. As reviewed above, the ETC will stop pumping protons and ATP will not be made from ATP synthase.  

2. Due to the stoppage of the ETC, there is a build up of “high energy intermediates” (NADH+ specifically) which inhibit aspects of the TCA cycle (refer to Exam 4 section to review)

3. This also ends up inhibiting the PDH due to build up of unused molecules. 

4. Since glycolysis occurs in the cytoplasm, it is interacting with a different pool of NADH than the stores in the mitochondria. Therefore, rather than shutting down completely, fermentation will occur, as pyruvate will not be converted to acetyl-CoA by the PDH.