Why FADH2 but Not NADH in Brain and Skeletal Muscle Cells?

[crays]

Hi, my book says that in the brain and skeletal muscle cells, it only yields a total of 36 ATP because NADH + H⁺ cannot enters the brain and skeletal muscle cells' mitochondria. But why FADH₂ can is not stated. Can someone tell me why?

Also, since NADH + H⁺ is formed inside the mitochondria why would NADH + H⁺ cannot be used in the brain? They are still formed inside the mitochondria right?

Also, i would like to ask if my understanding is correct. NADH + H⁺'s H⁺ is used to push hydrogen against the concentration so that it will go through a gradient and ATP is synthesized from the ATP synthetase. Is this right?

Lots of question, thanks for the help.

 

[Ygggdrasil]

Two NADH + H⁺ are produced during glycolysis, which occurs in the cytoplasm of cells. For the cell to use these molecules in oxidative phosphorylation (which occurs inside the mitochondria), the cell must have some way of transporting the "energy" stored by these cytoplasmic NADH + H⁺ into the mitochondria. There are two means by which this can occur, the glycerophosphate shuttle and the malate-aspartate shuttle.

The NADH and FADH₂ molecules that are produced during the Krebs cycle do not have these problems as the Krebs cycle occurs in the mitochondrial matrix.

Yes, the energy from the NADH is used to push hydrogen against its concentration gradient. Proton gradient provides the energy for the ATP synthase enzyme to generate ATP.

 

[crays]

Thanks Ygggdrasil. one more question, in the glycolysis process. Does the WHOLE NADH + H⁺ enters the mitochondrian or just the electrons? or just the H⁺? Because they can only be changed to ATP in the mitochondria right?

 

[Ygggdrasil]

It's just the electrons that enter the mitochondria.

 

[crays]

if its the electrons, why does the electrons choose FAD as it's carrier inside the mitochondria but not NAD⁺

 

[Ygggdrasil]

When the cell uses the malate-aspartate shuttle, the electrons from cytosolic NADH are transferred to mitochondrial NAD⁺. However, when the cell uses the glycerophosphate shuttle, the electrons from cytosolic NADH are transferred to mitochondrial FAD.

I'm not completely sure why cells have evolved and maintained the glycerophosphate shuttle as it is less energy efficient than the malate-aspartate shuttle. Perhaps the relevant issue is speed and the cell requires the combination of the two shuttles in order to transport the electrons from NADH quickly enough to meet demand (regeneration of NAD⁺ in the cytosol is important for glycolysis to continue).

 

[crays]

Thanks ygggdrasil, but do you have reading material like from wiki or anything to backup your statement that in the brain it uses the glycerophosphate shuttle? thanks~

 

[Ygggdrasil]

I don't know whether one mechanism is preferred or not in the brain.