[aleaiactaest]

I'm a little confused about one step regarding glycolysis and the glycerol phosphate shuttle, I'm going to walk you through my thoughts..
 
* During glycolysis we get 2 NADH for every glucose molecule
* NADH needs a shuttle system to pass the inner membrane of the mitochondria
* The first step of the glycerol phosphate shuttle is when the GPDH-C enzyme catalyzes the following redox reaction:
  DHAP + NADH -> 3PG + NAD+
* As I see it we are getting down on lower energy state regarding both 3PG and NAD+. NAD+ is obviously at a lower energy sate as NADH is carrying energy in form of electrons. Looking at the glycolysis process we gain 1 NADH and 1 ATP by going from DHAP to 3PG so it looks to me as 3PG should therefore be at a lower energy state than DHAP.
* Now as we continue in the shuttle system with GPDH-M we go from 3PG back to DHAP (lower energy to higher) and at the same time FAD+ -> FADH2 (lower energy to higher)
* It would make sense to me that 3PG was in fact at a higher energy state than DHAP, but looking at the glycolysis process i just can't see how.
 
I hope my explanations are decent enough

[aleaiactaest]

To put it in more simple words

When you oxidize you are lowering the total energy as you are taking away electrons

Apparently we are oxidizing by going from 3PG -> DHAP (catalysed by GPDH), hence 3PG must have a higher energy.

But how can 3PG have higher energy than DHAP if you gain 1 NADH and 1 ATP in the process of going from DHAP -> 3PG (in glycolysis). Something just doesn't add up. What am I missing?

[Yggdrasil]

Glycolysis involves 3-phosphoglycerate and the glycerophosphate shuttle involves glycerol-3-phosphate.  These are different molecules.

[Babcock_Hall]

There are several problem areas.  One is that the substrate is for the enzyme GAPDH is glyceraldehyde 3-phosphate, not DHAP.  Two is that what you mean by "energy state" is unclear and is possibly misleading.  I would look at the Gibbs' free energy of each reaction (most biochemistry textbooks have a table that gives this for each reaction in glycolysis).  Three, I suggest drawing out the molecules and assign oxidation numbers to the carbon atoms.  That way one can see the oxidations or reductions more clearly.  https://facultystaff.richmond.edu/~jbell2/p1428.pdf