Aerobic Glucose Metabolism

All Those Electrons

From the complete oxidation of glucose to CO2 we have a few ATP and buckets of NADH and a couple reduced flavin. The flavin and NAD+ are cofactors/sosubstrates that are in limited supply and are not allowed to be transported from cell to cell. These must all, obviously, be returned back to back to the oxidized state to keep glycolysis and the Krebs cycle running.

Enter oxidative Phosphorylation. Here inside the mitochondrial inner matrix, the electrons that are held in NADH and the flavin get dumped off to the final electron sink .... molecular oxygen.
It is also the pathway where the great majority of our ATP is synthesized.

Oxidative phosphorylation is an oxymoron of the highest order. The phosphorylation refers of course to the synthesis of ATP from ADP and inorganic phosphate This is a condensation reaction (reverse of hydrolysis) and has NOTHING to do with a net transfer of electrons that is implied by the term "oxidative"

What the term is referring to is that the synthesis of ATP through this pathway requires two apparently unrelated steps.

  1. The transfer of electrons from NADH and flavin to O2 to produce H2O
  2. The synthesis of ATP from ADP and PO4=.

The trick comes in that these apparently unrelated steps are obligately coupled through a third process.

ATP ADP + PO4=     ΔG°' = -31kJ/Mole

This means that at least 31 kJ/mole of energy input is required to make ATP from ADP and phosphate. We already know one way to make reactions go "backwards". That is to have "product" concetrations build up to the point where the ΔG is appropriate. BUT, we also already know that for hydrolysis reactions this is not really feasible PLUS this is a really BAD idea in this case. Cells NEED ATP concetration to be high relative to ADP concentration to keep other processes going well.

We should find out if the transfer of electrons from NADH to O2 contains enough energy to "drive" ATP synthesis. This means that we need to look at the thermodynamics again. Now we need to compare redox energy to chemical energy. For this we need a fudge factor.