16.4.1. Some uncouplers are :
1. 2, 4-dinitrophenol (DNP).
It does not effect the substrate-level phosphorylation of glycolysis. At pH 7.0 DNP exists mainly as the anion which is not soluble in the lipids. In its protonated form, it is lipid-soluble and hence can pass through inner membrane, carrying a proton and transport the proton to other side. The proton (H+) carried by DNP is discharged on the other side of the membrane. The phenolate ion then diffuses back towards the cytosol side, where it picks up a proton to repeat the process. In this way, uncouplers prevent formation of H+ gradient across the membrane. Dinitrophenol also stimulates the activity of the enzyme ATPase., which is normally inactive as a hydrolytic enzyme in mitochondria. Actually, ATP is never formed in the presence of DNP, since the high-energy intermediate is attacked i.e., it acts prior to the step of ATP synthesis.
It has an action identical to that of 2,4-dinitrophenol. Dicoumarol is also an antagonist of vitamin K function.
3. m-chlorocarbonyl cyanide phenylhydrazone (CCCP)
Its action is also similar to that of 2, 4-dinitrophenol but it is more active than the DNP.
16.5. Energy Balance
ATP obtained from a complete catabolism of glucose
1. From Glycolysis (in cytoplasm)
For each glucose 2 ATP’s used -2
4 ATP’s formed +4
The NADH produce in glycolysis can yield either 2.5 ATP or 1.5 ATP depending upon the shuttle system.
2NADH (Glycolysis) 5 or 3
2. 2 Pyruvate → 2 acetyl-CoA
2 NADH molecules formed (2.5 ATP) +5
(This NADH is already in the mitochondria and no transport is necessary.)
3. Citric Acid Cycle (and Electron transport chain)
From each acetyl-CoA we get 3 NADH, 1 FADH2 and 1 ATP. Two acetyl-CoA enter the cycle (if we started with 1 glucose).
6 NADH (2.5 ATP) +15
2 FADH2 (1.5 ATP) +3
2 ATP +2
Total (for two) 30/32
16.6. Shuttle system :
The glycerophosphate shuttle. The electrons of cytosolic NADH are transported to the mitochondrial electron-transport chain in three steps : (1) Cytosolic oxidation of NADH by dihydroxyacetone phosphate catalyzed by glycerol-3-phosphate dehydrogenase.
This enzyme is present in cytosolic (2) Oxidation of glycerol-3-phosphate by flavoprotein dehydrogenase with the reduction of FAD to FADH2. (3) Reoxidation of FADH2 with the passage of electrons into the electron-transport chain.