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LET'S TALK LIFE-SCIENCE BIOCHEMISTRY

Suraj Prakash Sharma | Ekta Chotia

MECHANISMS OF OXIDATIVE PHOSPHORYLATION
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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.

2.            Dicoumarol

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.

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