Oxidative phosphorylation

overview

 

o       intermembrane space (pH 4) is 10 000 times more concentrated

              with protons than the matrix (pH 8) 

o       this osmotic and electrical pressure is referred to as a proton-motive force

o       when the protons diffuse down this gradient back into the matrix through a channel in the

             ATP synthase molecule, the force of this movement is enough to drive the uphill synthesis of ATP

             (chemiosmotic phosphorylation of ADP)

o       since oxygen must be present to accept electrons at the end of the ETC, the events after the Krebs cycle can be referred to   

     as oxidative phosphorylation

o       NADH produced in the mitochondrion drives the translocation of 3 pairs of protons into the intermembrane space

o       Three molecules of ATP are synthesized from each NADH. Compare this to the 4 ATP generated

               from glycolysis and the Krebs cycle!

o       FADH₂ carries electrons at lower energy levels, so it donates its electrons further down the chain

o       Two pairs of protons are translocated as it journeys down to oxygen, and 2 ATP are produced from each FADH₂

o       NAD⁺ and FAD remain in the  matrix to capture electrons from the conversion of pyruvate and the

               Krebs cycle intermediates.

 

SUMMARY: 3 pairs of H+ are pumped across per NADH + H+ molecule, and 2 pairs of H+ are pumped across per FADH₂

 

    The cytoplasmic NADH (don't forget!) from glycolysis transfers its electrons to a shuttle that behaves like FADH₂

   The ATP yield from each cytoplasmic NADH is 2 in most eukaryotic cells, not all. 

   The NAD⁺ remains in the cytoplasm to accept more hydrogen from the glycolytic pathway. 

 

All loose ends are tied up!

 

KEEP THE ATP YIELDS STRAIGHT!!

mitNADH ® 3ATP

FADH₂® 2ATP

cytNADH ® 2ATP

 

    The equation

 

    C₆H₁₂O₆  +  6O₂  ®6CO₂ + 6H₂O + energy

    D G = -686 kcal/mol

 

    Free energy is trapped in ATP molecules. Very little energy remains in

    the carbon dioxide and water, while the rest is converted to heat.

 

    The balance sheet

 

    ATP yields must be viewed as estimates only. You must be able to

    account for each and every ATP molecule synthesized, including the

    process, location, and type of phosphorylation.

 

    For each glucose entering the cell

 

    Glycolysis  (cytoplasm)  net yield of             2 ATP   (SLP)

                                           2 NADH              4 ATP    (OP)

 

    Conversion of pyruvate  (mitochondrial matrix)

                                           2 NADH               6 ATP   (OP)

 

    Krebs cycle (mitochondrial matrix)                 2 ATP   (SLP)

                                           6 NADH               18 ATP   (OP)

                                           2 FADH2               4 ATP   (OP)

 

    TOTAL = 36 ATP per glucose (38 possible depending on cell and cytNADH shuttle)

 

    Efficiency

 

    The hydrolysis of ATP releases 7.3 kcal/mol.  Efficiency calculations

    take the number of ATP generated times the energy content of each

    ATP and divide this total by the free energy contained in glucose.

 

    fermentation   (2 X 7.3)/686  = about 2%

 

    cellular respiration (36 X 7.3)/686 = about 40%

 

 (may be as high as  60%, since it is believed that each ATP releases 10-12 kcal/mol under

  cellular conditions.)  Compare this to 25% efficiency for high-performance cars!