KREB'S CYCLE

 

  1. Kreb's cycle - introduction
    1. The Kreb's cycle converts pyruvate to CO2 and reducing energy (NADH and FADH2) and phosphorylated energy (GTP).

      2. 2 pyruvate + 2 GDP + 2 H3PO4 + 4 H2O + 2 FAD + 8 NAD+ ----> 6 CO2 + 2 GTP + 2 FADH2 + 8 NADH
    2. The reduced energy can be used to generate ATP using the electron transport chain in the presence of oxygen.
    3. It is a cyclic process in that oxaloacetate reacts with acetyl CoA to form citrate which starts a series of several other reactions. The final reaction in the series involves the regeneration of oxaloacetate!
    4. NAD and FAD; both are rather small nucleotide molecules that are electron carriers. FAD can transfer its reducing power to FP. The reduced and oxidized forms of NAD are not clear; thus oxidized NAD can be called => NADox; NAD+; or NAD; Reduced NAD can be called => NADred; NADH; and NADH2.
  2. Kreb's cycle steps:
    1. The three carbon pyruvate reacts whereby a carbon is lost as CO2. The reaction releases energy such that NAD+ is reduced to NADH. The remaining two carbon compound is not allowed to be free but rather binds a special molecule (coenzyme A) to form acetyl CoA.
      1. This is essentially an irreversible reaction due to the loss of CO2.
      2. This reaction is probably more than one step, considering all of the reactants. However, even to this day the intricacies of this reaction are not well understood.
      3. It is not clear in eukaryotic cells exactly what carbon form passes from the cytoplasm to the mitochondrial matrix (pyruvate? acetyl CoA? an intermediate?) The process looks very complex.
      4. The binding of CoA-SH to the two carbon molecule is probably a type of handle making it easier for enzymes to act upon it.
    2. The two carbon chain is released from acetylCoA and binds to the four carbon oxaloacetate to make the six carbon citrate ( a tricarboxylic acid)
      1. The six carbon molecule is much more manageable by enzymes. (analogy : like eating an ice cream off an ice cream stick). The two carbon molecule will eventually be totally decomposed to again regenerate oxaloacetate, the later of which can be reused again.
    3. The next reaction removes water from citrate which is followed by a reaction which adds back water!!! The purpose is to rearrange the molecule to make it more convenient to extract the energy.
    4. The next reaction releases energy to form reduced NADH.
    5. The reaction of oxalosuccinate (6 carbon molecule) to alpha-ketogluterate (5 carbon molecule) leads to the release of gaseous carbon dioxide.
    6. The next reaction again utilizes CoA-SH as a handle for one reaction. Energy is also released again as NADH. At this point we also lose the last carbon in the form of CO2.
    7. The reaction of succinyl-CoA to succinate leads to a phosphorylation of GDP to GTP. A GTP molecule can be easily converted to an ATP by one more reaction.
    8. Succinic acid to fumaric acid yields a reduced flavoprotein. This FADH2 can be passed to FPH2 and used in the electron transport chain (in the presence of oxygen) to presumably yield two ATP molecules.
    9. The final step of malate to oxaloacetate wrings out the last bit of energy (as NADH) of our original glucose. The regenerated oxaloacetate is now ready to react with another acetyl CoA and the whole vicious cycle can be repeated.
  3. Overall energy balance per glucose molecule:
  4. A.
    5.  

    Aerobic

    Anaerobic

    2 ATP used in glycolysis ==================>

    -2 ATP

    -2 AT P
         

    4 ATP formed in glycolysis ================>

    +4 ATP

    +4 ATP
         

    2 NADH2 formed in glycolysis ====via e.t.==>

    +6 ATP

    		  
         

    8 NADH2 formed in Kreb's cycle ===via e.t.==>

    +24 ATP

    		  
         

    2 GTP in Kreb's cycle =====================>

    +2 ATP

    		  
         

    2 FADH2 in Kreb's cycle ===via e.t.=========>

    +4 ATP

    		  
         

    Total:

    38 ATP

    2 ATP
    1. Frequently 36 ATP are quoted because it is known that in eukaryotic cells that the reduced NAD formed by glycolysis in the cytoplasm must be actively transported across the mitochondrial membrane to be made available to the electron transport chain. The cost of such active transport is one ATP for each NADH transported. Thus, the net gain for each cytoplasmic NADH is only two ATP rather than three. However if one plays this game, then one should also consider the active transport of other molecules as well (pyruvate? phosphate? Mg++? etc.). The actual net gain of ATP is unknown but must be regarded as less than 36.
  5. In eukaryotic cells the Kreb's cycle occurs in the liquid part of the mitochondria (matrix) while the electron transport chain occurs in the inner membrane (cristae).

The outer membrane is very porous and has almost no proteins; it seems to have no function and can almost be ignored.

In the endosymbiotic theory the inner membrane would correspond to the cell membrane of bacterial prokaryotes and the matrix would correspond to the cytoplasm of bacterial prokaryotes.