Anaerobic Oxidation

Embden Meyerhof pathway

The initial  stage of biological oxidiation  includes  a series of nine  main sequential   steps  of reactions  known  as Embden  Meyerhof pathway, after  the name  of the  german  biochemist  who worked out  these  steps. Enzyme  required  -for this pathway  are should  components  ofcytosol .Hence  this process  occur  in cytosol  and  requires no oxygen. Anaerobes  some  lower animals  and plants  and some animals tissues  produce  energy only by the above  pathway  due to the absence of O₂    .This process  is then called sugar  fermentation .In  other organisms  using O₂

And hence called aerobes  also  this process occurs as such  in cytosol  only  as preliminary  stage  of cellurlar  respiration  In this  case it is termed  glycolysis ,Presumably therefore  this process   is more  primitive  perhaps the earliest method   of intracellular  energy  production  adopted  by cells during  early  evolution ,The  main difference in  sugar  fermentation  and glycolysis   is that  of the  endproucts  lactic  acid or ethyl alcohol in the case of sugar fermentation  and pyruvic acid  in the case of glycolysis .

 The nine steps of Embden Meyerhof pathway   are as follows

1.      In  first  reaction a glucose  molecule is phosphorylated  to glucose 6- phosphate  by the  terminal  phosphate bond of an ATP molecule  which is consequently hydrolysed to ADP, The  enzyme  hexokinase  activates  the glucose  molecule and catalyzes  this reaction.

C₆H₁₂O₆+ATP → glucose 6-phosphate +ADP

2.      In the second  step.  Phosphohexose  isomerise enzyme  catalyzes  conversion  of glucose 6- phosphate into  its structural isomer fructose  fructose 6- phosphate.

3.      Now the enzyme, phopshofructokinase , catalyzes further phosphorylation  of fructose  6- phosphate into fructose 1,6- disphosphate  using  the terminal bond of another ATP molecule.

4.      In  fourth   step aldolase   emzyme  catalyzes the splitting of fructose 1,6-diphosphogly molecule  into 3 carbon  molecules one of glyceraldehydes  3 phosphate  or 3 phoshoglyceraldehde  (PGAL=C₃H₅O₃ -phosphate)  and one of DHAP which j is reconverted to PGAL by  triose phosphate isomerase.

5.       Now  each PGAL molecule is oxidized into 1,3 diphospholyceric acid  with  the help  of 3 phosphoglyceraldehyde  dehydrogenase enzyme in  presence of  inorganic phosphate (p_i)   and NAD⁺  ,This reaction  yields  two electrons and twoprotons (equiavalent  to two  hydrogen atoms) from each PGAL molecule  acting  as the hydrogen acceptor NAD⁺   ( nicotinamide adenine dinucleotide  ) accepts  two ,electrons and one proton  the remaining  proton  being liberated into the cytosol. NAD ⁺   is the  thus , reduced to NADH;

2(PGAL) + 2(H₃ PO₄) +2NNAD⁺→ 2(1,3,diphosphoglyceric acid ) + 2NADH + 2H⁺

6.      1,3- diphosphoglyceric acid  is a high  energy compound  the enzyme  phosho glyceric acid  kinase, now  catalyzes  the transfer of the high energy phosphate  group  of this acid to ADP   forming  a molecule  each of 3- phosphoglyceric acid  and ATP.

7.      Now the enzyme  e phosphoglyceromutase , catalyzes the transformation of 3- phosphoglyceric  acid to 2- pphosphoglyceric acid

8.      Next ,the  enzyme  enolase,  catalyzes dehydration  of 2- phosphoglyceric acid, resulting into  the formation  again  of a high energy  compound phosphoenolpyruvic acid.

9.      The  enzyme  pyruvic  acid  kinase, now  catalyzes  the transfer  of the  high energy phosphate  group  of phosphoenolpyruvic  acid  to ADP .Thus a molecule  each  of pyrivic  acid  and ATP  are formed .

According  to the  above  account, Embden  Meyerhof pathway  brings  about a  gradual degradation  of glucose molecule  into two molecules  of pyruvic  acid   In  anaerobic  conditions.  Each pyruvic  acid  molecule is reduced by NADH. H_⁺ to  ethyl alcohol  (and CO₂)   in yeast  cells , and to lactic acid  some  bacteria and  animals  tissues  ( muscles and RBC_S) certain  bacteria  convert milk  into curd to  producing  lactic  acid in this  fashion, NADH. H+  obviously reverts back to its   original oxidized  form (NAD)  to be used  again for  the same  function.

Efficiency of embden  Meyerhof pathway : As  is clear from the above account  the balance sheet of anaerobic oxidation  is that two molecules  of ATP  are spent in it per glucose  molecule , and  four mols of ATP are  formed. Thus  there  is a net again of two  molecules. Thus,or 14600 clories (14.6keal) of energy about 2.0% of the total energy content  of a glucose molecule.