Krebs Cycle

The acetyl  group  of acetyl   co A  is now completey  degraded  stepwise  into  carbon  and hydrogen atoms  in the mitochondria  by a cyclic  sequence of  eight main  reaction steps. The  carbon atoms combine with O₂ forming  CO₂   which    is discharged out  of the body  as an endproduct of respiration .Hydrogen atoms are. On  the other hand ,taken  up by hydrogen acceptor  molecules. This series or  pathway  is known as krebs cycle ( after hans krebs  who first decribed it in 1937)  or citric acid cycle ( because  formation of citric acid is the  first step  in this cycle) or tricarboxylic  acid( TCA)   Cycle ( because  many  intermediate compounds  formed in the cycle have  three carboxyl groups.) The enzyme of this cycle  mostly occur  in the  mitochondrial matrix,  the successive  steps  of the cycle  are as follow .

1.      Krebs cycle  begins  with  linking  of acetyl   group (2-  carbon compound)  of  acetyl Co.A  to oxaloacetic acid (a-4 carbon compounds ), using a molecule of H₂O and  forming  citric acid (a-6  carbon compound ) , This  is a condensation  reaction  catalyzed by  citrate synthsase.

Subsequent, reaction in the cycle invole a series of structural  rearrangements , leading  to dehydrogenations  and decarboxylations

2.      In second step,  the citric acid is  converted to its isomer  isocitric acid  by   two  reaction  catalyzed by the enzyme aconitase.

3.      Next, the  is citric acid  undergoes oxidative decarboxylation in two  steps, It is first  converted to oxalosuccinic acid  and then to 5 carbon  ketoglutaric acid.    These  reactions are catalyzed by is citrate  dehydrogenase, an  enzyme that  uses NAD⁺ as a coenzyme . This  step is obviously  linked  to the  release  of a molecules  of CO₂ and two  hydrogen  atoms  which  reduce  a molecule  of NAD⁺ to NADH. H⁺.

4.      A  further oxidative decarboxylation  results in conversion of  ketoglutaric  acid to high  energy 4- carbon  compound  succinly coenzyme A in a multistep reaction  catalyzed by three enzymes  collectively  called ketoglutarate  dehydrogenase complex. A molecule of carbon  dioxide and a pair  of hydrogen atoms  are again  released. Hydrogen atoms  are  again  accepted by  a molecule of NAD ⁺  which is  reduced to NADH. H ⁺

5.      Succinyl Co A reacts with GDP ( guanosine  diphosphate  )  and a phosphate  group  to form succinic  acid,  phosphorylating GDP into GTP( guanosine  triphosphate  ).   This reactionis catalyzed by  succinae thiokinase  (= succinyl Co A  synthetase) GTP is a  high energy  compounds  like  ATP. It either  supplies its  energy  to certain cellular reactions  or transfers  its terminal  high energy phosphate bond  to ADP, forming ATP.

6.      Now  succinct  acid is dehydrogenated to fumaric acid   by a reaction  catalyzed by succinate  dyhydrogenase. The   latter is the  only enzyme  of kerbs cycle  occurring  in the  inner  membrane  of mitochondrial wall , instead of in  the mitochondrial matrix. The pair  of hydrogen atoms  released in this   reaction  is accepted by a molecule  of FAD ( flavin  adenine dinucleotide)  which is conjugated to  succinate dehydrogenase as a prosthetic group  .Hence, FAD is  converted to FADH₂ .

7.      The enzyme  fumarase  now  catalyzes addition  of a molecules  of H2O (hydration ) to fumaric acid , forming  malic acid.

8.      Malic acid is dehydrogenated to from  oxaloacetic  acid .This  reaction is  catalyzed by a NAD- linked  enzyme  malate dehtdrogenase, It result  in reduction of a molecule  of NAD^+. The  oxaloacdetic  acid now  starts  next drebs cycle  by reacting  with  acetyl CoA. 

9.      Oxidative Phosphorylation

At three sites (complexes  I, lll  and lV)  along a respiratory  chain . each  electron  pair  loses   so much  of its  free energy that  at each site  a phosphate  group  binds  with an ADP molecule  by a high  energy  bond, forming  a molecule  of  ATP, Thus  oxidation  and phosphorylatin occur  simulataneously  to from ADP. This  is, therefore  called  oxidative  phosphorylation .Obviously  an electron   pair  originating  from NADH. H+   yields  only  two molecules  of ATP, because  it is  fed into  the chain  at CoQ stage.

Modern  scientists  have  discovered that the ATP synthesis in mitochondria     is brought  about  by ATP synthesizing  enzyme complex  which  are also  components  of  inner mitochondrial  membrane .Each  such complex  called ATP aynthetase  or F₀,F₁  ATPase  has two major  components  F₀ and F₁ component  protrudes like a  knob into  the matrix from  the inner  membrane .It is atteached by  a stalk to F0  component  ( basis  piece )  which  is embedded in the  membrane  and extends across it.  Scanning its thickness.

Chemiosmotic  mechanism :  The  actual  mechanism  of ATP  synthesis  by F0, F1  ATPase  complexes has been  postulated by Peter Mitchell (1961)  in his  chemiosmotic  hypothesis .According  to this postulate, the free energy  of  electron , released during  electron  transport,  is used   in pumping proton (H⁺)  pair from  mitoxhondriall  matrix across inner  mitochondrial  membrane  into the  perichondrial  space  of cristae  against  a diffusion  gradient of H⁺. This    proton pump  generates  an electrochemical  H⁺ gradient  (  a diffusion  gradient  and a membrane  potential )  between  the mitochondrial  matrix  and perichondrial  space. The  potential  energy  of this drives  the protons back  into  the matrox  through  channels is in F₀, F₁  ATPase  complexes . The free energy  released as  protons flow  back through   a channel  causes synthesis  of ATP from  a coupled  reaction  between ADP  and phosphate radical .