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 .