Discuss the various steps of Krebs cycle or TCA cycle. Explain the electron transport system.

The pyruvic acid formed during glycolysis enters mitochondria where it is further oxidised.  Following two steps of aerobic oxidation occur within mitochondria.

  1. Formation of Acetyl coenzyme A.
  2. Krebs cycle or citric acid or tricarboxylic acid cycle (TCA).




Pyruvic acid molecules produced during glycolysis move into mitochondria and all reactions of cellular respiration take place within these tiny power houses.  Here each three carbon molecule of pyruvic acid is decarboxylated and dehydrogenated.  Due to release of carbon dioxide,3 carbon molecule of pyruvic acid is converted to a 2 carbon acetyl group which reacts with a large complex enzyme, called coenzyme A.  This results in the formation of a compound called, Acetyl coenzyme A (CoA).  This process requires five cofactors namely, magnesium (mg++) thiamine pyrophosphate (TPP), NAD, coenzyme A and lipoic acid.  The overall reaction of conversion pyruvic acid into Acetyl coenzyme A can be written as:


2 Pyruvic acid+2 CoA+2 NAD. →2 Acetyl CoA+2 NADH+2H+


Thus, each molecules of pyruvic acid (3 carbon compound) forms one molecule of carbon dioxide (1 carbon compound) and one molecule of Acetyl coenzyme A (2 carbon compound).  The NADH2 molecule formed in this process enters the electron transport system of mitochondria to release energy




The reactions of these cycles were worked out by Sir. Hans Krebs, hence, the name Krebs cycle.  The important  steps of Krebs cycle are as follows:

  1. Formation of citric acid: Oxaloacetic acid accepts acetyl group of the acetyl coenzyme A and forms citric acid in the presence of enzyme, citrate synthetase.


Oxaloacetic acid + Acetyl Co A + H2O→Citric acid+CoA


  1. Formation of cis-aconitic acid: Citric acid (6 carbon compound) by losing one molecule of water is changed into another 6 carbon compound cis-aconitic acid in the presence of enzyme, aconitase.


Citric acid →Cis-aconitic acid+H2O


  1. Formation of iso-citric acid: Cis-aconitic acid is converted into iso-citric acid by utilizing water in the presence of enzyme, aconitase.


Cis-aconitic acid+H2O→Iso-citric acid


  1. Formation oxalosuccinic acid: Iso-citric acid on oxidation gives rise to another 6 carbon compound oxalosuccinic acid in the presence of enzyme, iso-citrate dehydrogenase. Two hydrogen atoms removed in this process are accepted by NAD.


Iso-citric acid+NAD→Oxalosuccinic acid+NADH2


  1. Formation of α-ketoglutaric acid: This step involves decarboxylation of oxalosuccinic acid. Decarboxylation of oxalosuccinic acid results in the formation of a 5 carbon compound α-ketoglutaric acid in the presence of enzyme, decarboxylase.


Oxalosuccinic acid→α-ketoglutaric acid+CO2


  1. Formation of succinyl Co A: Oxidative decarboxylation of α -ketoglutaric acid results in the formation of succinyl Co A, which is a 4 carbon compound in the presence of enzyme, α-ketoglutarate dehydrogenase.  In this step, one molecule of NAD is reduced to NADH2 and one molecule of CO2 is released.


α-ketoglutaric acid+CoA→Succinyl CoA+CO2+NADH2


  1. Formation of succinic acid: In this step, coenzyme A splits off from succinyl CoA and forms succinic acid in the presence of enzyme, succinyl thiokinase.  In this process, one molecule of GTP is formed, which later gives one molecule of ATP.  This reaction is also referred to as substrate phosphorylation.


Succinyl CoA+GDP+H3PO4+ H2O→Succinic acid+GTP+CoA



  1. Formation of fumaric acid: Oxidation of succinic acid results in the formation of fumaric acid in the presence of enzyme, succinate dehydrogenase.  Two hydrogen atoms are released, which are picked up by the hydrogen carrier molecules, FAD (flavin adenine dinucleotide).


Succinic acid+FAD→Fumaric acid+FADH2


  1. Formation of malic acid: With the addition of water, fumaric acid is converted into malic acid in the presence of enzyme, fumarase.


Fumaric acid+H2O→Malic acid


  1. Regeneration of oxaloacetic acid: In the last step of Krebs cycle, oxaloacetic acid is regenerated by oxidation of malic acid in the presence of enzyme, malate dehydrogenase.


Malic acid+NAD→Oxaloacetic acid+ NADH2


Oxaloacetic acid picks up another molecule of activated acetate to repeat the cycle.  A molecule of glucose yields two molecules of NADH2, 2 ATP and 2 pyruvate while undergoing glycolysis.  The 2 molecules of pyruvate are completely degraded in Krebs cycle to form 2 molecules of ATP, 8 NADH2 and 2 FADH2.


Krebs cycle 


The inner mitochondrial membrane contains groups of electron and proton transporting enzymes.  In each group, the enzymes are arranged in a specific series called, electron transport chain (ETC) or mitochondrial respiratory chain or electron transport system (ETS).  An electron transport chain or system is a series of coenzymes and cytochrome that take part in the passage of electron from a chemical to its ultimate acceptor.  The passage of electron from one enzyme or cytochrom to the next results in a loss of energy at each step.  At each step, the electron carriers include flavins, iron sulfur complexes, quinones, and cytochromes.  Quinones are highly mobile electron carrier.  4 enzyme complexes are involved in electron transport –

(i)         NADH-Q reductase or NADH dehydrogenase complex.

(ii)        Succinate Q-reductase complex.

(iii)       QH2-cytochrome c reductase complex.

(iv)       Cytochrome c oxidase complex.


NADH-Q has 2 prosthetic groups, flavin mononucleotide (FMN) and iron sulfur (Fe-S) complexes.  Both electrons and protons pass from NADH2 to FMN.and the latter is reduced.  However, FMNH2 breaks to release protons and electrons.  Protons are passed out through the inner mitochondrial membrane to outer chamber.

Electrons now move to the iron sulfur complex (Fe-S) and from there, to quinone   The common quinone is coenzyme Q, also called ubiquinone (UQ).

Charged quinone picks the proton from mitochondrial matrix and passes it into the outer chamber with the help of cytochrome b.

FADH2 produced during the reduction of succinate passes its electrons and protons to coenzyme Q through iron sulfur complex (Fe-S).  The enzyme is succinate Q reductase complex.

Q H2-cytochrome c reductase complex has 3 components- cytochrome b, iron sulfur complex (Fe-S) and cytochrome c1 .  Coenzyme Q may also be involved between iron sulfur complex (Fe-S) and cytochrome c1.

 Cytochrome c1 hands over its electron to cytochrome c.   Like Coenzyme Q, cytochrome c is also mobile carrier of electrons.

Cytochrome c oxidase complex contains cytochrome a and cytochrome a3 .Cytochrome a3 also possess 2 copper centres, which helps in the transfer of electrons to oxygen.  Oxygen is the ultimate acceptor of electrons.  It becomes reactive and combines with protons to form metabolic water.

Energy released during passage of electrons from one carrier to the next is made available to specific transmembrane complexes which pump protons from the matrix side of the inner mitochondrial membrane to the outer chamber.  This increases proton concentration in the outer chamber or outer surface of the inner mitochondrial membrane.  The difference in the proton concentration on the outer and inner sides of the inner mitochondrial membrane is known as proton gradient.


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