Oxidative decarboxylation:
- Pyruvic acid formed in the cytoplasm enters into mitochondria.
- Pyruvic acid is converted into Acetyl CoA in presence of pyruvate dehydrogenase complex.
- The pyruvate dehydrogenase catalyses the reaction require several coenzymes, including NAD+ and Coenzyme A.
- During this process two molecules of NADH are produced from metabolism of two molecules of pyruvic acids (produced from one glucose molecule during glycolysis).
- The Acetyl CoA (2c) enters into a cyclic pathway, tricarboxylic acid cycle.
Tri Carboxylic Acid Cycle (Krebs cycle) or Citric acid Cycle :
- This cycle starts with condensation of acetyl group with oxaloacetic acid and water to yield citric acid. This reaction is catalysed by citrate synthase.
- Citrate is isomerised to form isocitrate.
- It is followed by two successive steps of decarboxylation, leading to formation of α-ketoglutaric acid and then succinyl-CoA.
- In the remaining steps the succinyl CoA oxidized into oxaloacetic acid.
- During conversion of succinyl CoA to succinic acid there is synthesis of one GTP molecule.
- In a coupled reaction GTP converted to GDP with simultaneous synthesis of ATP from ADP.
- During Krebs cycle there production of :
- 2 molecule of CO2
- 3 NADH2
- 1 FADH2
- 1 GTP.
- During the whole process of oxidation of glucose produce:
- CO2
- 10 NADH2
- 2 FADH2
- 2 GTP.( 2 ATP)
Electron transport system and oxidative phosphorylation :
- The metabolic pathway, through which the electron passes from one carrier to another, is called Electron transport system.
- it is present in the inner mitochondrial membrane.
- ETS comprises of the following:
- Complex I – NADH Dehydrogenase.
- Complex II – succinate dehydrogenase.
- Complex III – cytochromes bc1
- Complex IV – Cytochromes a-a3 (cytochromes c oxidase).
- Complex V – ATP synthase.
- NADH2 produced in the citric acid cycle oxidized by NADH Dehydrogenase, and electrons are then transferred to ubiquinone located in the inner membrane.
- FADH2 is oxidized by succinate dehydrogenase and transferred electrons to ubiquinone.
- The reduced ubiquinone is then oxidized with transfer of electrons to cytochrome c via cytochromes bc1 complex.
- Cytochrome c is small protein attached to the outer surface of the inner membrane and acts as a mobile carrier for transfer electrons from complex III and complex IV.
- When electrons transferred from one carrier to another via complex I to IV in the electron transport chain, they are coupled to ATP synthase for the synthesis of ATP from ADP and Pi.
- One molecule of NADH2 gives rise to 3 ATP.
- One molecule of FADH2 gives rise to 2ATP.
- Oxygen plays a vital role in removing electrons and hydrogen ion finally production of H2O.
- Phosphorylation in presence of oxygen is called oxidative phosphorylation.
Total ATP Production -
Process Total ATP produced :
- Glycolysis 2ATP + 2NADH2 (6ATP) = 8ATP
- Oxidative decarboxylation 2NADH2 (6ATP) = 6ATP
- Krebs’s Cycle 2GTP (2ATP) + 6NADH2 (18ATP) + 2FADH2 (4ATP) = 24 ATP
- Energy production in prokaryotes during aerobic respiration = 38 ATP
- Energy production in eukaryotes during aerobic respiration = 38 − 2 = 36 ATP
- (2ATP are used up in transporting 2 molecule of pyruvic acid in mitochondria.)
Abbreviations :
ATP − Adenosine tri phosphate
ADP − Adenosine di phosphate
NAD − Nicotinamide Adenine dinucleotide
NADP − Nicotinamide Adenine dinucleotide Phosphate
NADH − Reduced Nicotinamide Adenine dinucleotide
PGA − Phosphoglyceric acid
PGAL − Phospho glyceraldehyde
FAD − Flavin adenine dinucleotide
ETS − Electron transport system
ETC − Electron transport chain
TCA − Tricarboxylic acid
OAA − Oxalo acetic acid
FMN − Flavin mono nucleotide
PPP − Pentose phosphate pathway
