With the help ofschematic representation explain Krebs cycle.
The cyclic process through which acetyl Co-A is completely oxidised and CO2 is released in a step-wise manner is called Krebs cycle.
Steps in Krebs cycle:-
- Acetyl Co-A (2C) reacts with oxaloacetate (4C) to form citric acid or citrate (6C).
- The reaction takes place in the presence of citrate synthetase.
- One water molecule is used in this reaction.
- A molecule of Co-A released is recycled for the acetylation of another molecule of pyruvate.
OAA + Acetyl Co - A H2O`->`Citrate + Co `->` A
- Citric acid is isomerised into isocitric acid in the presence of the enzyme aconitase and Fe2+.
- This occurs in two steps:-
- Dehydration during which citrate loses water molecules to form cisaconitate (6C). Citrate Cis `->` aconitate + H2O
- Hydration during which cis-aconitate combines with a water molecule to form iso-citrate (6C).Cis `->` aconitate + H2O Iso - citrate
- Iso-citrate (6C) is oxidised in the presence of the enzyme iso-citrate dehydrogenase and Mn2+ ions to form an intermediate called oxalosuccinate (6C).
- Two hydrogen atoms released during this step are taken up by the coenzyme NAD to form NADH2.
Iso - citrate + NAD`->`Oxalosuccinate + NADH2
- Oxalosuccinate (6C) undergoes decarboxylation in the presence of the enzyme decarboxylase to form α-ketoglutarate (5C).
Oxalosuccinate`->`α - Ketoglutarate + CO2
5.Oxidative decarboxylation (Dehydrogenation-II and Decarboxylation-II):
- α-Ketoglutarate (5C) undergoes simultaneous dehydrogenation and decarboxylation to form the four-carbon compound succinyl Co-A.
- The reaction takes place in the presence of α-ketoglutarate dehydrogenase.
α - Ketoglutarate + Co `->` A + NAD`(alpha - `Succinyl Co `->` A + NADH2 + CO2
6.Hydration and phosphorylation:
- Succinyl Co-A (4C) gets hydrated using a water molecule to form succinate (4C) in the presence of succinate thiokinase.
- GDP is phosphorylated to form GTP.
Succinyl Co - A + H2O`->`Succinate + Co - A
GTP + ADP → ATP + GDP
- Succinate (4C) is oxidised to form fumarate (4C) in the presence of succinate dehydrogenase.
- The hydrogen atoms released in this reaction are accepted by FAD and it is reduced to FADH2.
Succinate + FAD`->`Fumarate + FADH2
- A molecule of water is added to fumarate (4C) to form malate (4C).
- The reaction takes place in the presence of fumarase.
Fumarate + H2O`->`Malate
- Malate (4C) is oxidised by the removal of hydrogen atoms to form oxaloacetate (4C) in the presence of malate dehydrogenase.
- Released hydrogen atoms are accepted by NAD and it is reduced to NADH2.
Malate + NAD`->`Oxaloacetate + NADH2
Two turns of the Krebs cycle produce six molecules of NADH2, two molecules of FADH2 and two molecules of ATP.
During each turn, three molecules of water are used up and two molecules of CO2 are released.
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