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Kreb's cycle
🎯 Kreb's cycle includes various steps of breaking the bond of carbon-carbon into CO2 which take place in mitochondria.
🎯 Kreb's cycle also known as the Citric acid cycle (1st product of the reaction) and as the Tricarboxylic acid cycle (The nature of organic acid produced in the reaction).
🎯 Under aerobic conditions Pyruvic acid is oxidised through TCA Cycle (Tricarboxylic acid cycle) given by Krebs.
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Process
Pyruvic acid cannot be directly involved in Kreb's cycle hence NAD and CoA
react with Pyruvic acid for their activation oxidative decarboxylation takes
place in this reaction and one molecule of Co2 is released and NAD is reduced
to NADH2, leading to the formation of Acetyl CoA. All enzymes (Pyruvate Dehydrogenase) of this reaction are present in the Matrix of mitochondria.
Pyruvic acid + CoA + NAD → Acetyl CoA + NADH + H+
Pyruvate Dehydrogenase
This reaction is a link reaction. Which is not a part of Kreb's cycle. The
actual Krebs cycle starts when Acetyl CoA enter the Matrix of mitochondria.
1. Acetyl CoA (2 carbon ) with oxaloacetic acid to form
citric acid (6 carbon) in the presence of citrate synthetase. The CoA
group is released and a water molecule is absorbed.
Acetyl CoA + H2O+ oxaloacetic acid → Citric acid
citrate synthetase
2. Citric acid is dehydrated in the presence of enzyme aconitase and
cis aconitic acid is formed.
Citric acid → Cis aconitic acid
aconitase
3. Cis aconitic acid is hydrated in the presence of enzyme aconitase to
yield Isocitric acid (6 carbon)
Cis aconitic acid → Isocitric acid
aconitase
4. Isocitric acid undergoes dehydrogenation in the presence of the
enzyme Isocitrate dehydrogenase to form Oxalosuccinic acid. Here
hydrogen atom pair is taken by NDA+ to form NADH + H+.
NDA+ → NADH + H+
Isocitric acid → Oxalosuccinic acid
Isocitrate dehydrogenase
5. Oxalosuccinic acid loses its one carbon atom in the form of CO2
to form ɑ(Alpha)-Ketoglutaric acid, a 5-carbon compound. The enzyme is
carboxylase.
Oxalosuccinic acid → ɑ-Ketoglutaric acid
carboxylase
6. ɑ-Ketoglutaric acid undergoes simultaneous decarboxylation and
dehydrogenation. CoA is added to it and the CO2 molecule is liberated. Energy
is released in the form of one NADH + H+ molecule. The reaction ends in a
4-carbon compound Succinyl-CoA. The enzyme needed is ɑ-Ketoglutaric
dehydrogenase.
CoA → CO2
NAD+ → NADH + H+
ɑ-Ketoglutaric acid → Succinyl-CoA
ɑ-Ketoglutaric dehydrogenase
7. CoA is released from Succinyl CoA and Succinic acid is formed
in the presence of the enzyme Succinic thiokinase. Here one molecule of GTP or
ITP is formed from GDP or IDP. This GTP is converted into ATP. It may be noted
that this is only a reaction of TCA Cycle production ATP.
GDP → GTP
Succinyl CoA → Succinic acid
Succinic thiokinase
(GTP + ADP → GDP + ATP)
8. Succinic acid again undergoes dehydrogenation to form
Fumaric acid in the presence of succinate dehydrogenase. Here one pair
of hydrogen is taken by FAD to form FADH2. One FADH2 is equal to 2 ATP.
FAD → FADH2
Succinic acid → Fumaric acid
succinate dehydrogenase
9. An H2O molecule is added to Fumaric acid to form
Malic acid in the presence of Fumarase.
Fumaric acid + H2O → Malic acid
Fumarase
10. Malic acid again undergoes dehydrogenation to form
oxaloacetic acid. Which was the beginning compound of this cycle and
thus, complete one molecule cycle. One NADH + H+ is formed, and the enzyme is
Malate Dehydrogenase.
NAD+ → NADH + H+
Malic acid → oxaloacetic acid
Malate Dehydrogenase
Net Gain of ATP in Respiration
A). Glycolysis → 8 ATP
B). In the oxidation of Pyruvic acid → 6 ATP ( 2 × 1NADH + H+)
C). Kreb's cycle → 24 ATP
I). 3 NADH + H+ = 9 ATP (1NADH + H+ = 3 ATP)
So 2* × 9 ATP = 18ATP
II). 1 FADH2 = 2 ATP
SO 2* × 2 ATP = 4 ATP
III). 2*× 1 ATP = 2 ATP
(* 2 Molecule of Acetyl CoA)
So Total ATP gain in Respiration is → 38 ATP
Important Note
- All reaction of Kreb's cycle is reversible, but the link reaction is not reversible.
- In Kreb's cycle Decarboxylation, Oxidation and Phosphorylation occur.
- No oxygen is taken up in Kreb's cycle itself but it is taken up in the respiratory chain.
- Oxalo Succinic acid and ɑ-keto glutamic acid are sources of CO2 in Kreb's Cycle.
- The total no. Of CO2 produced in Kreb's Cycle from 2 molecules of Acetyl CoA - 4CO2
- Decarboxylase Enzyme - For removal of CO2
- Dehydrogenase Enzyme - For removal of Hydrogen.
- NAD+ (Major acceptor) and FAD (Minor acceptor) are H acceptors.
- Total no. of 2H released in Kreb's Cycle - 2H × 8 = 16H
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