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Krebs Cycle


Krebs cycle was discovered by Hans Krebs. Krebs cycle is also known as Citric Acid Cycle, because the first intermediate compound formed in it is Citric acid (Citric Acid is first stable intermediate compound), in which 6 carbons are present.


As well as, Kreb’s cycle is also called TCA cycle (TCA-Tri Carboxylic Acid Cycle).
The process of Krebs cycle takes place in the matrix of mitochondria. 

After completion two turns of Krebs cycle there are total 6 molecules of NADH2, Two molecules of FADH2 and Two molecules of ATP are formed 

During this complete oxidation of two molecules of Pyruvic acid take place in mitochondrial matrix in Krebs cycle.
Although oxygen is not used directly in this process, but if there is oxygen in the cell, then only this process starts.
a 2-carbons acetyl coenzyme-A is needed to start this process.
2-carbons acetyl coenzyme-A is made from the oxidation of pyruvic acid in mitochondrial matrix.
This whole process is called Oxidative Phosphorylation (conversion of pyruvic acid to acetyl coenzyme-A) , and it also takes place in the matrix of the mitochondria.
The entire Krebs Cycle is also a 10 steps cyclic reaction.
All the important enzymes used in Oxidative Decarboxylation and Krebs cycle are present in the matrix of mitochondria.
Krebs cycle is also similar to glycolysis but cyclic reactions.

Steps of Kreb’s Cycle

There are following steps of Krebs cycle.

1- In the first steps, acetyl coenzyme-A, which is a two-carbon compound enters the Krebs cycle, and reacts with the four-carbon compound Oxaloacetic acid (OAA) to form a six-carbon citric acid molecule.
Because citric acid is the first product of Krebs cycle. That’s why Krebs cycle is also called citric acid cycle. This reaction is catalyzed by the enzyme synthase.

2- In the second step, citric acid is converted into cis-aconitate (6-carbons compound) with the help of enzyme Aconitase. In this process a water molecule is releases.

3- In this step, cis aconitate is converted into Isocitric acid (6-carbons compound) in the presence of Aconitase enzyme.

4- In this step Isocitric acid is converted into Oxalo-Succinate in the presence of dehydrogenase enzyme. Here a molecule NADH2 is formed.

5- In the next step the Oxalo-succinate is converted to alpha-ketoglutarate, which has five carbons.

This reaction is also called decarboxylation reaction. Because a molecule CO2 is released in it.

6- In this reaction, alpha-ketoglutarate (α-ketoglutarate), is converted into succinyl-coenzyme-A in the presence of enzyme, dehydrogenase (5-carbons α –Ketoglutarate Convert into 4 carbons Succinyl Co – A),

A molecule NADH2 is also formed in this process and a molecule CO2 is also released.

7- In this step succinyl coenzyme-A is converted into succinic acid (4-carbons compound) . Enzyme synthase is used in this process.

Along with this, an ATP is also formed. Here ATP is produced by substrate level oxidation.

8- Succinic acid is converted to fumarate (4-carbons compound) and a molecule of  FADH2, is also formed. Enzyme dehydrogenase is used in this process.
9-Now fumarate changes to malate (4-carbons compound) in the presence of fumarase enzyme.

In the last step, malate is again converted to oxaloacetate in the presence of enzymes, and thus the process of Krebs cycle continues.

Note- To completely oxidize 2 molecules of Acetyl coenzyme-A, the Krebs cycle will run twice. So in total 6NADH2, 2FADH2 and 2ATP molecules are formed.


Question – What are the other name of Krebs cycle?
Answer – Krebs Cycle is also known as Citric Acid Cycle or TCA (Tri Carboxylic Acid Cycle).

Question – Name of Biologist, who discovered Kreb’s Cycle?
Answer – This cycle was discovered by Hans Krebs, for which he awarded by the 1954 Nobel Prize.

Question – Where does the Krebs cycle take place?
Answer – Krebs cycle take place Mitochondrial matrix.

Question- How many ATP are forms by substrate level phosphorylation in Krebs cycle?
Answer – 2ATP is produced by substrate level phosphorylation in Krebs cycle.

Question – How many NADH2 and FADH2 molecules are formed in the Krebs cycle?
Answer – There are 6NADH2 and 2FADH2 molecules are formed in the Krebs cycle.

Question – How many steps requires in Krebs cycle?
Answer – There are total 10 steps require in Krebs cycle.


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