What is Citric Acid Cycle?

Citric Acid Cycle is an essential catabolic cycle of cellular respiration involves pyruvate as an end product from glycolysis to form energy in terms of ATP, NADH and FADH2. Which in further processes to provide ATP.

The Citric Acid Cycle takes place in Mitochondria and are coupled with Electron Transport Chain to produce ATP.

Hence Mitochondria is the power house of the cell. The Citric Acid Cycle was discovered by Hans Kreb in 1937 from animal enzymes.

Therefore, it can be named as the Krebs’s Cycle or Tricarboxylic Acid Cycle (Contains 3 carboxyl group in the component).

Citric Acid Cycle Feature

1. Takes place during aerobic respiration:

Pyruvate under aerobic condition produces energy and under anaerobic condition they undergo a process of fermentation forming lactic acid or alcohol

2. TCA Cycle is irreversible at 3 steps: producing high amount of negative energy.

The 3 steps involve the enzymes: citrate synthase, α – ketoglutarate dehydrogenase and isocitrate dehydrogenase.

3. Pyruvate; an end product of glucose metabolism; is transported from cytosol to mitochondria to become Acetyl Co – A.

Pyruvate is transported to mitochondria through Voltage gated channel crossing outer mitochondrial membrane. Mitochondrial Pyruvate Carrier then takes pyruvate through Inner Mitochondrial Membrane.

Pyruvate Dehydrogenase Complex; an enzyme oxidizes Pyruvate to form Acetyl Co – A.

4. Cycle involves C6 to C4 in a series of steps:

Tricarboxylic Acid (C6 – 6 carbon compound) is formed by the combination of Acetyl Co – A (C2) and Oxaloacetate (C4).

Condensation process takes place between Oxaloacetate and Acetyl Co – A to form C6 compound.

Further the C6 compound catabolize to produce reduced equivalents (NADH, FADH2).

Oxaloacetate + Acetyl Co – A à Citrate (C6)

5. Eight enzymes are involved in 4 oxidations:

On each oxidation, number of carbo atoms present reduces gradually leaving out C4 compound. C4 the condense to form C6 and he cycle continues.

6. Two sources for Acetyl Co – A:

Acetyl Co – A is not only derived from Glycolysis. Fatty Acid oxidation also provides Acetyl Co – A as its direct end product which can be utilized by the Citric Acid Cycle.

7. A cycle with various intermediate forms:

Citric acid cycle forms various intermediates which provides substrates for different biosynthetic process in plants.

For Example: α – keto glutarate forms glutamate which is essential in Nucleic Acid formation

8. Citric Acid Cycle coupled with Electron Transport Chain:

The reduced equivalents from the cycle is directly transported to complexes present in the inner cell membrane of mitochondria to yield ATP.

Mechanism of Citric Acid Cycle

The Citric Acid Cycle has two steps:

1. Pyruvate Decarboxylation

2. Citric Acid Formation

3. TCA Cycle

Step: 1 Pyruvate Decarboxylation

Enzyme Complex involved – Pyruvate Dehydrogenase Complex (combination of enzymes, coenzymes and cofactors)

Pyruvate Dehydrogenase Complex (PDC) consists of 3 enzymes and 3 cofactors.

They are:

a. Pyruvate decarboxylase(E) + Thiamine pyrophosphate (CoF) – Complex 1

b. Dihydrolipopyl Transacetylase (E) + Lipoic Acid (CoF) – Complex 2

c. Dihydrolipopyl Dehydrogenase (E) + Flavin Adenine Dinucleotide (CoF) – Complex 3

The enzyme complex catalyses the oxidation reaction of Pyruvate to Acetyl Co – A inside the mitochondria with the release of CO2 and reduction of NAD → NADH2

Step: 2 Citric Acid Formation

Enzyme: Citrate Synthase This step is the 1st step of the Citric Acid cycle

I – Citric Acid, a six-carbon compound is formed by condensation and hydrolysis. Acetyl Co – A and Oxaloacetic Acid combines along CH3 and COO- group of acetyl Co – A and Oxaloacetic Acid respectively. On hydrolysis, Coenzyme A is released and Citric Acid is formed.

Step: 3 TCA Cycle

Citrate is the tricarboxylic compound from which the cycle proceeds.

II – Formation of Isocitrate

Enzyme: Aconitase

Enzyme aconitase mediates dehydration and rehydration of citrate molecule forming an isomer of citrate which is isocitrate.

III – α – Ketoglutarate Formation

Enzyme: Isocitrate Dehydrogenase

Dehydrogenase is associated with oxidation of the isocitrate to form α – ketoglutarate with a release of CO2 and formation of carboxyl group. This oxidation involves a reduction of NAD → NADH2. The six – carbon compound now becomes five carbon compound (C5).

IV – Succinyl Co – A synthesis

Enzyme: α – Ketoglutarate Dehydrogenase Complex

The enzyme functions similarly to that of Pyruvate Dehydrogenase accounts for the oxidation of α – Ketoglutarate to Succinyl Co – A. CO2 is released and NAD → NADH2. In this step 4 the five – carbon compound reduces to four – carbon compound (C4).

V – Succinate

Enzyme: Succinyl Co – A synthase

The mitochondrial matrix provides a phosphate group to be attached to the Co – A releasing Coenzyme from the compound. On hydration (i.e. addition of water) succinate is formed. ADP → ATP

VI – Fumarate

Enzyme: Succinate Dehydrogenase

The enzyme catalysis the formation of Fumarate by reducing FAD → FADH2. A double bond is formed between 2nd and 3rd carbon.

VII – Malate

Enzyme: Fumarase

Adding Fumarase and hydrating Fumarate results in the formation of Malate.

VIII – Oxaloacetate formation

Enzyme: Malate dehydrogenase

As dehydrogenase is coupled with formation of reduced equivalents, oxaloacetate is formed by reducing NAD → NADH2. This is the last and the fourth oxidation step in the TCA cycle.

Significance of Citric Acid Cycle

1. Citric Acid Cycle mainly provides ATP as energy. This is the important function of the citric acid cycle.

Acetyl Co – A + FAD + 3 NAD + ADP + H2O →

→ CO2 + ATP + 3 NADH + FADH + Co – A

One NADH = 3 ATP

One FADH2 = 2 ATP

ATP = 1

Total = 3(3) + 2 + 1 = 9+2+1 = 12

Citric Acid Cycle provides 12 ATP but glycolysis produces 2 pyruvate molecule the above calculation accounts only for one cycle of pyruvate.

Two cycle of Citric Acid then provides 2(12) = 24 ATP molecules

2. Citric Acid cycle is Amphibolic. Amphibolic means a process or a reaction which can be both Catabolic and Anabolic.

Catabolic nature is the breakdown of pyruvate for energy.

Anabolic nature is that the intermediates of Citric Acid Cycle can be used for the synthesis of amin acids, purine, porphyrin and pyrimidines

3. As Citric Acid cycle provides intermediates for synthesis of amino acid, purines etc., the cycle cannot run in a proper way.

To make the cycle to run in a proper way, many replenishing reactions are taking place to fill the missing intermediates. This process is called the anaplerosis.

4. Citric Acid cycle is the final common oxidative for energy production of carbohydrates, lipids and proteins.