TCA Cycle (Tricarboxylic Acid Cycle)

“TCA cycle is the series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl CoA derived from carbohydrates, fats, and proteins into ATP.”

TCA cycle or Tricarboxylic Cycle is also known as Kreb’s Cycle or Citric Acid Cycle. It is the second stage of cellular respiration that occurs in the matrix of mitochondria. All the enzymes involved in the citric acid cycle are soluble.

It is an aerobic pathway because NADH and FADH₂ produced transfer their electrons to the next pathway which will use oxygen. If the transfer of electrons does not occur, no oxidation takes place. Very little ATP is produced during the process directly.

The TCA cycle is a closed loop. The last step of the pathway regenerates the first molecule of the pathway.

Also Read: Glycolysis

Steps of TCA Cycle

Following are the important steps of the TCA cycle:

Step 1

Acetyl Co-A combines with a four-carbon compound, oxaloacetate, and releases the CoA group resulting in a six-carbon molecule called citrate.

Step 2

In the second step, citrate gets converted to isocitrate, an isomer of citrate. This is a two-step process. Citrate first loses a water molecule and then gains one to form isocitrate.

Step 3

The third step involves oxidation of isocitrate. A molecule of carbon dioxide is released leaving behind a five-carbon molecule, ɑ-ketoglutarate. NAD⁺ gets reduced to NADH. The entire process is catalyzed by the enzyme isocitrate dehydrogenase.

Step 4

Here, ɑ-ketoglutarate is oxidized reducing NAD⁺ to NADH and releasing a molecule of carbon dioxide.

CoA is picked up by the remaining four-carbon molecules forming an unstable compound succinyl CoA. ɑ-ketoglutarate dehydrogenase catalyzes the entire process.

Step 5

CoA from succinyl CoA is replaced with a phosphate group. It is then transferred to ADP to make ATP. Succinate, a four-carbon molecule is produced in this step.

Step 6

Succinate is oxidized to fumarate. Two hydrogen atoms are transferred to FAD to produce FADH₂. FADH₂ transfers its electrons directly to the electron transport chain since the enzyme carrying out the reaction is embedded in the inner membrane of mitochondria.

Step 7

A water molecule is added to fumarate which is then converted to malate.

Step 8

The oxidation of malate regenerates oxaloacetate, a four-carbon compound, and another molecule of NAD⁺ is reduced to NADH in this step.

End Products of TCA Cycle

Following are the end products of TCA cycle:

1. 6 NADH
2. 2 ATPs
3. 2 FADH₂

Also Read: Amphibolic Pathway

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