Citric Acid Cycle

Citric Acid Cycle: Introduction

The Citric Acid Cycle is the primary metabolic pathway for all the aerobic processes in an animal tissue. It was discovered in 1937 and is also known as the Krebs cycle or the tricarboxylic cycle (TCA Cycle). It is a series of reactions that are important for the living cells. They provide complete oxidation of the C2 units or Acetyl-CoA that is derived from fats, carbohydrates and lipids. The process of oxidation gives Carbon Dioxide and water. The citrus acid cycle is a series of enzyme catalyzed chemical reactions, which take place as part of the cellular respiration. Precisely it occurs in the Matrix of the mitochondria present in a cell. It is part of the metabolic pathway, which generates fuel for the human body to use.

Reactions in Citrus Acid Cycle: Overview

As mentioned earlier Citrus Acid Cycle is the complete breakdown of food to generate energy from the energy rich molecules that are the end products of the reaction. The organism uses the energy produced from these molecules as the fuel. The Acetyl coenzyme that is produced by the breakdown of the sugars, fatty acids and some amino acids, reacts with the Oxaloacetic acid to further produce Citric Acid that is again converted into Oxaloacetic acid after a series of enzyme catalyzed chemical reactions. In the whole process of chemical conversion, Carbon dioxide and water are produced at different stages and ATP molecules are also formed which are the storehouses of energy.

Step by Step Illustration of the Chemical Reactions

• The Citric Acid Cycle starts when the Coenzyme A transfers its 2-carbon acetyl group to the 4-carbon compound Oxaloacetate and forms a 6-carbon molecule citrate.
• The citrate is then rearranged to form an isomeric form (Isomer is a molecule with the same number and types of atoms as another molecule but the structure is different.). Thus it forms an Isocitrate.
• The 6-carbon isocitrate is then oxidized and a molecule of carbon dioxide is removed by producing the 5-carbon molecule alpha-ketuglutarate. During this process of oxidation NAD is converted into NADH (an equivalent of 3 ATP) + H.
• Then the Coenzyme A is removed so that succinate can be produced. The energy that is released is used to make guanosine triphosphate or GTP from guanosine diphosphate or GDP. This GTP is then converted to ADP.
• Then the Succiante that was produced in the last step is converted into fumarate by the process of oxidation. During this process, FAD is reduced to FADH2.
• Malate is formed from Fumarate when water is added.
  
  Then Malate goes through the process of oxidation and Oxaloacetate is produced which is the starting compound of the Citric Acid cycle. During this process of oxidation NAD+ is reduced to NADH+ H.

This is the gist of Citric Acid Cycle and is often called an amphibolic pathway as it involves both catabolism and anabolism. The Citric Acid Cycle occurs when the body is in need of energy. Apart from providing energy for the body it also supplies metabolic intermediates for biosynthetic process.