A Summary of Krebs Cycle

What is it?

Krebs cycle, named after Sir Hans Adolf Krebs – a British biochemist who recognized it in 1937, refers to a chain of enzymatic chemical reactions that are crucial for all living cells that adopt oxygen for their cellular respiration. Broadly speaking, the Krebs cycle is one of three prime stages of cellular respiration, other stages being glycolysis and electron transport chain.

Cellular respiration

Cellular respiration is simply a process by which food is broken down by the cells of the body in order to secrete a high energy molecule in the form of adenosine triphosphate (ATP). Also known as the Citric Acid Cycle and the tricarboxylic acid cycle, the Krebs cycle follows the Glycolysis stage and comes prior to the electron transport chain. In plants and animals including humans, Krebs cycle usually occurs in the cell’s mitochondrial matrix and is primarily fueled by fatty acids and pyruvic acid – a product of Glycolysis.

Krebs cycle equation

Krebs cycles’ processes are quite complicated and involve a number of steps that are executed in a chain of reactions. The overall chemical reaction of Krebs cycle can be summarized in the form of an equation, which is as follows:  

Acetyl-CoA + 3 NAD+ + Q + GDP + Pi + 2 H2O --> CoA-SH + 3 NADH + 3 H+ + QH2 + GTP + 2 CO2

Krebs cycle equation

Also referred to Citric Acid, Krebs cycle has close relationship with the citric acid, as it is the first product formed as a result of chemical reactions in the various stages of Krebs cycle. Further, citric acid is also the final reactant in the Krebs cycle. 

Steps of Krebs cycle

Krebs cycle is one of the important stages of cellular respiration, and involves a chain of chemical reactions in the living cells that break down food molecules into water, energy, and carbon dioxide. For the food to enter this stage of cellular respiration, it must be broken down into CH3CO or acetyl groups. In other words, there is a transitional phase prior to the beginning of Krebs cycle. Hence, before jumping into the steps of Krebs cycle, it is important to take a quick look at this transitional phase, in which the pyruvic acid is converted into acetyl coenzyme A (acetyl CoA). This is a three step process. The first step is the removal of carbon from pyruvic acid and releasing of carbon dioxide gas, which in turn is exhausted by the lungs. At the next step, oxidation takes place via hydrogen atom removal. It is then absorbed by NAD+ and results in the release of the acetic acid. Eventually, the acetic acid integrates with coenzyme A to form acetyl CoA. 

Once the acetyl CoA is produced, it steps into the Krebs cycle in order to get completely fragmented. This in turn is a process that involves eight steps, which is as mentioned below:

  1. Firstly, coenzyme A sends an acetyl group that contains two carbons to the oxaloacetate – a compound consisting of four carbons. This leads to the formation of a molecule citrate containing six carbons.
  1. This citrate adjusts itself in such a way as to produce isocitrate with six carbons.
  1. As a result, a molecule named alpha-ketoglutarate that contains five carbons is formed. It then fragments NAD+ into NADH and H+.
  1. The next step is the oxidization of the alpha-ketoglutarate when carbon dioxide is detached and replaced with coenzyme A. The result is the formation of succinyl-CoA – a compound with four carbons. As in the case of the third step, the process leads to the breaking down of NAD+ into NADH and H+.
  1. In this step, succinate is produced as a result of the removal of coenzyme A from succinyl-CoA.
  1. The oxidization of succinate takes place and the result is fumarate.
  1. When fumarate is combined with water, it leads to the formation of oxaloacetate.
  1. During the last oxidation, NAD+ is converted into NADH and H+.

In short, in every stage of Krebs Cycle, two molecules of CO2 (carbon dioxide) are produced and oxaloacetate is reinforced. These processes enable the next stage of cellular respiration, that is, the electron transport chain.