Named after Hans Kreb, the Nobel Prize Winner, the Krebs cycle is a sequence of events pertaining to metabolic reactions occurring in the mitochondrial matrix of eukaryotic cells. This corresponds to saying that Krebs cycle is observed in fungi, animals and plants only and not in bacteria as they do not possess the cellular machinery required for conducting this mechanism.
Living entities derive energy from the ATP (adenosine triphosphate) which is metabolized from the glucose molecules. In the body, this glucose can be stored in many forms – dietary carbohydrates, fats and proteins comprise components which can be metabolized to glucose, glycogen stored in liver cells and muscles are a long chain of molecules of glucose and so on. The tricarboxylic acid (TCA) cycle or the citric acid cycle is one of the first steps in the aerobic pathway, operating to produce adequate oxaloacetate for the cycle to continue.
The resultant of the mitochondria is the oxidation of acetyl CoA which releases carbon dioxide and hydrogen atoms ultimately forming water. The cycle is known as citric acid cycle as the first metabolic intermediate that forms in the cycle is the citric acid.
The criteria for this cycle to occur is the availability of aerobic conditions, this is because the FAD and NAD+ (energy-rich molecules) are recovered from their reduced versions only after they pass the electrons to molecular oxygen. This cycle is the common pathway for oxidation of proteins, biomolecules, carbohydrates, fatty acids. It is via the acetyl CoA, the molecules enter this cycle from other pathways and cycles.
Furthermore, the significance of this cycle lies in the fact that it supplies high-energy molecules for the production of water and ATPs to the electron transport chain.
Steps In Krebs Cycle
The Krebs cycle is a series of enzyme-catalyzed reactions. The Krebs cycle steps are as listed below:
- Acetyl CoA is condensed with oxaloacetate
- Citrate isomerized to isocitrate
- Oxidative decarboxylation of isocitrate
- Oxidative decarboxylation of α-ketoglutarate
- succinyl-CoA is converted into succinate
- Succinate dehydrated to fumarate
- Fumarate hydrated to malate
- L-malate dehydrogenated to oxaloacetate
The purpose of this cycle is the complete oxidation of glucose. These were the Krebs cycle steps. The cycle yields energy-rich molecules which generate ATPs in the electron transport chain.
What are the enzymes involved in Krebs cycle?
As aforementioned, the citric acid cycle is an enzyme-catalyzed cycle. The enzymes required for catalysing the reactions taking place in the Krebs cycle in the eukaryotic cells are found in the mitochondrial matrix except for succinate dehydrogenase and aconitase that are found in the inner mitochondrial membrane.
One common trait found in all the enzymes participating in this cycle is that almost all enzymes need Mg2+.
Enzymes required in this reaction which catalyze through various steps through the process of the cycle are:
- Succinyl-CoA synthetase
- Malate dehydrogenase
- Succinate dehydrogenase
- Isocitrate dehydrogenase
- Citrate synthase
This was a brief on Krebs cycle. The next topic suggested for you would be the CAM pathway. Learn how this adaptation seen in plants helps them fix carbon dioxide in dry conditions.
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