Explain the megnism of Krebs cycle
The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid (TCA) cycle, is a series of chemical reactions that occur in the mitochondria of eukaryotic cells. It is a central metabolic pathway involved in the oxidative metabolism of carbohydrates, fats, and proteins. The primary purpose of the Krebs cycle is to generate energy in the form of ATP and reducing power in the form of NADH and FADH2.
The mechanism of the Krebs cycle involves a series of eight reactions, which occur in a cyclic manner. These reactions are as follows:
1. Acetyl-CoA Production:
- Before entering the Krebs cycle, pyruvate (a product of glycolysis) is converted into acetyl-CoA.
- This reaction is catalyzed by the enzyme pyruvate dehydrogenase, and it releases carbon dioxide and reduces NAD+ to NADH.
2. Citrate Formation:
- Acetyl-CoA combines with oxaloacetate to form citrate, a six-carbon compound.
- This reaction is catalyzed by the enzyme citrate synthase, and it releases coenzyme A (CoA).
3. Isomerization of Citrate:
- Citrate is isomerized into its isomer, isocitrate.
- This reaction is catalyzed by the enzyme aconitase, and it does not involve any redox reactions.
4. Oxidative Decarboxylation of Isocitrate:
- Isocitrate is oxidized to form α-ketoglutarate, releasing carbon dioxide and reducing NAD+ to NADH.
- This reaction is catalyzed by the enzyme isocitrate dehydrogenase.
5. Decarboxylation of α-Ketoglutarate:
- α-ketoglutarate is oxidatively decarboxylated to produce succinyl-CoA, releasing carbon dioxide and reducing NAD+ to NADH.
- This reaction is catalyzed by the enzyme α-ketoglutarate dehydrogenase.
6. Substrate-level Phosphorylation:
- Succinyl-CoA is converted into succinate, and a high-energy phosphate group is transferred to ADP, forming ATP.
- This reaction is catalyzed by the enzyme succinyl-CoA synthetase.
7. Oxidation of Succinate:
- Succinate is oxidized to form fumarate, and FAD is reduced to FADH2.
- This reaction is catalyzed by the enzyme succinate dehydrogenase, which is embedded in the inner mitochondrial membrane and is the only enzyme of the Krebs cycle that is part of the electron transport chain.
8. Formation of Oxaloacetate:
- Fumarate is hydrated to form malate, and malate is oxidized to regenerate oxaloacetate.
- These reactions are catalyzed by the enzymes fumarase and malate dehydrogenase, respectively.
- In the process, NAD+ is reduced to NADH.
At the end of one cycle of the Krebs cycle, one molecule of glucose (or two molecules of pyruvate) has been completely oxidized, resulting in the production of three molecules of NADH, one molecule of FADH2, one molecule of ATP, and two molecules of carbon dioxide. These electron carriers (NADH and FADH2) then enter the electron transport chain, where they donate their electrons to produce ATP through oxidative phosphorylation.