University:
Liberty University
Course:
BIOM 500 | Introduction to Biomedical Sciences
Academic year:

2024
Views:

88

Pages:

6
Author:

Sheha A.

Citric Acid Cycle It is also called as Krebs cycle or tricarboxylic acid cycle. It is a series of 8-step enzyme-catalyzed reactions in the matrix of mitochondria. Definition: It is a cyclic pathway of oxidation of acetyl CoA to CO2 and H2O. Steps: Formation of Citrate: Acetyl CoA condenses with oxaloacetate to form citrate. Isomerization of Citrate: Citrate is isomerized to isocitrate. Oxidation of Isocitrate: Isocitrate is oxidized to α-ketoglutarate, releasing CO2 and NADH. Oxidative Decarboxylation of α-ketoglutarate: α-ketoglutarate is oxidized to succinyl CoA, releasing CO2 and NADH. Substrate-level Phosphorylation: Succinyl CoA is converted to succinate, releasing a high-energy phosphate bond that is used to form GTP. Oxidation of Succinate: Succinate is oxidized to fumarate, releasing FADH2. Hydration of Fumarate: Fumarate is hydrated to malate. Oxidation of Malate: Malate is oxidized to oxaloacetate, regenerating the starting material and producing NADH. 4. α-Ketoglutarate: Is converted into succinyl CoA with the help of α-ketoglutarate dehydrogenase complex. This is an oxidative decarboxylation reaction. 5. Succinyl CoA: Succinyl CoA is converted to succinate. In this reaction, a high-energy phosphate bond is generated. This energy is used to form GTP, which can then be converted to ATP. 6. Succinate: Succinate is converted into fumarate. This step involves the transfer of two hydrogen atoms to FAD, forming FADH2. 7. Hydration of Fumarate: Fumarate is hydrated to malate. This reaction involves the addition of a water molecule to fumarate. 8. Oxidation of Malate: Malate is oxidized to oxaloacetate. In this step, one molecule of NADH is produced. 9. Malate is oxidized to oxaloacetate: This reaction involves the oxidation of malate by NAD+, producing NADH. This completes the cycle, regenerating oxaloacetate for the next round of acetyl-CoA oxidation. The citric acid cycle is summarized as follows: Oxaloacetate is oxidized to malate. Two molecules of CO2 are released. Four pairs of electrons are transferred to NAD+, forming four molecules of NADH. One molecule of FAD is reduced to FADH2. One molecule of GTP (equivalent to ATP) is generated. The overall equation for the citric acid cycle is: Acetyl-CoA + 3NAD+ + FAD + GDP + Pi + H2O → 2CO2 + 3NADH + FADH2 + GTP + CoA + 2H+