The Citric Acid Cycle
The citric acid cycle is also called the tricarboxylic acid (TCA) cycle and the Krebs cycle. It is the final common catabolic pathway for the oxidation of fuel molecules. Two carbons enter the citric acid cycle as acetyl CoA and two carbons leave as CO2. In the course of the cycle, four oxidation-reduction reactions take place to yield reduction potential in the form of three molecules of NADH and one molecule of FADH2. A high energy phosphate bond (GTP) is also formed.
Krebs Cycle/Citric Acid Cycle/Tricarboxylic Acid (TCA) Cycle
Krebs Cycle: Decarboxylates pyruvate generated in glycolysis under aerobic conditions to form carbon dioxide and reduced coenzymes. These reactions (with the exception of succinate dehydrogenase) take place in the matrix of the mitochondrion. No ATP is generated directly by Krebs cycle. Reduced coenzymes used to generate ATP in electron transport chain powered oxidative phosphorylation.
2 pyruvates + 4 CoA-SH + 8 NAD+ + 2 FAD + 2 GDP + 2 Pi + 8 H20 D 6 CO2 + 4 CoA-SH + 8 NADH + 8 H+ + 2 FADH2 + 2 GTP + 2 H2O
The Krebs Cycle is arranged in three phases.
1. Transition reaction - 2 carbons from pyruvate are bonded to coenzyme A to be transferred into the Krebs cycle proper. One carbon dioxide per pyruvate is formed.
2. Decarboxylation reactions - 6C compound formed and two more carbon dioxides per pyruvate formed to generate 4 carbon compound.
3. Regeneration of oxaloacetate - 4C compound backbone rearranged to reform oxaloacetate.