OVERVIEW
The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle or citric acid cycle, is the series of chemical reactions that generates energy through the oxidation of acetate. It was identified in 1937 by Hans Krebs, who was responsible for elucidating most of the pathway. This process occurs in the matrix of the mitochondrion in eukaryotic cells. In prokaryotic cells, this reaction occurs in the cytosol with the proton gradient for ATP production being across the cell's surface (plasma membrane) rather than the inner membrane of the mitochondrion.
(Inputs and outputs increased by a factor of two to represent the Krebs cycle's total output for each glucose molecule that undergoes glycolysis.)
The Krebs cycle, also known as the tricarboxylic acid (TCA) cycle or citric acid cycle, is the series of chemical reactions that generates energy through the oxidation of acetate. It was identified in 1937 by Hans Krebs, who was responsible for elucidating most of the pathway. This process occurs in the matrix of the mitochondrion in eukaryotic cells. In prokaryotic cells, this reaction occurs in the cytosol with the proton gradient for ATP production being across the cell's surface (plasma membrane) rather than the inner membrane of the mitochondrion.
- Inputs: 2 Acetyl CoA, 6 NAD+, 2 FAD, and 2 ADP+Pi
- Outputs: 4 CO2, 6 NADH, 6 H+, 2 FADH2, 2 ATP, and 2 CoA
(Inputs and outputs increased by a factor of two to represent the Krebs cycle's total output for each glucose molecule that undergoes glycolysis.)
ANALYZING THE CYCLE
The Krebs cycle begins as acetyl-CoA (2-carbon molecule) combines with oxaloacetate (4-carbon molecule) to form citrate (6-carbon molecule). Citrate then undergoes transformation into cis-aconitate and then into isocitrate. It is after this step that one of the carbon atoms breaks off and is released as CO2. An NAD+ molecule is also reduced to NADH at this point in the cycle (as the isocitrate dehydrogenase enzyme is active). As alpha-ketoglutarate (5-carbon molecule) is formed, we yet again lose a carbon atom in the form of CO2, and another NADH molecule is produced, this time in the presence of the alpha-ketoglutarate dehydrogenase complex (multiple enzymes present). |
Alpha-ketoglutarate becomes succinyl-CoA (4-carbon molecule) which then becomes succinate, and GTP (essentially an equivalent to ATP) is created at this point in the cycle. Succinate is converted to fumarate as FADH2 is produced in the presence of succinate dehydrogenase. Fumarate becomes malate, and the cycle is completed as the malate transitions back into the oxalacetate we began with as malate dehydrogenase is the active enzyme. The third and final NADH is produced during this phase. The oxaloacetate is then available to combine with any available acetyl-CoA to kick off the cycle all over again. |
ADDITIONAL RESOURCES
For another source of information on the Krebs cycle, check out these great walkthrough videos:
For another source of information on the Krebs cycle, check out these great walkthrough videos:
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PRACTICE MAKES PERFECT
If you think you've got a good understanding of the Krebs cycle, head on over to our practice page and quiz yourself using the flashcards we have provided. Good luck!
If you think you've got a good understanding of the Krebs cycle, head on over to our practice page and quiz yourself using the flashcards we have provided. Good luck!