Acetyl-CoA as the central hub of lipid metabolism
Acetyl-CoA occupies a central position at the crossroads of fat breakdown and synthesis. During lipid catabolism, it is formed via fatty acid β-oxidation, after which it enters the tricarboxylic acid cycle for complete combustion.
At the same time, when substrates are in excess, acetyl-CoA serves as the basic building block for anabolic processes:
- Fatty acid biosynthesis (de novo lipogenesis);
- Synthesis of endogenous cholesterol.
In addition to lipid pathways, this same metabolite is formed from pyruvate during carbohydrate oxidation and from the breakdown of ketogenic amino acids, ensuring the mutual conversion of macronutrients.
Plastic role of citrate in lipid synthesis
The link between catabolism and lipid anabolism is clearly demonstrated by citrate. When an excess of substrates accumulates in the mitochondrial matrix, citrate is transported across the membrane into the cell cytosol.
In the cytoplasm, citrate is cleaved into two components:
- Oxaloacetate — used in further metabolic reactions (including the gluconeogenesis pathway via phosphoenolpyruvate carboxykinase);
- Acetyl-CoA — involved directly in the assembly of new fatty acid chains.
Thus, citrate acts as a regulatory bridge, switching the cell from oxidizing energy substrates to storing energy in the form of lipids.
Features of odd-chain fatty acid catabolism
The breakdown of fatty acids with an odd number of carbon atoms has important specific features regarding their entry into the TCA cycle. Unlike the standard pathway, which supplies only two-carbon fragments, their oxidation leads to the formation of intermediate compounds:
- The end product of odd-chain shortening enters the matrix and is converted into succinyl-CoA (similar to the metabolic pathway of the amino acids valine and isoleucine via propionyl-CoA);
- Next, succinyl-CoA is oxidized in the cycle to fumarate.
This mechanism links lipid metabolism not only with the energy cycle, but also with the utilization pathways of other metabolites.
Integration of metabolic pathways via TCA cycle intermediates
Lipid metabolism does not exist in isolation and is synchronized with protein and carbohydrate metabolism through common nodal molecules.
| Metabolite | Connection to lipid metabolism | Connection to other types of metabolism |
|---|---|---|
| Acetyl-CoA | Fatty acid β-oxidation, cholesterol synthesis | Pyruvate oxidation (glycolysis), breakdown of ketogenic amino acids |
| Citrate | Export to the cytosol for fatty acid anabolism | Initial intermediate of the TCA cycle |
| Succinyl-CoA | Product of odd-chain fatty acid metabolism | Oxidation of valine and isoleucine, heme synthesis |
| Fumarate | Generated from odd-chain fat breakdown products | Ornithine cycle (urea cycle), breakdown of tyrosine and phenylalanine |
| Pyruvate / Oxaloacetate | Participate in supplying substrates for lipogenesis | Gluconeogenesis, transamination (alanine, aspartate) |
Remember the transport mechanism for fat synthesis: "CITrate goes to the CITosol to provide acetyl-CoA for fatty acids."
Class and exam questions
How are fatty acid β-oxidation and the tricarboxylic acid cycle connected?
During β-oxidation, fatty acids are broken down into acetyl-CoA molecules. The resulting acetyl-CoA enters the TCA cycle, where it undergoes complete oxidation with the release of energy.
How are TCA cycle intermediates used for fatty acid biosynthesis?
When energy is in excess, citrate exits the mitochondrial matrix into the cytosol and is cleaved to release acetyl-CoA. This cytosolic acetyl-CoA serves as a substrate for the synthesis of new fatty acids.
In the form of which TCA cycle metabolite are odd-chain fatty acids utilized?
They are oxidized to form propionyl-CoA, which is then converted into succinyl-CoA and oxidized in the TCA cycle to fumarate.
What else to study on this topic
- Energy balance and reaction equations of the tricarboxylic acid cycle
- Connection of the Krebs cycle with the ornithine urea cycle via fumarate
- Anaplerotic reaction of pyruvate carboxylase and oxidative decarboxylation of pyruvate
- Role of oxaloacetate and phosphoenolpyruvate carboxykinase in gluconeogenesis
- Transamination and glutamate dehydrogenase reaction of α-ketoglutarate
- Involvement of succinyl-CoA in heme biosynthesis
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Lipid and Fatty Acid Metabolism" and 3,616 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the "Additional Topics" section
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.