How the Liver Synthesizes Fats
The synthesis of triacylglycerols (TAGs) in hepatocytes is activated after a meal. Cells use glucose not only to generate energy, but also as a structural building block. The entire process can be divided into three sequential steps:
- Fatty acid preparation. Excess acetyl-CoA, which the cell receives during glycolysis, is directed toward building fatty acid chains. Ultimately, they are activated, converting into acyl-CoA.
- Glycerol backbone formation. The cell requires glycerol-3-phosphate, which is synthesized via two pathways. The first is the reduction of the glycolytic intermediate dihydroxyacetone phosphate with the obligatory participation of the coenzyme NADH. The second is the direct phosphorylation of free glycerol, which requires ATP consumption.
- Molecule assembly. Fatty acid radicals (acyl-CoA) attach to glycerol-3-phosphate, forming phosphatidic acid. It is then dephosphorylated (losing a phosphoric acid group) to become diacylglycerol (DAG). Final acylation of DAG completes the process, producing a TAG molecule.
Since the liver does not store synthesized fat, TAG molecules are packaged into very-low-density lipoproteins (VLDLs) and secreted into the bloodstream.
Regulation and Storage in Adipose Tissue
Adipocytes (fat tissue cells) specialize in energy storage. This process occurs during the absorptive period, when insulin dominates the blood.
Adipose tissue is insulin-dependent, and the hormone acts on it through multiple mechanisms:
- Stimulates glucose uptake. Insulin prompts cells to insert GLUT-4 transporters into the membrane, opening the path for glucose to enter.
- Activates glycolysis. Glucose breakdown provides the cell with ATP for synthetic reactions and glycerol-3-phosphate (the backbone of future fat).
- Triggers the pentose phosphate pathway. This shunt supplies NADPH, which is critical for de novo fatty acid synthesis.
- Regulates blood enzymes. Insulin initiates the synthesis and translocation of lipoprotein lipase (LPL) to the capillary endothelium. LPL hydrolyzes circulating blood lipoproteins, allowing adipocytes to take up fatty acids for storage.
Tissue Distribution of Fatty Acids
The fate of fatty acids extracted from the blood via lipoprotein lipase depends directly on the tissue's functional needs:
- In adipose tissue, they are re-esterified (converted back into TAGs) and stored in depots.
- In the myocardium and skeletal muscles, they serve as the preferred fuel: here, fatty acids undergo oxidation, generating vast amounts of ATP for muscle contraction.
If the body requires energy and fats are hydrolyzed (broken down), two functionally distinct products are formed. Fatty acids are oxidized by most tissues. Glycerol travels to the liver for gluconeogenesis—the synthesis of new glucose, which is vital for the brain and other glucose-dependent cells during starvation.
Energy Depots: Fats vs. Glycogen
Both fats and glycogen serve as energy reservoirs, but their characteristics differ significantly. Fats vastly surpass carbohydrates in caloric density.
Capacity and lifespan:
- Glycogen reserves last approximately one day of fasting.
- Fat reserves can sustain life for many weeks.
Consumption and turnover rates:
- Glycogen turns over rapidly (in less than two days) and is consumed continuously by cells (except for a brief period after a meal).
- Fat depots are inert: only about one-hundredth of the reserve is renewed daily. With regular, rhythmic feeding, stored fats may not be consumed at all, as organs manage to obtain fatty acids from circulating lipoproteins (chylomicrons and VLDLs). Energetically, these lipoprotein transport forms act as short-term reserves, functioning more like glycogen than true fat.