Substrate Preparation and Transport
The creation of fats begins with the breakdown of glucose. During glycolysis in the cell cytosol, a glucose molecule is converted into pyruvate. The resulting pyruvate must enter the mitochondria, where it is converted into acetyl-CoA and oxaloacetate, which then condense to form citrate.
Since further molecular assembly takes place in the cytosol, citrate is transported out of the mitochondrion. Once in the cytoplasm, it is cleaved back into acetyl-CoA and oxaloacetate.
For synthesis reactions to proceed successfully, the cell crucially requires reducing equivalents in the form of NADPH. The body obtains these from two main sources:
- The pentose phosphate pathway (beginning with the transformation of glucose-6-phosphate);
- The malate-to-pyruvate conversion reaction catalyzed by the specific malic enzyme.
Tissue Specificity of Backbone Formation
To assemble a complete triacylglycerol molecule, two components must be joined: glycerol-3-phosphate (serving as the backbone) and acyl-CoA (activated fatty acids). The origin of glycerol-3-phosphate strictly depends on the tissue type where synthesis is occurring.
In liver cells, two mechanisms operate simultaneously to obtain this substrate:
- Reduction of dihydroxyacetone phosphate, an intermediate metabolite of glycolysis.
- Phosphorylation of free glycerol delivered via the bloodstream, a reaction catalyzed by the enzyme glycerol kinase.
In adipose tissue, the situation is different. The enzyme glycerol kinase is completely absent in adipocytes. For this reason, adipose tissue has only one pathway for obtaining the fat backbone: from dihydroxyacetone phosphate. This has fundamental clinical significance: for adipocytes to store fats, glucose must enter them and glycolysis must proceed uninterruptedly.
Mechanism of Fat Deposition in Adipocytes
During the absorptive period, adipose tissue actively stores triacylglycerols. Unlike the liver, adipocytes prefer not to synthesize fatty acids from scratch, but rather to take up pre-formed ones from the bloodstream. In the blood, lipids circulate within specialized transport complexes: chylomicrons (CM), very-low-density lipoproteins (VLDL), and intermediate-density lipoproteins (IDL).
On the inner wall of blood capillaries supplying adipose tissue lies the enzyme lipoprotein lipase (LPL). It hydrolyzes passing triacylglycerols, breaking them down into free fatty acids and glycerol.
The subsequent fate of these products differs:
- Fatty acids diffuse into the adipocyte and are immediately activated, converting into acyl-CoA.
- The released glycerol cannot be utilized by the adipocyte (due to a lack of the enzyme) and is carried away via the bloodstream to the liver.
Simultaneously, glucose enters the cell via insulin-dependent GLUT-4 transporters. Inside the cell, it is broken down into dihydroxyacetone phosphate, which is reduced to glycerol-3-phosphate. At the final stage, acyl-CoA and glycerol-3-phosphate combine to form triacylglycerol molecules, which are stored in the lipid droplet.
Regulatory Role of Insulin
Insulin acts as the master conductor of lipid anabolism, stimulating this process through two parallel pathways:
- Activation of glucose transport. The hormone stimulates GLUT-4 transporters, significantly accelerating glucose entry into adipocytes. Without this step, the cell cannot synthesize glycerol-3-phosphate.
- Activation of intravascular lipolysis. Insulin induces the synthesis and subsequent translocation of lipoprotein lipase to the capillary endothelium of adipose tissue, thereby ensuring a continuous and robust influx of fatty acids into cells for storage.