Fats as the Primary Energy Reserve
During the active digestive (absorptive) period, the body stores excess nutrients in the form of fats, which are deposited in specialized cells called adipocytes. When a fasting period begins or when a person performs prolonged physical exertion, these stores become actively utilized.
Fats are the most energy-dense substances in the human body. This colossal energy yield is explained by the chemical structure of fatty acids: their molecules are highly reduced because they contain a large number of -CH2- bonds. For this reason, the energy yield from their oxidation is more than double that of carbohydrates (9.7 kcal versus 4.7 kcal per gram of substance).
Activation Cascade: The Adenylyl Cyclase System
Fat mobilization does not occur spontaneously—it is a tightly regulated hormonal process. The main stimulators are glucagon and epinephrine. When they bind to specific receptors on the adipocyte membrane, a complex signal transduction cascade is triggered inside the cell:
- Receptor Activation: The hormone interacts with the receptor, leading to the activation of a G-protein.
- Enzyme Action: The active G-protein stimulates the enzyme adenylyl cyclase (AC).
- Second Messenger: Adenylyl cyclase catalyzes the conversion of ATP into its cyclic form—cAMP.
- Kinase Activation: The accumulation of cAMP leads to the activation of Protein Kinase A.
- Phosphorylation: Active Protein Kinase A attaches a phosphate group to hormone-sensitive lipase (HSL), transitioning it from an inactive to an active form.
The Protective Role of Perilipin
At rest, prior to hormonal action, active mobilization is prevented not only by enzyme inactivity. The fat droplets (triacylglycerols) stored in the adipocyte cytoplasm are securely protected by a specialized protein—perilipin.
In its dephosphorylated form, perilipin densely coats the lipid droplet, creating a physical barrier that prevents any enzymes from accessing the substrate. However, when a hormonal signal triggers the action of Protein Kinase A, this enzyme phosphorylates not only the lipase but also perilipin itself. As a result of phosphorylation, perilipin changes its conformation and detaches from the lipid droplet. Only then do the triacylglycerol molecules become fully accessible for breakdown.
The Hydrolysis Reaction Pathway
Once the perilipin barrier is removed and HSL is activated, direct lipid hydrolysis begins:
- First Step: Active hormone-sensitive lipase (TAG lipase) cleaves the first fatty acid molecule from the triacylglycerol (TAG). This reaction yields diacylglycerol (DAG) and a free fatty acid (R-COOH).
- Process Completion: The remaining ester bonds in diacylglycerol and monoacylglycerol (MAG) are hydrolyzed by other intracellular lipases present in the cell.
The final outcome of this multi-step process is the complete breakdown of stored fat into free fatty acids and a glycerol molecule, which can then be utilized by the body for energy production.