Lipolysis begins with hormonal signaling that triggers a cascade of intracellular events.
The Initiation of Lipolysis: A Step-by-Step Breakdown
Lipolysis, the breakdown of stored triglycerides (TAGs) into glycerol and free fatty acids, is a tightly regulated process that is crucial for energy homeostasis. Its initiation is a complex process. Here's a breakdown of how it gets started:
- Hormonal Signals: Lipolysis is triggered by counterregulatory hormones such as adrenaline. These hormones signal the body's need for energy.
- Adenylate Cyclase Activation: The counterregulatory hormones bind to receptors on the cell membrane and activate adenylate cyclase.
- cAMP Production: Activated adenylate cyclase catalyzes the conversion of ATP to cyclic AMP (cAMP). cAMP acts as a second messenger.
- Protein Kinase A (PKA) Activation: The elevated levels of cAMP activate protein kinase A (PKA).
- Protein Phosphorylation: PKA then phosphorylates a series of proteins on the lipid droplet surface. These proteins include perilipin and hormone-sensitive lipase (HSL). Phosphorylation of perilipin allows for access by lipases.
- Lipase Access: Providing access by lipases to hydrolyse TAGs
In summary, the process starts with a hormonal signal, leading to a chain reaction involving cAMP and PKA, which then modifies proteins on the lipid droplet, making the stored triglycerides accessible to lipases for breakdown.
| Step | Key Player(s) | Action |
|---|---|---|
| 1. Hormonal Trigger | Adrenaline, other hormones | Binds to receptors on the cell membrane |
| 2. Adenylate Cyclase Activation | Adenylate Cyclase | Activated, converts ATP to cAMP |
| 3. cAMP Production | cAMP | Acts as a second messenger |
| 4. PKA Activation | Protein Kinase A (PKA) | Activated by cAMP |
| 5. Protein Phosphorylation | PKA, Perilipin, HSL | Phosphorylates proteins on the lipid droplet surface |
| 6. Lipase Access | Lipases | Gains access to triglycerides, initiating hydrolysis and the release of fatty acids and glycerol. |
