Lipolysis

What is lipolysis?

The process of hydrolyzing (breaking down) triglycerides into glycerol and free FA which are used for energy

Where does lipolysis occur?

In adipose tissue

What are the 5 steps in lipolysis?

• Activation of Hormone Sensitive Lipase (HSL)

• Triglyceride Hydrolysis

• Release and transport of FFAs

• Glycerol utilisation

• Fatty acid oxidation

1. Activation of Hormone Sensitive Lipase

• During fasting, what occurs to increase cAMP levels

• cAMP then causes what activation train

During fasting, what occurs to increase cAMP levels: Glucagon and epinephrine bind to their receptors on adipocytes

cAMP then causes what activation train: Activates protein kinease A (PKA) which phosphorylates and activates HSL

Triglyceride Hydrolysis

What occurs?

• ATGL: Adipose triglyceride lipase

• HSL: Hormone sensitive lipase

• MGL: Monoacylglycerol lipase

• ATGL: Triglyceride → Diglyceride + FFA

• HSL: Diglyceride → Monoglyceride + FFA

• MGL: Monoglyceride → Glycerol + FFA

Release and Transport of FFA

• How is FFA transported?

Diffused out of adipocytes and bind to albumin in bloodstream for transport to energy demanding tissues

Glycerol Utilisation

• Transported to

• How is it used?

Transported to: Liver

Used by: Entering gluconeogenesis or glycolysis after conversion to glycerol-3-phosphate by glycerol kinase

Fatty Acid Oxidation

• FFA activated to

• Transported to

• Undergo

  • Generating

FFA activated to: Fatty acyl-CoA

Transported to: Mitochondria via carnitine shuttle

Undergo: Beta-oxidation

• Generating: Acetyl-CoA, NADH and FADH2 for ATP production by krebs cycle and oxidative phosphorylation

Regulation of lipolysis

• How is it activated?

• Glucagon, epinephrine, norepinephrine by cAMP/PKA pathway

• Cortisol because it enhances expression of lipolytic enzymes

Regulation of lipolysis

• How is it inhibited?

• Insulin because it stimulates phosphodiesterase activity to degrade cAMP, reducing HSL activation

• High glucose and TG availability