Triacylglycerols and Body Energy Reserve

Lipids and Body Fuel Reserve

• Lipids make up approximately 15-20% of human body weight.
• Triacylglycerols (TG) comprise 85-90% of body lipids and are the most abundant lipids.
• TG is stored in adipose tissue and serves as the body's energy reserve.

Why Fat is the Fuel Reserve

• Triacylglycerols are a highly concentrated form of energy, yielding 9 Cal/g.
• Fatty acids in TG are in the reduced form, which allows for efficient energy storage.
• Triacylglycerols are non-polar and hydrophobic, allowing them to be stored in an anhydrous form without water.
• In contrast, glycogen and proteins are polar and require water for storage.

Triacylglycerols and Energy Storage

• One gram of fat stored in the body yields nearly six times as much energy as one gram of (hydrated) glycogen.
• In a healthy 70 kg adult, approximately 10-11 kg of fat is stored, which corresponds to a fuel reserve of 100,000 Cals.

Other Important Body Lipids

• Phospholipids, glycolipids, and cholesterol are major components of cell membranes.
• Cholesterol is a precursor for bile acids and steroid hormones.

Mobilization of Fat from Adipose Tissue

• Triacylglycerol lipase removes fatty acids from TG to form diacylglycerol and eventually glycerol and free fatty acids.
• This process is known as lipolysis.

Fate of Free Fatty Acids

• Free fatty acids are transported in the circulation bound to albumin and enter various tissues for energy production.
• About 95% of the energy obtained from fat comes from the oxidation of fatty acids.

Triacylglycerol/Fatty Acid Cycle

• During starvation, TG stored in adipose tissue is hydrolyzed to free fatty acids for oxidation and energy production.
• About 65% of these FFA are converted to TG and sent back to adipose tissue for deposition.

Fatty Acid Oxidation

• Fatty acids are mostly oxidized by B-oxidation, which involves the sequential removal of a two-carbon fragment, acetyl CoA.
• B-oxidation involves three stages: activation of fatty acids, transport of fatty acids into mitochondria, and oxidation proper in the mitochondrial matrix.

Coenzyme A and Fatty Acid Oxidation

• The coenzyme A used for activation is different from the one that finally combines with fatty acid in the mitochondria to form acyl CoA.
• The cell has two separate pools (cytosolic and mitochondrial) of coenzyme A.