Biochemistry of Triglycerides

Biochemistry of Triglycerides

• The lecture covers the metabolic pathways of triglycerides.

Specific Objectives

• Students should be able to explain metabolic pathways of triglycerides.

Triglyceride Absorption and Transport

• Bile salts emulsify large fat droplets from the stomach.
• Pancreatic lipase and colipase break down fats into monoglycerides and fatty acids.
• Monoglycerides and fatty acids move into intestinal cells by diffusion.
• Cholesterol is transported into cells.
• Absorbed fats combine with cholesterol and proteins in intestinal cells to form chylomicrons.
• Chylomicrons are removed from the lymphatic system.
• The mixture of absorbed lipids migrates to the endoplasmic reticulum.
• Biosynthesis of complex lipids takes place, synthesizing triglycerides (TAG).
• Newly synthesised TAGs and cholesterol esters are hydrophobic and aggregate to form chylomicrons.
• Chylomicrons are small fat globules composed of protein and lipid secreted into blood and lymphatic vessels.

Fate of Absorbed Fat/Triglycerides

• Triglycerides are transported in the blood as chylomicrons.
• Blood becomes milky after lipid-rich meals due to elevated chylomicron levels.
• Adipose tissue and the liver absorb chylomicrons.
• The liver synthesizes endogenous triglycerides.
• Endogenous triglycerides are transported in blood as VLDL (very low-density lipoproteins).
• VLDL are deposited in adipose tissue.
• Lipoprotein lipase breaks down triglycerides in lipoproteins into smaller fatty acids and monoglycerides.
• Smaller fatty acids and monoglycerides are transported into tissues.
• Some are burned for fuel, others are re-assembled into triglycerides for storage.

Triacylglycerols (TAG)

• TAGs are stored in adipose cells and are the body's major energy reserve.
• TAGs are composed of three fatty acids and one glycerol molecule.

Triglyceride Hydrolysis

• The mobilization of stored fat requires the hydrolytic release of fatty acids and glycerol from TAGs.
• This process is initiated by hormone-sensitive lipase, like epinephrine or glucagon, which removes fatty acids from carbon 1 and/or 3 of the TAG.
• Additional lipases that target diacylglycerol or monoacylglycerol remove remaining fatty acids.

Fate of Glycerol

• Glycerol released during TAG degradation cannot be metabolized in adipocytes. This is due to a lack of glycerol kinase.
• Glycerol is transported via the blood to the liver for phosphorylation.
• Phosphorylated glycerol can be used to form TAGs in the liver or participate in glycolysis and gluconeogenesis.

Fate of Fatty Acids

• Free fatty acids move across the adipocyte cell membrane, binding to plasma albumin.
• Fatty acids reaching the liver are oxidized to release energy.

This quiz explores the metabolic pathways involved in the absorption and transport of triglycerides. Students will learn about the roles of bile salts, pancreatic lipase, and the formation of chylomicrons. Mastering these concepts is crucial for understanding lipid metabolism in the human body.