BIOCHEM L4C | Metabolism of Lipids Review

Lipid Functions and Digestion

• Triglycerides serve as the major source of energy and precursors for specialized regulatory molecules.
• Lipid digestion is completed in the small intestine.
• Formation of mixed micelles maximizes exposure to pancreatic lipases for lipid hydrolysis during digestion.
• A continuous source of digested dietary products for absorption at enterocyte brush-border membranes is provided by mixed micelles.

Dietary Lipids and Mutations

• Trans fats are not mentioned as part of the human diet.
• The most common mutation in lipoprotein lipase (LPL) is a point mutation affecting enzyme activity.

Lipoproteins and Their Functions

• Very Low-Density Lipoprotein (VLDL) carries triglycerides (TGs), phospholipids (PLs), and cholesterol esters from the liver to other tissues.
• Apolipoprotein B-100 is the major protein retained by Low-Density Lipoprotein (LDL) in VLDL metabolism.
• LDL is known as the major cholesterol carrier in human blood.
• Lipoprotein(a) [Lp(a)] is particularly atherogenic when complexed with apo(a) protein.

HDL Functions and Reactions

• After binding to scavenger receptor BI on hepatocyte membranes, HDL-3 particles undergo endocytosis and are internalized.
• The enzyme lecithin-cholesterol acyltransferase (LCAT) is rate-limiting for the efflux of free cholesterol to apolipoprotein AI (apoAI).

Apolipoproteins and Lipoprotein Interactions

• Chylomicrons acquire two further apolipoproteins, apoCII and apoE, from HDL molecules after entering the bloodstream.
• Chylomicrons, made by the liver, transport lipids to various tissues, especially adipose tissue.
• Structural similarity of apo(a) to plasminogen allows it to inhibit plasminogen activation on Lp(a) particles.
• Apolipoprotein C-II facilitates the binding of lipoprotein lipase (LPL) to chylomicrons in the capillaries of adipocytes.

Metabolism Dynamics

• LPL acts on VLDL in lipoprotein metabolism to produce intermediate-density lipoproteins (IDL).
• Components such as bile salts and phospholipids are part of mixed micelles, while cholesterol is not a component.
• The main function of mixed micelles is to enhance lipid absorption by increasing the solubility of lipids in the intestinal lumen.
• Micelles are significantly smaller than emulsion droplets, aiding in the effective transport of lipids.

Absorption Mechanisms

• Monoglycerides and fatty acids enter enterocytes via passive diffusion or transporter proteins.
• Acylation of dietary phospholipids to form phosphatidic acid is facilitated by the enzyme lysophosphatidic acid acyltransferase.
• Microsomal triglyceride transfer protein (MTP) plays a role in the assembly and secretion of lipoproteins by transferring lipids to apolipoproteins.
• Chylomicrons are responsible for transporting dietary lipids to adipose tissue and the liver.

Test your knowledge on the metabolism of lipids, including digestion, absorption, and the metabolic pathways for different lipoproteins. Understand the importance of lipids as structural components of cell membranes and as a major source of energy.