U01.01.132 Key enzymes in lipid transport

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Learning Objectives

  • Differentiate the functions of the major lipases in various tissues.
  • Explain the biochemical role of LCAT and CETP in HDL maturation and cholesterol transport.
  • Understand the mechanism of PCSK9 and its clinical significance in LDL regulation.
  • Connect enzyme deficiencies to specific lipid accumulation patterns.

1. The Lipase Family

Lipases are enzymes that break down triacylglycerols (TGs) into free fatty acids and glycerol, but they operate in distinct locations.

Enzyme Location Function
Pancreatic Lipase Small Intestine Degrades dietary TGs so they can be absorbed by enterocytes.
Lipoprotein Lipase (LPL) Vascular Endothelium Degrades TGs in circulating chylomicrons and VLDL; activated by Apo CII.
Hepatic Lipase (HL) Liver Capillaries Degrades remaining TGs in IDL (converting it to LDL) and chylomicron remnants.
Hormone-Sensitive Lipase (HSL) Adipocytes Degrades stored TGs to release FFAs for energy and glycerol for gluconeogenesis.

2. HDL Maturation and Cholesterol Transfer

HDL acts as a “scavenger” for cholesterol, but it requires specific enzymes to process and distribute that cholesterol.

  • LCAT (Lecithin-cholesterol acyltransferase): Catalyzes the esterification of 2/3 of plasma cholesterol. This process “traps” cholesterol inside the HDL core, moving it from a nascent (flat) state to a mature (spherical) state.
  • CETP (Cholesteryl ester transfer protein): Mediates the transfer of these cholesteryl esters from HDL to other lipoproteins like VLDL, IDL, and LDL in exchange for TGs.

Activity: Enzyme Function Matching

3. PCSK9 and LDL Receptor Regulation

PCSK9 is a critical regulator of serum LDL levels.

  • Mechanism: PCSK9 binds to the LDL receptor on the surface of hepatocytes and promotes its degradation in the lysosome.
  • Effect of High PCSK9: Decreased LDL receptors on the cell surface lead to increased Serum LDL.
  • PCSK9 Inhibitors: By inhibiting PCSK9, the liver can recycle its LDL receptors back to the surface, significantly decreasing Serum LDL.

Clinical Notes & Corrections:

  • Apo CII Deficiency: If Apo CII is missing, LPL cannot be activated. This leads to Type I Familial Dyslipidemia, characterized by milky blood and pancreatitis due to extreme TG levels.
  • Statins vs. PCSK9: While statins inhibit cholesterol synthesis, PCSK9 inhibitors work by increasing the clearance of cholesterol from the blood.

Activity: Clinical Lipid Scenarios

Memory Hook:

LCAT Loads Cholesterol into HDL. CETP Cholesteryl Ester Transfer Protein.  HSL is for Hunger.

Activity: