Lipids and Lipoproteins

Lipids Overview

  • Definition: Insoluble in water, contain nonpolar carbon-hydrogen bonds.

  • Composition (from food):

    • 95%95\% triglycerides (fatty acids and glycerol).

    • 2%2\% cholesterol, phospholipids, steroids, fat-soluble vitamins.

  • Functions:

    • High energy source.

    • Hormones and precursors (e.g., cholesterol for cortisol, sex hormones).

    • Cell membrane components (phospholipids, cholesterol).

    • Organ insulation and protection (triglycerides).

Lipid Physiology Phases

  • Digestive Phase:

    • Dietary triglycerides broken down by lipase (from pancreas).

    • Aided by bile acids (from liver, stored in gallbladder) to form micelles (tiny fat droplets) for absorption.

  • Absorptive Phase:

    • Lipids (free fatty acids, monoglycerides) moved across small intestine cells.

  • Transport Phase:

    • Longer fatty acids and cholesterol reassembled into chylomicrons, transported via lymphatic system to blood.

    • Short fatty acids bound to albumin for transport.

Cholesterol

  • Source: Mostly made endogenously in hepatocytes from 22 acetyl CoA groups (<2\% from diet).

  • Functions: Precursor for essential hormones (e.g., cortisol, estrogen, testosterone); essential cell membrane constituent, providing structure and fluidity.

  • Circulation: As cholesterol esters, often packaged in HDL and LDL.

  • Synthesis Key Enzyme: HMG CoA reductase is the rate-limiting enzyme; targeted by statin drugs to lower cholesterol.

Triglycerides

  • Structure: Glycerol molecule + 33 fatty acid