Medical Biochemistry Lipids

Function of Lipids (4 main)

Energy: source and storage (TG)

Structure: cholesterol and phospholipids

Hormones: Cholesterol forms steroids

Digestion: of fatty foods (cholesterol derivative in bile)

Nerve conduction in CNS
Insulation and protection

Lipid Classification (4)

Fatty acids (FA)

Triglycerides (TG)

Phospholipids (PL)

• Lecithin

• sphingomyelin

Sterols (Cholesterol based)

Fatty Acid Structure

Saturated: No C=C

Unsaturated:

• Monounsaturated (one C=C)

• Polyunsaturated (>= 2 C=C)

• Arrangements:

  • Cis: Natural

  • Trans: Man-made

Fatty Acid Source

Nonessential: Can be synthesized in the body (typically saturated or mono unsaturated)

Essential: Cannot be synthesized

Essential FA (2)

Linoleic acid (Omega-6)

Alpha-linolenic acid (Omega-3)

Both from plants, required for biosynthesis of prostaglandins

Triglyceride Structure

3 FA attached to 1 glycerol via ester bond

hydrophobic

stored in adipose tissue

Triglyceride Source (2)

Diet

Lipogenesis

TG Function

Store energy: (95% of adipose tissue)

Energy via beta-oxidation (lipolysis)

Excess beta-oxidation forms ketones

Phospholipid Structure

Complex lipids

phosphoric acid and nitrogen

amphipathic

emulsifiers

Eg. Lecithin and sphingomyelin

Phospholipid Function

Cell membrane

Myelin sheath (sphingomyelin)

Lipoproteins: lipid carriers

Lung surfactant (lecithin)

Lecithin / Sphingomyelin (L/S) ratio (significance and range)

>2 = mature lung

<2 = increased risk of respiratory distress syndrome (RDS)

Cholesterol Structure

Unsaturated steroid alcohols

4 rings, hydroxyl group position 3, single side chain tail position 17

made almost exclusively by animals

Cholesteryl Esters Structure and percentage of total cholesterol

long chain FA esterified to cholesterol

70% of total cholesterol

more hydrophobic

Cholesteryl ester production reactions

Cholesterol + FA → ACAT → cholesterol ester (intracellular)

Cholesterol + Lecithin → LCAT → cholesterol ester (a/w HDL and LDL in blood)

Cholesterol Source and Risk

Dietary (exogenous): < half daily needs (from meat, eggs, seafood)

In vivo (endogenous):  Major site in liver, synthesized from acetyl CoA

Statin Drugs Function

Inhibit HMG-CoA reductase

Lower cholesterol synthesis and blood levels

Cholesterol Function (4)

Structure: cell membrane