Lipids: Biochemistry and Metabolism

1. Classification of Lipids

• Saturated fatty acids have no double bonds (e.g., palmitic acid).
• Unsaturated fatty acids have one or more double bonds (e.g., oleic acid has one; linoleic acid has two — an essential fatty acid).

2. Dietary Lipid Digestion & Absorption

1. Lingual/gastric lipase begins hydrolysis in the stomach.
2. Pancreatic lipase (with colipase) cleaves triglycerides to 2-monoglycerides + free fatty acids in the small intestine.
3. Bile salts emulsify lipids, forming micelles that allow uptake by enterocytes.
4. Inside enterocytes, monoglycerides and fatty acids are resynthesized into triglycerides → packaged into chylomicrons with apolipoprotein B-48.
5. Chylomicrons enter the lymphatics (lacteals) → thoracic duct → venous blood at the jugular-subclavian junction.

• Guyton and Hall Textbook of Medical Physiology, p. 842

3. Lipid Transport: Lipoproteins

Lipoprotein Classes

• Guyton and Hall Textbook of Medical Physiology, p. 843
• Lippincott's Biochemistry, p. 673

Key Enzymes in Lipoprotein Metabolism

• Lipoprotein lipase (LPL): On capillary endothelium; activated by Apo C-II; hydrolyzes TG in chylomicrons and VLDL → releases fatty acids for storage or energy.
• Hepatic lipase: Converts IDL → LDL.
• LCAT (Lecithin-Cholesterol Acyltransferase): Activated by Apo A-I; esterifies free cholesterol in HDL (core of HDL grows).
• CETP (Cholesterol Ester Transfer Protein): Transfers cholesterol esters from HDL to VLDL/LDL.

Reverse Cholesterol Transport (RCT)

4. Fatty Acid Oxidation (β-Oxidation)

• Short- and medium-chain FAs enter freely.
• Long-chain FAs require carnitine shuttle:
  • Acyl-CoA + Carnitine → Acylcarnitine (catalyzed by CPT-I on outer membrane)
  • Translocase shuttles acylcarnitine across inner membrane
  • CPT-II regenerates Acyl-CoA in the matrix
  • Carnitine is synthesized from lysine + methionine.

• Ganong's Review of Medical Physiology, p. 37

5. Ketone Body Formation & Metabolism

1. 2 Acetyl-CoA → Acetoacetyl-CoA
2. Acetoacetyl-CoA + Acetyl-CoA → HMG-CoA (mitochondrial HMG-CoA synthase)
3. HMG-CoA → Acetoacetate + Acetyl-CoA (HMG-CoA lyase)
4. Acetoacetate → β-Hydroxybutyrate (reversible) or Acetone (irreversible, volatile)

• Guyton and Hall Textbook of Medical Physiology, p. 846

6. Fatty Acid Synthesis (De Novo Lipogenesis)

1. Acetyl-CoA (mitochondrial) → exported to cytoplasm as citrate via the citrate shuttle
2. Acetyl-CoA + CO₂ → Malonyl-CoA (catalyzed by Acetyl-CoA Carboxylase, ACC; requires biotin) — the committed, rate-limiting step
3. Fatty Acid Synthase (FAS) elongates the chain by 2 carbons per cycle using malonyl-CoA as donor
4. NADPH is supplied by the pentose phosphate pathway (HMP shunt)
5. The primary product is Palmitate (C16:0); further elongation/desaturation occurs in the ER

• Insulin activates ACC (stimulates lipogenesis)
• Glucagon/epinephrine inactivate ACC via PKA phosphorylation
• AMPK phosphorylates and inactivates ACC when energy is low
• Malonyl-CoA inhibits CPT-I, preventing simultaneous synthesis and oxidation

7. Cholesterol Biosynthesis

4 Stages:

• 2 Acetyl-CoA → Acetoacetyl-CoA → HMG-CoA (cytosolic HMG-CoA synthase)
• HMG-CoA + 2 NADPH → Mevalonate (HMG-CoA reductase) ← rate-limiting step; target of statins

• 3 ATP phosphorylate mevalonate → decarboxylation → isopentenyl pyrophosphate (IPP, Δ³-isopentenyl-PP)

• 6 isoprene units condense (via geranyl-PP, farnesyl-PP) → Squalene (requires NADPH)

• Squalene cyclizes → lanosterol → multiple steps → cholesterol

Regulation of HMG-CoA Reductase

1. SREBP-2 (Sterol Regulatory Element-Binding Protein 2): When intracellular cholesterol is low, SREBP-2 is activated → increases HMG-CoA reductase gene transcription.
2. Accelerated protein degradation: High cholesterol promotes enzyme degradation.
3. AMPK phosphorylation: Inactivates the enzyme when energy is low.
4. Insulin activates; glucagon inhibits.
5. Statins: Competitive inhibitors of HMG-CoA reductase → reduce hepatic cholesterol → upregulate LDL receptors → lower plasma LDL.

• Lippincott's Biochemistry, p. 672; Basic Medical Biochemistry 6e

8. Cholesterol Utilization & Elimination

1. Bile acid synthesis: Rate-limiting step = cholesterol-7α-hydroxylase (inhibited by bile acids — feedback)
2. Steroid hormone synthesis: Cholesterol → Pregnenolone (P450scc, rate-limiting) → cortisol, aldosterone, sex hormones
3. Vitamin D synthesis: Skin + liver + kidney
4. Direct secretion into bile (risk: gallstones / cholelithiasis if bile salts insufficient)

9. Regulation Summary: Fed vs. Fasted State

10. Clinical Correlations

Digestion and Absorption of Lipids

Overview

1. Emulsification — mechanical disruption of fat globules
2. Intraluminal digestion — enzymatic hydrolysis by lipases
3. Micellar solubilization — bile salts package digestion products
4. Mucosal uptake — absorption by enterocytes
5. Re-esterification & chylomicron assembly — packaging for lymphatic transport

1. Dietary Lipid Composition

90% of dietary fat is triacylglycerols (TAG / triglycerides) — long-chain fatty acyl esters of glycerol. The remainder includes:

• Phospholipids (~5%) — from cell membranes in animal products
• Cholesterol and cholesteryl esters (~5%)
• Fat-soluble vitamins (A, D, E, K)

• Nonpolar, water-insoluble: TAG, cholesteryl esters, carotene — remain as oil droplets
• Polar amphiphiles: phospholipids, bile acids — form monolayers and micelles
• Medical Physiology (Boron & Boulpaep), p. 1375

2. Emulsification

• Mastication (chewing) and cooking begin the process
• Gastric churning: antral peristalsis against a closed pylorus grinds food into fine particles
• Retrograde propulsion through the contracted pylorus further reduces droplet size
• Intestinal peristalsis mixes luminal contents with pancreatic/biliary secretions

• Biliary phospholipids and cholesterol (amphiphilic — polar heads project into water)
• Denatured proteins and dietary polysaccharides
• Products of early digestion (fatty acids, monoacylglycerols from gastric lipase)

• Medical Physiology, p. 1377

3. Intraluminal Digestion

A. Lingual Lipase (Minor in Adults)

• Secreted by serous glands at the base of the tongue
• Active in the mouth and continues in the stomach (stable at acidic pH)
• Preferentially cleaves sn-3 position of TAG → fatty acid + 1,2-DAG
• Minor role in adults; more important in neonates (before pancreatic lipase matures)

B. Gastric Lipase

• Secreted by gastric chief cells (stimulated by gastrin)
• 42-kDa glycoprotein; optimum pH 3–6
• ~15% of total fat digestion occurs in the stomach
• Preferentially cleaves the sn-3 position of TAGs; leaves intact 1,2-DAG
• Clinically important in pancreatic insufficiency: compensates for up to 1/3 of fat digestion
• Products (fatty acids) reach the duodenum → stimulate CCK and GIP release
• Medical Physiology, p. 1377–1378

C. Pancreatic Lipase (Major Enzyme)

• Alkaline pH (~6–7; pancreatic HCO₃⁻ neutralizes gastric acid)
• Colipase cofactor (10 kDa; secreted as procolipase, activated by trypsin)
• Ca²⁺, fatty acids, bile salts (in the right concentration)

• Active only at the oil-water interface of TAG droplets
• Surface emulsifiers (phospholipids, proteins) inhibit pancreatic lipase by blocking the interface
• Bile-salt micelles would also displace lipase — this is reversed by colipase
• Colipase anchors lipase to the lipid interface by penetrating the phospholipid coating
• Crystal studies show lipase has a "lid" over its catalytic cleft; colipase-induced conformational change opens the lid, exposing the active site
• Pancreatic lipase cleaves sn-1 and sn-3 positions of TAG → 2-monoacylglycerol (2-MAG) + 2 free fatty acids (the primary products)

D. Other Pancreatic Esterases

• PLA2 is secreted as a pro-enzyme, activated by trypsin; requires bile salts
• Bile salt–stimulated lipase (in human breast milk) also digests DAGs, MAGs, cholesterol esters, and vitamin esters — important in breast-fed neonates
• Medical Physiology, p. 1378–1380

4. Micellar Solubilization — The Role of Bile Salts

Bile Salt Chemistry

• Hydrophobic steroid backbone (faces lipid)
• Hydrophilic hydroxyl groups and charged conjugated amino acid (glycine or taurine, facing water)

From Emulsion Droplet to Mixed Micelle

1. Emulsion droplet (A): Lipases and biliary lipids adsorb to the surface; TAG in core is hydrolyzed
2. Multilamellar vesicle (B): Lipolytic products (MAGs, fatty acids, lysophospholipids, cholesterol, bile salts) build up at the surface and bud off as multilayered liquid-crystalline vesicles
3. Unilamellar vesicle (C): Bile salts thin the multilamellar coating → single lipid bilayer vesicle
4. Mixed micelle (D): Further bile salts completely convert to mixed micelles — hydrophobic tails inward, polar heads outward (~4–8 nm diameter)

• Medical Physiology, p. 1381

5. Mucosal Uptake by Enterocytes

Barriers to Cross

1. Mucous gel layer (95% water, but limits diffusion of large vesicles)
2. Unstirred water layer (UWL) — the disequilibrium zone adjacent to the brush border; diffusion is rate-limiting for very lipophilic molecules
3. Apical brush-border membrane of enterocytes

How Lipids Enter

• The acidic microclimate (pH ~5.5–6.0) at the brush-border surface causes bile salts to dissociate from the micelle (bile salts become protonated at low pH and less micellar)
• Fatty acids and MAGs become protonated and diffuse passively across the apical membrane

• Long-chain fatty acids (LCFAs) — primarily passive diffusion down concentration gradient; possibly also protein-mediated (FATP4/CD36 transporters on brush border)
• 2-MAG — passive diffusion
• Cholesterol — partially passive, partially via NPC1L1 transporter (target of ezetimibe)
• Short/medium-chain fatty acids (SCFAs/MCFAs) — water-soluble enough to pass directly into the portal blood without re-esterification

6. Intracellular Re-esterification and Chylomicron Assembly

• LCFAs are activated to Fatty acyl-CoA (by acyl-CoA synthetase)
• Two pathways to reform TAG:
  • Monoacylglycerol pathway (dominant during feeding): 2-MAG + 2 acyl-CoA → TAG
  • Phosphatidic acid (glycerol-3-phosphate) pathway (fasting): De novo synthesis
• Cholesterol is re-esterified by ACAT (acyl-CoA:cholesterol acyltransferase)
• Lysophospholipids are re-acylated to form phosphatidylcholine (lecithin)

• Lipid droplets form in the SER cisternae
• Apolipoproteins (especially Apo B-48) are synthesized in the RER → move to SER to associate with lipid droplets
• MTP (Microsomal Triglyceride Transfer Protein) is essential — transfers lipids onto nascent Apo B-48; MTP mutations → abetalipoproteinemia

• Nascent chylomicrons → Golgi apparatus (for glycosylation of apolipoproteins, addition of Apo A-I)
• Transport vesicles bud from trans-Golgi → move to basolateral membrane
• Vesicles fuse with basolateral membrane → exocytosis of chylomicrons into the lamina propria

• Chylomicrons (~80–500 nm) are too large to enter blood capillary fenestrae
• They pass through larger interendothelial channels into lymphatic capillaries (lacteals)
• Lacteal → cisterna chyli → thoracic duct → left subclavian vein → systemic circulation

• Medical Physiology, p. 1383–1385

7. Cholesterol and Phospholipid Absorption (Summary)

8. Enterohepatic Circulation of Bile Salts

• Bile salts are efficiently reabsorbed in the terminal ileum (>95%) via the ASBT (apical sodium-dependent bile acid transporter)
• Returned to liver via portal vein → re-secreted into bile
• Normal bile salt pool: ~3–5 g; recirculates 6–10 times/day
• Interruption (ileal resection, cholestyramine) → increased synthesis from cholesterol → lowers cholesterol; also → fat malabsorption if pool is too depleted
• Costanzo Physiology 7e, p. 387

9. Clinical Consequences of Impaired Lipid Digestion/Absorption

Summary Diagram of the Complete Process

Dietary TAG (>90%)
      │
      ▼
EMULSIFICATION (stomach churning + biliary phospholipids)
      │
      ▼
GASTRIC LIPASE → ~15% TAG hydrolyzed → 1,2-DAG + FA
      │ (CCK released in duodenum)
      ▼
PANCREATIC LIPASE + COLIPASE → 2-MAG + 2 FA  ← PLA2 (phospholipids → lyso-PC)
                                               ← Cholesterol ester hydrolase
      │
      ▼
BILE SALTS form MIXED MICELLES (fatty acids, 2-MAG, lyso-PC, cholesterol)
      │
      ▼ (diffuse through unstirred water layer)
ENTEROCYTE BRUSH BORDER UPTAKE
  LCFAs + 2-MAG → passive diffusion / FATP4/CD36
  Cholesterol → NPC1L1
  SCFAs/MCFAs → portal blood directly
      │
      ▼
RE-ESTERIFICATION in SER (TAG, phospholipids, cholesterol esters)
      │
      ▼
CHYLOMICRON ASSEMBLY (Apo B-48, MTP) in SER/RER → Golgi → vesicles
      │
      ▼
EXOCYTOSIS at basolateral membrane → LACTEALS → thoracic duct → blood
      │
      ▼
Bile salts → terminal ileum ASBT reabsorption → portal vein → ENTEROHEPATIC CIRCULATION