Learning Objective: Explain the structure, composition, and function of elastin; describe its synthesis and degradation process; and relate α₁-antitrypsin deficiency to the development of emphysema (COPD).
Overview
Elastin is a stretchy structural protein found in tissues that require elasticity. It allows structures such as lungs, arteries, and skin to stretch and recoil without losing shape.
Structure and Composition
| Feature | Description |
|---|---|
| Protein Type | Stretchy, amorphous protein providing elasticity |
| Major Locations | Skin, lungs, large arteries, elastic ligaments, vocal cords, epiglottis, ligamenta flava |
| Amino Acid Content | Rich in nonhydroxylated proline, glycine, and lysine |
| Compare with Collagen | Collagen contains hydroxylated proline and lysine |
| Precursor Molecule | Tropoelastin |
| Scaffolding Protein | Fibrillin provides the structural framework for elastin deposition |
Synthesis and Cross-Linking
Elastin is synthesized as tropoelastin, which is secreted into the extracellular space and then cross-linked to form mature elastin fibers.
Key Enzyme: Lysyl Oxidase
- Catalyzes oxidative deamination of lysine residues.
- Creates desmosine cross-links → provides elastic recoil.
- Requires copper as a cofactor.
Elastic Property
- The ability to stretch and recoil depends on randomly coiled elastin molecules that resume their shape after extension.
Degradation
Elastin is degraded by elastase, an enzyme released by:
- Neutrophils, during inflammation
- Macrophages
Normally, α₁-antitrypsin (AAT) inhibits elastase to protect lung tissue.
Pathophysiology: α₁-Antitrypsin Deficiency
| Feature | Effect |
|---|---|
| Defect | Deficiency of α₁-antitrypsin → unopposed elastase activity |
| Consequence | Destruction of alveolar walls → panacinar emphysema (especially in lower lobes) |
| Liver Involvement | Misfolded AAT accumulates in hepatocytes → liver cirrhosis |
| Clinical Presentation | Dyspnea, cough, early-onset emphysema in a non-smoker, elevated liver enzymes |
| Histology | PAS-positive globules in hepatocytes |
Key Points
- Elastin = stretchy protein rich in non-hydroxylated proline, glycine, lysine
- Cross-linking (by lysyl oxidase) occurs extracellularly
- α₁-antitrypsin = elastase inhibitor → deficiency → emphysema + liver disease
- Fibrillin = scaffold for elastin deposition (defective in Marfan syndrome)
- Elastase released by neutrophils and macrophages, destroys elastin fibers if unchecked
