Overview
This lecture reviews the structure, types, and biological functions of milk caseins, emphasizing their role in nutrition, micelle formation, and allergenicity in humans.
Milk Caseins: Composition and Types
- Caseins make up about 80% of milk proteins and are the main allergens in cow's milk.
- The primary casein types are αS1-casein (Bos d 9), αS2-casein (Bos d 10), β-casein (Bos d 11), and κ-casein (Bos d 12).
- Cow's milk has a higher proportion of caseins (76–86%) compared to human breast milk (40%).
- Casein types differ in genetic variants, amino acid composition, and post-translational modifications (glycosylation, phosphorylation).
Structural Organization of Caseins
- Caseins are phosphoproteins with high proline content and flexible, random-coil structures.
- The tertiary structure is a disordered globule; the quaternary structure is an ordered micelle.
- Micelles consist of a hydrophobic core (mainly αS- and β-caseins) and hydrophilic surface (κ-casein).
- Casein micelles are stabilized by calcium phosphate bridges and electrostatic interactions, with κ-casein forming a protective outer layer.
Biological Functions
- Micelles supply essential proteins, calcium, and phosphates to newborns.
- During digestion, caseins release bioactive peptides (casomorphins) with physiological effects.
- Caseins act as molecular chaperones, stabilizing other milk proteins under stress.
Human vs. Cow's Milk Caseins
- Human κ-casein has higher carbohydrate content (~55%) than bovine κ-casein (~5%).
- Human casein micelles are generally smaller (60–80 nm) than cow's (150–180 nm).
- Differences in micelle composition and glycosylation impact infant digestion and allergy risk.
Allergenicity and Milk Allergy
- Caseins are weak allergens due to their flexible, noncompact structure and effective digestion.
- Allergenic epitopes are mainly linear and located in conserved protein regions.
- Cross-reactivity is high among ruminant milks (cow, sheep, goat).
- Milk allergy in children often resolves, but casein-specific allergies may persist into adulthood.
Key Terms & Definitions
- Casein — Main phosphoprotein in milk, forming micelles; includes αS1, αS2, β, and κ-types.
- Micelle — A spherical protein aggregate stabilized by calcium phosphate bridges.
- Phosphorylation — Addition of phosphate groups to serine residues in caseins.
- Glycosylation — Attachment of carbohydrate chains to proteins, notable in κ-casein.
- Epitope — Protein region recognized by the immune system, triggering allergies.
- Casomorphin — Bioactive peptide derived from casein digestion with opioid-like effects.
Action Items / Next Steps
- Review the structure and function of casein micelles.
- Study differences in casein composition between human and cow's milk.
- Understand the role of casein in milk allergy and cross-reactivity.
- Prepare for exam questions on micelle formation and casein structure.