Primary structure of kappa-casein isolated from mares’ milk.

This research paper reports the first-ever purification and mapping of the complete primary structure of kappa-casein (a milk protein) from horse milk. The study also looks at how this protein in horse milk compares to its counterparts in other animals, its particular post-translational modifications, and how it interacts with a particular lectin.

Purification and Complete Primary Structure

• The horse milk casein, a family of closely related proteins, was separated into four fractions using a method called Reverse Phase-High Performance Liquid Chromatography (RP-HPLC).
• The fastest-eluting protein from chromatography was used for sequence analysis, which ultimately revealed the complete primary structure of kappa-casein.

Comparison with Other Kappa-Casein

• The detailed sequence of kappa-casein from the horse milk was found to be 95% identical to the sequence reported for the C-terminal portion of the zebras’ kappa-casein.
• The horse milk kappa-casein also demonstrated high similarity with kappa-caseins derived from non-Equidae sources (other animals outside the horse family), which confirmed the isotyped nature of the protein.

Post-Translational Modifications

• The research also investigated the presence and nature of post-translational modifications in equine kappa-casein.
• Using mass spectrometry, after enzymatic dephosphorylation, two main modified components were found.
• Notably, the smaller modified component was more abundant than the larger one.

Interactions with Lectin and Structural Cohesion

• Lectin – a specific protein that can bind to specific carbohydrates – was found to recognize equine kappa-casein, signifying interaction.
• The specific lectin in this case was directed towards a particular type of sugar bond found in bovine kappa-casein.
• Upon comparing sequence patterns with previous studies, it was found that the distribution of electrical charge and hydrophobic regions in equine kappa-casein were similar, but not identical, to bovine kappa-casein. These regions play a vital role in forming and stabilizing the micellar structure of casein in milk.