κ-Casein terminates casein micelle build-up by its "soft" secondary structure

Area of Science:

• Biochemistry
• Colloid Science
• Food Science

Background:

• Previous work demonstrated stepwise assembly of alpha-casein aggregates with Ca-phosphate nanoclusters, mimicking casein micelles.
• The mechanism for terminating casein micelle growth remained unclear.

Purpose of the Study:

• To investigate the role of kappa-casein in terminating casein aggregate growth.
• To understand the structural changes of casein proteins during aggregation.

Main Methods:

• Utilized a multilayered model system for protein adsorption.
• Incorporated calcium-phosphate nanoclusters during protein adsorption steps.
• Analyzed secondary structure changes using spectroscopic methods.

Main Results:

• Kappa-casein adsorption onto alpha-casein surfaces, with or without Ca-phosphate, induced significant changes toward beta-structures.
• In the absence of Ca-phosphate, kappa-casein adsorption terminated further protein aggregation.
• Alpha-casein secondary structure remained largely unaffected during adsorption.

Conclusions:

• Calcium-insensitive kappa-casein likely forms the outer layer of casein micelles due to its structural flexibility.
• The altered secondary structure of kappa-casein may occlude interaction sites, halting aggregation.
• Findings offer insights into casein micelle formation and stability in vivo.