Self-Assembly of Bovine β-Casein below the Isoelectric pH

• Published in: Journal of agricultural and food chemistry 2008-03, Vol.56 (6), p.2192-2198
• Main Authors: Portnaya, Irina, Ben-Shoshan, Einav, Cogan, Uri, Khalfin, Rafail, Fass, Deborah, Ramon, Ory, Danino, Dganit
• Format: Article
• Language: English
• Subjects: acidic pH; Animals; Biological and medical sciences; Caseins - chemistry; Cattle; cryo-TEM; Food Chemistry/Biochemistry; Food industries; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; Isoelectric Point; ITC; Micelles; micellization; Milk and cheese industries. Ice creams; Scattering, Radiation; self-assembly; Thermodynamics; X-Rays; β-casein
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/jf072630r

β-Casein is an intrinsically unstructured amphiphilic protein that self-assembles into micelles at neutral pH. This paper reports that β-casein self-organizes into micelles also under acidic conditions. The protein association behavior and micelle characteristics at pH 2.6, well below the pI, are presented. The pH was found to strongly affect the micelle shape and dimensions. Cryogenic transmission electron microscopy (cryo-TEM) experiments revealed disk-like micelles of 20–25 nm in length and ∼3.5 nm in height in acidic conditions. An aggregation number of 6 was determined by sedimentation equilibrium under these conditions. Isothermal titration calorimetry experiments verified the association below the pI and allowed determination of the micellization enthalpy, the critical micellar concentration, and the micellization relative cooperativity (MR). Small-angle X-ray scattering results at concentrations below the critical micellization concentration (CMC) suggest that the monomeric protein is likely in a premolten globule state at low pH. Calculations of the protein charge at acidic and neutral pH reveal a similar high net charge but considerable differences in the charge distribution along the protein backbone. Overall the results show that β-casein is amphiphilic at low pH, but the distribution of charge along the protein chain creates packing constraints that affect the micelle organization, leading at concentrations above the CMC to the formation of disk micelles.