Submitted to: Le Lait
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 18, 2007
Publication Date: November 12, 2007
Citation: Qi, P.X. 2007. Casein micelle structure: the past and the present. Le Lait. 87(4-5):363-383. Interpretive Summary: Milk and dairy products are the chief source of calcium and proteins for many people around the world. Curiously, the process whereby the calcium is carried in the milk is not well understood. At the concentrations present in milk, the calcium, and its companion phosphate, would normally precipitate, but this is prevented by a specific class of milk proteins, the caseins. These proteins interact with one another and carry the calcium tightly enough to keep it from precipitating, but not too tight as to prevent its controlled release during digestion. This unique protein-calcium-phosphate transport system is often termed as the casein micelles. The debate remains about the details in this highly complex system. To better understand how the caseins perform their functions of helping provide necessary nutrients to the neonate through a complex interaction mechanism, we examined the casein micelles using high resolution microscopy techniques. The results showed that only specific types of interactions among caseins lead to the formation and stabilization of a complete casein micelle system. These studies not only help us gain basic knowledge about the casein micelles but also suggest possible new applications for milk and dairy products.
Technical Abstract: At the heart of the milk system are the colloidal casein–calcium–transport complexes termed the casein micelles. The application of physical chemical techniques such as light, neutron, and X-ray scattering, and Electron Microscopy (EM) has yielded a wealth of experimental detail concerning the structure of the casein micelle. From these experimental data bases have arisen two conflicting models for the internal structure of the casein micelle. One model emphasizes protein submicellar structures as the dominant feature, while the other proposes that inorganic calcium phosphate nanoclusters serve this function. In this study, these models are critically examined in light of the two current primary dogmas of structural biology which are: protein structure gives rise to function and that competent and productive protein – protein interactions (associations) will lead to efficient transit through the mammary secretory apparatus. In this light an overwhelming argument can be made for the formation of proteinaceous complexes (submicelles) as the formative agents in the synthesis of casein micelles in mammary tissue. Whether these submicelles persist in milk has been questioned. Recently we have carried out studies on casein micelles and submicelles using Atomic Force Microscopy (AFM) and high resolution Transmission Electron Microscopy (TEM) to gain insights on the nature of protein-protein interactions in submicelles and micelles from a structural biology perspective. The results provide experimental evidence that protein - protein interactions are important in the formation and stabilization of casein micelles.