An energy-minimized casein submicelle working model

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Abstract
To develop a molecular basis for structure-function relationships of the complex milk protein system, an energy-minimized, three-dimensional model of a casein submicelle was constructed consisting ofκ-casein, fourα s1-casein, and fourβ-casein molecules. The models for the individual caseins were from previously reported energy-minimized, three-dimensional structures. Docking of oneκ-casein and fourα s1-casein molecules produced a framework structure through the interaction of two hydrophobic antiparallel sheets ofκ-casein with two small hydrophobic antiparallel sheets (residue 163–174) of two preformedα s1-casein dimers. The resulting structure is approximately spherically symmetric, with a loose packing density; its external portion is composed of the hydrophilic domains of the fourα s1-caseins, while the central portion contains two hydrophbic cavities on either side of theκ-casein central structure. Symmetric and asymmetric preformed dimers ofβ-casein formed from the interactions of C-terminalβ-spiral regions as a hinge point could easily be docked into each of the two central cavities of theα-κ framework. This yielded two plausible energy-minimized, three-dimensional structures for submicellar casein, one with two symmetricβ-casein dimers and one with two asymmetric dimers. These refined submicellar structures are in good agreement with biochemical, chemical, and solution structural information available for submicellar casein.