Three-Dimensional Molecular Modeling of Bovine Caseins

Three-dimensional (3 -D) structures derived from X-ray crystallography are important in elucidating structure- function relationships for many proteins. However, not all food proteins can be crystallized. The casei ns of bovine milk are one class of non-crysta11izable proteins (a, 1-, K-, and /3-). The complete primary and partial secondary structures of these proteins are known, but homologous proteins of known crystallographic structure cannot be found. Therefore , sequence based predictions of secondary structure were made and adjusted to conform with data from Raman and Fourier-transformed infra- red spectroscopy. With this information, 3-D structures for these caseins were built using the Sybyl molecular modeling programs. The K-casein structure contained two anti-parallel P-sheets which are predominately hydrophobic. The a,1-casein structure also contained a hydrophobic domain composed of .B-sheets as well as a hydrophilic domain ; these two are connected by a segment of ex- helix . Both the K- and a,1-caseins represent unrefined models in that they have been manipulated to remove unrealistic bonds but have not been energy-mini mized . Nevertheless the models account for the tendency of these caseins to associate. The .B-casein model appears to follow a divergent structural pattern. When subjected to energy minimization, it yielded a loosely packed structure with an ax.ial ratio of 2 to I, a hydrophobic C-terminal domain , and a hydrophilic N-terminal end. All three casein structures showed good agreement with literature concerning their global biochemical and physico-chemical properties

Three-Dimensional Molecular Modeling of Bovine Caseins

Abstract Three-dimensional (3 -D) structures deri ved from X-ray crystallography are important in elucidating structure-function relationships for many proteins. However , not all food proteins can be crystallized. The casei ns of bovine milk are one class of non-crysta11izable proteins (a, 1 -, K-, and /3-). The complete primary and partial secondary structures of these proteins are known, but homologous proteins of known crystallographic structure cannot be found. Therefore , sequence based predictions of secondary structure were made and adjusted to conform with data from Raman and Fourier-transformed infra-red spectroscopy . With this information, 3-D structures for th ese caseins were built using the Sybyl molecular modeling programs. The K-casein structure contained two anti-parallel P-sheets which are predominately hydrophob ic. The a, 1 -casei n structure also contained a hydrophobic domain composed of .B-sheets as well as a hydrophilic domain ; the se two are connected by a segment of ex-helix . Both the K-and a, 1 -caseins represent unrefined models in that they have been manipulated to remove unrealistic bonds but have not been energy-minimized . Nevertheless the models account for the tendency of these caseins to associate. The .B-casein model appears to follow a divergent structural pattern. When subjected to energy minimization, it yielded a loosely packed structure with an ax.ial ratio of 2 to I, a hydrophobic C-terminal domain , and a hydrophilic N-terminal end. All three casein structures showed good agreement with literature concerning their global biochemical and physico-chemical properties

Comparative Thermodynamic Linkage Study of the Calcium-Induced Solubility of Bovine and Caprine Caseins

In this paper, the calcium-induced solubility profiles of isolated caprine αs1-casein and both bovine and caprine whole caseins, containing known amounts of αs1, have been analyzed and compared using Wyman\u27s theory of thermodynamic linkage in conjunction with nonlinear regression analysis. All of the solubility profiles could be described by an initial salting-out followed by salting-in. These events were quantified by a salting-out constant, k1, and a salting-in constant, k2. These constants are correlated with association constants for salt binding so that n and m, the number of moles of calcium bound to different classes of sites which induce the respective changes in solubility, could also be quantitated. The purified αs1-casein from caprine milk differs from its bovine counterparts in that the parameters k1 and n are somewhat smaller, indicating less salt binding at both 24 and 1 °C. Conversely, for whole caseins, k1 was 6 times larger for caprine, which contains significantly less αs1-casein, than for bovine whole casein. This suggests stronger Ca2+-casein interactions in caprine whole casein as compared to whole bovine casein. Binding of Ca2+ ions to stronger affinity sites results in greater “salting-out” for caprine whole casein. Studies on temperature and ionic strength dependence of the comparative solubilities confirm this hypothesis. © 1993, American Chemical Society. All rights reserved