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

κ-Carrageenan Interaction with Bovine and Caprine Caseins As Shown by Sedimentation and Nuclear Magnetic Resonance Spectroscopic Techniques

The solubility and hydration characteristics of κ-carrageenan-casein systems from bovine and caprine milk with incorporated salt (NaCl) were determined by means of sedimentation and 17O nuclear magnetic resonance (NMR) experiments. Relative salt interaction parameters for both caseins alone and caseins in mixtures with κ-carrageenan were assessed by nonlinear regression analysis from the characteristics of solubilization of the systems. The κ-carrageenan-casein interactions appear to depend largely on the ratios of κ- to αs1-casein and possibly αs2-casein. Second virial coefficients (B0 values) and hydration products derived from 17O NMR data suggest that while soluble at high salt, the caprine casein mixtures exhibit-strong interactions, whereas the bovine counterparts do not. At lower salt concentrations the solubility data and the 17O NMR data are in agreement. Thus, a structural dependence upon protein components in salt-containing κ-carrageenan-casein solutions from bovine and caprine milk has been demonstrated

NMR of casein hydration and activity in solutions with ions

Oxygen-17 NMR studies of caseins isolated from fresh milk of several species were carried out in concentrated electrolyte solutions as a function of both protein and electrolyte concentration. Molecular dynamics simulations of ion and water binding to caseins are compared with the experimental observations by O-17 NMR of caseins in solutions with ions. Protein activities are also determined by utilizing Wyman\u27s theory of linked functions with a detailed model of ion binding to casein

Comparison of Calcium-Induced Associations of Bovine and Caprine Caseins and the Relationship of αs1-Casein Content to Colloidal Stabilization: A Thermodynamic Linkage Analysis

In milk, αs1-, αs2-, β-, and κ-caseins undergo association into colloidal complexes (casein micelles) that are visible with the electron microscope. Hydrophobic interactions and Ca2+ bonding are among the major causes of colloidal complex formation. In model systems, stable colloidal casein micelles can be obtained in a calcium caseinate solution by centrifugation at 1500 × g. The stabilities of colloidal complexes of bovine and two caprine caseins, selected for their αs1-casein contents, were tested and thermodynamically linked with the calcium-induced changes in the amount of stable colloid present. Analysis of the data according to this thermodynamic linkage approach for bovine and caprine caseins indicates colloid formation at low calcium concentrations (c.015 M). However, at increased calcium ion concentrations, these colloids are destabilized. Bovine casein (αs1-casein = 38% of total casein) was most stable with respect to added calcium ion concentration, and caprine casein, low in αs1-casein (5% of total casein), was least stable. The high caprine (αs1-casein = 17% of total casein) was intermediate in stability. After destabilization, bovine casein was not resolubilized at elevated calcium concentrations, but both caprine caseins were. More casein from the low αs1-casein sample could be resolubilized (salted in). These results suggest a role for casein composition in dictating the functional properties of milks from various species. © 1993, American Dairy Science Association. All rights reserved

Comparison of Hydration Behavior of Bovine and Caprine Caseins As Determined by Oxygen-17 Nuclear Magnetic Resonance: Effects of Salt

Oxygen-17 nuclear magnetic resonance (NMR) transverse relaxation measurements at 4.7 T were used to study the hydration properties of bovine and caprine casein solutions with and without NaCl. Measurements were carried out at 21 °C and pD 6.95. Nonlinear protein concentration dependences were observed for the oxygen-17 NMR transverse relaxation rates, but the fitting of the data did not require any higher order virial coefficients; only So was needed. This indicates that long-range, charge-charge repulsive interactions dominate entirely the protein activity in solution. Estimates of the bovine casein average “net” charge were obtained from the virial coefficient (Bo) and the partial specific volume; these are in accord with published amino acid sequence data. For bovine casein, the αS1- and β-components occur in nearly equal amounts, whereas in caprine casein, β-casein is the predominant species and αS1-casein varies from high to low values, suggesting altered protein-protein interactions. Hydration levels of caprine casein high in the αS1-casein component when compared with those of bovine casein (intermediate hydration) and the low αS1- caprine casein (low hydration) are interpreted in terms of “trapped water” and “preferential interactions” with water on the basis of quantitative differences in casein monomer contents. © 1995, American Chemical Society. All rights reserved

Oxygen-17 Nuclear Magnetic Resonance Studies of Bovine and Caprine Casein Hydration and Activity in Deuterated Sugar Solutions

The hydration of bovine and genetically variable caprine caseins in D2O solutions of sucrose and lactose was investigated by oxygen-17 NMR and fitted by nonlinear regression analysis. A charge-charge interaction model was employed to analyze the transverse relaxation (1/T2) data. Lactose caused increased hydration of the bovine casein and the caprine casein naturally low in αs1-casein, whereas sucrose led to increased hydration of the caprine casein naturally high in αs1-casein. At pD 7.20 and 21 °C the effect of charge-charge repulsive interactions on the native caseins generally leads to decreased protein stability in bovine and caprine caseins. However, addition of sugars causes stronger (attractive) interactions yielding more stable casein complexes with increased hydration. The calculated preferential binding term -(∂gs/∂gp) for casein mixtures suggests that sucrose and lactose are preferentially excluded from these milk proteins, yielding greater access to much of the aqueous compartment. This is consistent with the view that sugars lead to the stabilization of proteins in nonfrozen, aqueous systems