Disorder in Milk Proteins: Caseins, Intrinsically Disordered Colloids (original) (raw)
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Disorder in Milk proteins caseins, intrinsically disordered
This article opens a series of reviews on the abundance and roles of intrinsic disorder in milk proteins. The focus of this introductory article on caseins is symbolic, since caseins were among the first recognized functional unfolded proteins and since they are definitely the most disordered, the most abundant, and the most studied of all milk proteins. In eutherian milks, the casein family includes at least three and usually four major members (α s1 -, α s2 -, β-, and κ-caseins) that are unrelated in sequence. However, in some species, two different α S2 -casein genes are active, and therefore the total number of caseins can be as high as five. These proteins have found a number of uses in food industry. The functional repertoire of caseins ranges from nutritional function to involvement in the improving and/or maintaining cardiovascular health, to crucial contribution to the milk capacity to transport calcium phosphate, to serve as molecular chaperones, and to protect the mother's mammary gland against amyloidoses and ectopic calcification. An intricate feature of caseins is their ability to assemble to colloidal protein particles, casein micelles, serving to sequester and transport amorphous calcium phosphate. These and many other functions of caseins are obviously dependent on their intrinsically disordered nature and are controlled by various posttranslational modifications. Since various aspects of casein structure and function are rather well studied and since several recent reviews emphasized the functional roles of caseins' intrinsic disorder, the major goal of this article is to show how intrinsic disorder is encoded in the amino acid sequences of these proteins.
Molecules
The milk of mammals is a complex fluid mixture of various proteins, minerals, lipids, and other micronutrients that play a critical role in providing nutrition and immunity to newborns. Casein proteins together with calcium phosphate form large colloidal particles, called casein micelles. Caseins and their micelles have received great scientific interest, but their versatility and role in the functional and nutritional properties of milk from different animal species are not fully understood. Caseins belong to a class of proteins that exhibit open and flexible conformations. Here, we discuss the key features that maintain the structures of the protein sequences in four selected animal species: cow, camel, human, and African elephant. The primary sequences of these proteins and their posttranslational modifications (phosphorylation and glycosylation) that determine their secondary structures have distinctively evolved in these different animal species, leading to differences in their...
Milk Proteins - From Structure to Biological Properties and Health Aspects
InTech eBooks, 2016
Mammalian milk is a complex fluid mixture of various proteins, minerals, and lipids, which play an important role in providing nutrition and immunity to the newborn. Casein proteins, which form about 80% of the bovine milk proteins, form large colloidal particles with calcium phosphate to form casein micelles, which for many years have been an important subject of interest. Casein micelles are composed of four main types of proteins: α S1-casein, α S2-casein, β-casein, and k-casein. These constituent casein proteins lack well-defined secondary and tertiary structure due to large amount of propyl residues. These micelles are being extensively studied because of their importance in functional behavior of milk and various milk products. However, the exact structure and nature of these casein micelles are still under debate. These different casein proteins possess different functional properties due to their primary amino acid sequence.
Casein Proteins: Structural and Functional Aspects
Milk Proteins - From Structure to Biological Properties and Health Aspects, 2016
Mammalian milk is a complex fluid mixture of various proteins, minerals, and lipids, which play an important role in providing nutrition and immunity to the newborn. Casein proteins, which form about 80% of the bovine milk proteins, form large colloidal particles with calcium phosphate to form casein micelles, which for many years have been an important subject of interest. Casein micelles are composed of four main types of proteins: α S1-casein, α S2-casein, β-casein, and k-casein. These constituent casein proteins lack well-defined secondary and tertiary structure due to large amount of propyl residues. These micelles are being extensively studied because of their importance in functional behavior of milk and various milk products. However, the exact structure and nature of these casein micelles are still under debate. These different casein proteins possess different functional properties due to their primary amino acid sequence.
‘New views’ on structure–function relationships in milk proteins
Trends in Food Science & Technology, 2001
The molten globule state has been regarded as a major intermediate in protein folding. It is characterized by nativelike secondary structure with a compact molecular size but little specific tertiary structure. -lactalbumin under various denaturing conditions has been considered a paradigm of the classical molten globule state. It has been shown that caseins share many of the same properties and may therefore exist naturally in a molten globule-like state with defined secondary structure and limited fluctuating tertiary structure, which lead to their propensity for polymerization. The architectural concepts of tensegrity may be used to describe, in part, the structure of casein polymers. #
Journal of dairy science, 2013
A typical casein micelle contains thousands of casein molecules, most of which form thermodynamically stable complexes with nanoclusters of amorphous calcium phosphate. Like many other unfolded proteins, caseins have an actual or potential tendency to assemble into toxic amyloid fibrils, particularly at the high concentrations found in milk. Fibrils do not form in milk because an alternative aggregation pathway is followed that results in formation of the casein micelle. As a result of forming micelles, nutritious milk can be secreted and stored without causing either pathological calcification or amyloidosis of the mother's mammary tissue. The ability to sequester nanoclusters of amorphous calcium phosphate in a stable complex is not unique to caseins. It has been demonstrated using a number of noncasein secreted phosphoproteins and may be of general physiological importance in preventing calcification of other biofluids and soft tissues. Thus, competent noncasein phosphoprotei...
Purification and properties of a major casein component of rat milk
Biochimica et biophysica acta, 1981
A casein component (C2-casein) was purified by ion-exchange and gel filtration chromatography from rat milk, and the properties of this protein were examined. The molecular weight of C2-casein, as determined by Sepharose 4B gel filtration in 6 M guanidine hydrochloride, was 34 000 +/- 1000. The average hydrophobicity calculated from the amino acid composition showed that C2-casein is a rather hydrophilic protein. The alpha-helix content obtained from optical rotatory dispersion experiments was about 12%. In ultracentrifugation analyses, monomer and polymer peaks of C2-casein were both seen, and the monomer-to-polymer ratio was not affected by changing temperature conditions. C2-casein was precipitated by the presence of 2.5 mM CaCl2, and the precipitability was greatly decreased by the dephosphorylation of the protein. C2-casein was stabilized from Ca2+-dependent precipitation by the addition of another rat casein component (C3-casein) or of bovine kappa-casein.
UHT milk contains multiple forms of αS1-casein that undergo degradative changes during storage
Food Chemistry, 2012
Milk proteins are susceptible to chemical changes during processing and storage. We used proteomic tools to analyse bovine a S1-casein in UHT milk. 2-D gels of freshly processed milk a S1-casein was presented as five or more spots due to genetic polymorphism and variable phosphorylation. MS analysis after phosphopeptide enrichment allowed discrimination between phosphorylation states and genetic variants. We identified a new alternatively-spliced isoform with a deletion of exon 17, producing a new C-terminal sequence, K 164 SQVNSEGLHSYGL 177 , with a novel phosphorylation site at S 174. Storage of UHT milk at elevated temperatures produced additional, more acidic a S1-casein spots on the gels and decreased the resolution of minor forms. MS analysis indicated that non-enzymatic deamidation and loss of the N-terminal dipeptide were the major contributors to the changing spot pattern. These results highlight the important role of storage temperature in the stability of milk proteins and the utility of proteomic techniques for analysis of proteins in food.
Invited review: Understanding the behavior of caseins in milk concentrates
Journal of Dairy Science, 2019
The colloidal properties of the casein micelles play a major role in the structural properties of milk protein concentrates. Because of their great technological importance, the structural-functional relationships of casein micelles have been studied for decades in skim milk; however, novel ingredients are now available with higher protein concentrations and varying in composition. The colloidal behavior of caseins in these systems is not fully understood. Concentrates prepared with membrane technologies, and subjected to pre-or postmodifications that affect their technological functionality, have become increasingly widespread. This has created large opportunities for innovation and generation of value-added ingredients. The manner in which caseins interact with themselves and the other components in these concentrates will affect the structure of the final matrix. During concentration by filtration, the interparticle distance between the micelles decreases considerably, increasing their spatial correlation and decreasing their diffusivity. Rearrangements occur due to changes in environmental conditions, such as ionic composition, osmotic stress, shear, pH, or heating temperature. This will have important consequences on bulk viscosity of the concentrates, as well as on the mode of formation of structures' building blocks. This paper aims at highlighting some of the important factors affecting the colloidal structure of casein micelles, their destabilization and network formation, namely, processing history, volume fraction, composition of the serum phase, and ionic equilibrium. Understanding these factors will lead to a better quality control of dairy ingredients and to the development of a new generation of ingredients with targeted functionality.
Casein micelle structure: What can be learned from milk synthesis and structural biology?
Current Opinion in Colloid & Interface Science, 2006
At the heart of the skim 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 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. These models are critically examined in light of our current information regarding the biological processes of protein secretion. In addition two primary tenets of structural biology are applied: that protein structure gives rise to function and that competent protein-protein interactions (associations) will lead to efficient transit through the mammary secretory apparatus. However, a set of complex equilibria governs this process which may be completed only after the final step in the processes: milking. In this light an overwhelming argument can be made for the formation of proteinacious complexes (submicelles) as the formative agents in the synthesis of casein micelles in mammary tissue. Whether these submicelles persist in the milk has been questioned. However, perturbations in micellar equilibria allow for the reemergence of submicellar particles in dairy products such as cheese. Thus protein-protein interactions appear to be important in milk and dairy products from the endoplasmic reticulum to the cheese cutting board. Published by Elsevier Ltd.