Casein micelle structure: What can be learned from milk synthesis and structural biology?

Farrell, Harold M.; Malin, Edyth L.; Brown, Eleanor M.; Qi, Phoebe X.

doi:10.1016/j.cocis.2005.11.005

Cited by 215 publications

(152 citation statements)

References 44 publications

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“…By contrast, crosslinking with transglutaminase appears not to affect casein micelle size or substructure, as measured by a range of scattering methods, and produces particles that are resistant to drastic changes in solvent quality (de Kruif et al, 2012). In protein submicellar models of casein micelle structure, the average separation of the dense substructures, about 18 nm, has been interpreted as the diameter of the submicelles (Stothart and Cebula, 1982;Stothart, 1989;Hansen et al, 1996;Farrell et al, 2006a). Notwithstanding the many difficulties of sample preparation, a number of recent electron microscopical methods have yielded images of casein micelles that do not support any submicelle model but are consistent with nanocluster models (Holt et al, 1978;McMahon and McManus 1998;Marchin et al, 2007;Trejo et al, 2011).…”

Section: Casein Micelle Structurementioning

confidence: 99%

“…A diversity of views on the structure of the casein micelle have been expressed in recent reviews from the colloid and food science perspective Horne, 2008;Dalgleish and Corredig, 2012;McMahon and Oommen, 2012) and from a somewhat different biological perspective to that developed here (Farrell et al, 2006a). In this review, we consider the structure of the casein micelle in relation to 3 of its recognized biological functions.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

Holt

Carver

Ecroyd

et al. 2013

Journal of Dairy Science

393

240

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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 phosphoproteins have similar patterns of phosphorylation and the same type of flexible, unfolded conformation as caseins. The ability to suppress amyloid fibril formation by forming an alternative amorphous aggregate is also not unique to caseins and underlies the action of molecular chaperones such as the small heat-shock proteins. The open structure of the protein matrix of casein micelles is fragile and easily perturbed by changes in its environment. Perturbations can cause the polypeptide chains to segregate into regions of greater and lesser density. As a result, the reliable determination of the native structure of casein micelles continues to be extremely challenging. The biological functions of caseins, such as their chaperone activity, are determined by their composition and flexible conformation and by how the casein polypeptide chains interact with each other. These same properties determine how caseins behave in the manufacture of many dairy products and how they can be used as functional ingredients in other foods. aBStraCtA 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 phosphoproteins have similar patterns of phosp...

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Section: Casein Micelle Structurementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

Holt

Carver

Ecroyd

et al. 2013

Journal of Dairy Science

393

240

View full text Add to dashboard Cite

show abstract

“…6 The structure of casein micelles has been intensively investigated in the past, and numerous models have been postulated. [7][8][9][10][11][12] In a recent development, the internal assembly and structure of the casein micelle has been described by the dual-binding model. 8 Here two distinct forms of binding are involved in the micellar assembly.…”

Section: Introductionmentioning

confidence: 99%

The pH Induced Sol−Gel Transition in Skim Milk Revisited. A Detailed Study Using Time-Resolved Light and X-ray Scattering Experiments

et al. 2010

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We present a detailed study of the evolution of the size, structure and stability of casein micelles upon acidification of skim milk typically applied in yogurt-making processes using a combination of time-resolved light and small-angle X-ray scattering experiments. While most of the available light scattering studies on casein acidification have been restricted to transparent and therefore highly diluted samples, we now profit from a newly developed multiangle 3D light scattering instrument, which allows for time-resolved measurements in highly turbid samples. Our experiments clearly demonstrate the presence of two parallel pH-dependent processes, micellar reassembly and aggregation. Using a systematic investigation of the effect of casein concentration, acidification rate, and ionic strength, we are able to decouple these two processes and obtain detailed information about the pH-induced restructuration of the casein micelle structure that occurs prior to destabilization. Moreover, our experiments also unambiguously demonstrate that these micellar reassembly processes are highly concentration dependent, and that typical light scattering studies conducted under highly diluted conditions are resulting in findings that may not be relevant for the situation encountered in industrial processes at higher concentrations. Experiments conducted with covalently cross-linked micelles, where the pH-induced reassembly has been suppressed, further confirm our findings.

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“…Over the same period, several different competing models, attempting to reconcile and explain a growing body of diverse experimental observations have emerged (Dalgleish, 2011). Despite some fundamental differences between these different models (Farrell, Malin, Brown, & Qi, 2006;Horne, 2006), and various revisions that have been made to them over years, a strikingly common feature that has remained the same is the role that -casein (KC) plays in providing colloidal stability to casein aggregates. It is envisaged that most, if not all of KC fraction resides at the surface of casein micelles.…”

Section: Introductionmentioning

confidence: 99%

Interactions between casein layers adsorbed on hydrophobic surfaces from self consistent field theory: κ-casein versus para-κ-casein

2014

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Interactions mediated by layers of -casein and para--casein have been calculated and compared using the self consistent field (SCF) approach, at pH and electrolyte concentrations found in milk. Our results show the interaction potentials at close separation distances to be repulsive for -casein, while they are attractive for para--casein. We have also studied -casein chains stripped of their sugar moieties and highlighted the important role that the carbohydrate side chains play in the provision of steric forces. The point of transition from a repulsive to an attractive interaction is found to be rather sensitive to the degree of coverage of the surface by the protein chains. At =0.0025 chains per unit monomer area, the value is just over 40%, whereas at =0.0032 this increases to 87%. At the coverage values higher than those we estimate for the surface of casein micelles, no transition is detected and the interactions remain repulsive even when all of -casein has been converted to para--casein, as already shown by Mellema, Leermakers, & de Kruif (1999). At lower surface coverage values, the interaction potential curves for otherwise uncharged surfaces, covered with para--casein, posses an energy barrier. We have demonstrated that the origin of this energy barrier is electrostatic, with para--casein contributing a net positive charge to the surface.We have also explored the importance of the asymmetry in the distribution of charge between the para--casein and the glycomacropeptide sides of the protein. The glycomacropeptide is net negative while para--casein side is net positive at neutral pH. Our results suggest that on a negatively charged surface, such as the surface of casein micelles, this uneven distribution of charge is just as important in determining the conformation of -casein chains as is the difference in the hydrophobic and hydrophilic nature of the two sides of this protein.

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Casein micelle structure: What can be learned from milk synthesis and structural biology?

Cited by 215 publications

References 44 publications

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

Invited review: Caseins and the casein micelle: Their biological functions, structures, and behavior in foods

The pH Induced Sol−Gel Transition in Skim Milk Revisited. A Detailed Study Using Time-Resolved Light and X-ray Scattering Experiments

Interactions between casein layers adsorbed on hydrophobic surfaces from self consistent field theory: κ-casein versus para-κ-casein

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