Invited review: Modeling milk stability

Novel insights into the stability of milk and milk products during storage and processing result from describing caseins near neutral pH as hydrophilic, intrinsically disordered, proteins. Casein solubility is strongly influenced by pH and multivalent ion binding. Solubility is high at a neutral pH or above, but decreases as the casein net charge approaches zero, allowing a condensed casein phase or gel to form, then increases at lower pH. Of particular importance for casein micelle stability near neutral pH is the proportion of free caseins in the micelle (i.e., caseins not bound directly to nanoclusters of calcium phosphate). Free caseins are more soluble and better able to act as molecular chaperones (to prevent casein and whey protein aggregation) than bound caseins. Some free caseins are highly phosphorylated and can also act as mineral chaperones to inhibit the growth of calcium phosphate phases and prevent mineralized deposits from forming on membranes or heat exchangers. Thus, casein micelle stability is reduced when free caseins bind to amyloid fibrils, destabilized whey proteins or calcium phosphate. The multivalent-binding model of the casein micelle quantitatively describes these and other factors affecting the stability of milk and milk protein products during manufacture and storage.

Anti-cancer potential of casein and its derivatives: novel strategies for cancer treatment

Cancer is one of the leading causes of death worldwide, with over 10 million fatalities annually. While tumors can be surgically removed and treated with chemotherapy, radiotherapy, immunotherapy, hormonal therapy, or combined therapies, current treatments often result in toxic side effects in normal tissue. Therefore, researchers are actively seeking ways to selectively eliminate cancerous cells, minimizing the toxic side effects in normal tissue. Caseins and its derivatives have shown promising anti-cancer potential, demonstrating antitumor and cytotoxic effects on cells from various tumor types without causing harm to normal cells. Collectively, these data reveals advancements in the study of caseins and their derivative peptides, particularly providing a comprehensive understanding of the molecular mechanism of action in cancer therapy. These mechanisms occur through various signaling pathways, including (i) the increase of interferon-associated STAT1 signaling, (ii) the suppression of stemness-related markers such as CD44, (iii) the attenuation of the STAT3/HIF1-α signaling, (iv) the down-expression of uPAR and PAI-1, (v) the loss of mitochondrial membrane potential and reduced intracellular ATP production, (vi) the increase of caspase-3 activity, and (vii) the suppression of TLR4/NF-кB signaling. Therefore, we conclude that casein could be an effective adjuvant for cancer treatment.

Regenerated Fibers from Rennet-Treated Casein Micelles during Acidification

Micellar casein fibers of defined size and internal structure can be produced by the extrusion of cold-renneted casein micelles into a warm, calcium-rich coagulation bath. Calcium phosphate contacts within the casein matrix are important for fiber stability and production but become less important under acidic pH conditions. We demonstrate this with swelling experiments in media with pH < 2, which we adjust with citric acid of different molarities. In contrast to the simple swelling of dried casein fibers in water, a two-phase process takes place in citric acid similar to swelling in 1 N HCl. However, instead of a second deswelling step, we observe in citric acid that the fiber swells further. The observation is explained by a pH-dependent transition from a rennet casein gel to an acidified rennet gel. This can be simulated with a kinetic model that couples two second-order rate equations via a time-varying ratio. The final swelling values decrease with increasing proton concentration via a scaling relation, which is also confirmed by swelling in other acids (HCl or acetic acid) in this pH range. We attribute this to a decrease in the molecular weights of the aggregated casein structures within the strands of the gel network.

Nearly Reversible Expansion and Shrinkage of Casein Microparticles Triggered by Extreme pH Changes

Solvent flows in the fL/s range across the total surface of a casein microparticle cause its expansion and shrinkage. Microparticles prepared from the milk protein casein have a porous and flexible inner structure with water-filled channels and cavities. Solvent uptake occurs in two phases and results in disintegration if de-swelling is not triggered by acidification. So far, nothing is known about the reversibility of the swelling/de-swelling steps. We performed pH jump experiments between pH 11 and pH 1 on a single micro-particle and analyzed the swelling-induced size changes with system dynamics modeling. Both the swelling steps and the subsequent de-swelling process proceed reversibly and at an unchanged rate over a sequence of at least three pH exchange cycles. We observed that the duration of the first swelling step increased during the sequence, while the second step became shorter. Both of the time intervals are negatively correlated, while a statistical evaluation of only one swelling cycle for an ensemble of microparticles with different stabilities did not reveal any significant correlation between the two parameters. Our results indicate that the pH-induced swelling/shrinkage of casein microparticles is, to a large extent, reversible and only slightly influenced by the acid-induced decomposition of colloidal calcium phosphate.

Assessment of milk protein digestion kinetics: effects of denaturation by heat and protein type used

Knowledge about how molecular properties of proteins affect their digestion kinetics is crucial to understand protein postprandial plasma amino acid (AA) responses. Previously it was found that a native whey protein isolate (NWPI) and heat denatured whey protein isolate (DWPI) elicit comparable postprandial plasma AA peak concentrations in neonatal piglets, while a protein base ingredient for infant formula (PBI, a β-casein-native whey protein mixture) caused a 39% higher peak AA concentration than NWPI. We hypothesized that both whey protein denaturation by heat as well as changing protein composition by including β-casein, increases the rate of intact protein loss, and that changing the protein composition (by including β-casein), but not whey protein denaturation, yields a faster absorbable product release. Therefore NWPI (91% native), DWPI (91% denatured) and PBI hydrolysis was investigated in a semi-dynamic in vitro digestion model (SIM). NWPI and DWPI hydrolysis were also compared in a dynamic digestion model with dialysis (TIM-1) to exclude potential product inhibition effects that may occur in a closed vessel digestion model as SIM. In both models, the degree of hydrolysis (DH), loss of intact protein, and release of absorbable products (SIM: <0.5 kDa peptides and free AA, TIM-1: bioaccessible AA) were monitored. Additionally, in SIM, intermediate product amounts and their characteristics were determined. DWPI showed considerably faster intact protein loss, but similar DH and absorbable product release kinetics compared with NWPI in both models. Furthermore, more, relatively large, intermediate products were released from DWPI than from NWPI. PBI showed increased intact protein loss, similar DH, and absorbable product release kinetics, but more, relatively small, intermediate products than NWPI. In conclusion, both whey protein denaturation and β-casein inclusion increased the rate of intact protein loss without affecting absorbable product release during in vitro digestion. Our results suggest that intermediate digestion product characteristics are important in relation to postprandial AA responses.

The influence of intrinsic and extrinsic factors on protein-polyphenol interactions in dairy systems

The application of polyphenols in dairy products is becoming more widespread due to the rich protein content of dairy products reacting with polyphenols. Many studies have pointed to the positive effects of polyphenol-protein binding on dairy products. This article illustrated the mechanisms of how polyphenols react with proteins. The effects of the combination of the polyphenols and protein were described in terms of intrinsic and extrinsic factors, as well as the effects on food properties. Moreover, specific examples of the use of polyphenols in dairy products will also be proposed. Importantly, based on the reported findings, future processes that exploit and develop this interaction can be considered to enable the targeting of polyphenols and proteins in dairy products to secure a range of industrial benefits.

Comparative Peptidomics Analysis of Fermented Milk by Lactobacillus delbrueckii ssp. bulgaricus and Lactobacillus delbrueckii ssp. lactis

Few studies have investigated the peptidomics of fermented milk by Lactobacillus delbrueckii. The aim of the present study was to interpret the peptidomic pattern of the fermented milk by five strains of L. delbrueckii ssp. bulgaricus and ssp. lactis prior to and after the simulated gastrointestinal digestion in vitro. The results indicated variations in the peptidomics among the samples, particularly between the samples of different subspecies. The peptides originating from β-casein were abundant in the samples of ssp. bulgaricus, whereas the peptides derived from αs1-casein and αs2-casein were more likely to dominate in those of ssp. lactis. For β-casein, the strains of ssp. bulgaricus displayed extensive hydrolysis in the regions of (73–97), (100–120), and (130–209), whereas ssp. lactis mainly focused on (160–209). The digestion appears to reduce the variations of the peptidomics profile in general. Among the five strains, L. delbrueckii ssp. bulgaricus DQHXNS8L6 was the most efficient in the generation of bioactive peptides prior to and after digestion. This research provided an approach for evaluating the peptide profile of the strains during fermentation and digestion.

Per aspera ad chaos: a personal journey to the wonderland of intrinsic disorder

This perspective article describes some of the key points of my personal journey through the intriguing world of intrinsically disordered proteins (IDPs). It also shows the evolution of my perception of functional proteins from a standard lock-and-key theory, where a unique function is defined by a unique 3D structure, to the structure-function continuum model, where the structural heterogeneity and conformational plasticity of IDPs define their remarkable multifunctionality and binding promiscuity. These personal accounts of the difficult and lengthy transition from order to disorder paralleled the uneasy and challenging transition in the mind of the scientific community from disbelief in intrinsic disorder to acceptance of IDPs as real entities that play critical biological roles. I hope that this perspective will be of interest to the readers of this journal.

The effects of pH and NaCl concentration on the structure of β-casein from buffalo milk

In the present study, we aimed to investigate the effects of pH and sodium chloride (NaCl) concentration on the structure of β-casein (β-CN) purified from buffalo milk using circular dichroism (CD), intrinsic tryptophan, and anilino-8-naphthalene sulfonate (ANS) fluorescence spectroscopy. We found that NaCl concentration played a critical role in the stability of the secondary structure of β-CN. The CD negative peak had a redshift as the NaCl concentration was increased and accompanied by a decrease of β-sheet content and an increase of α-helix content. ANS fluorescence spectroscopy also indicated that higher NaCl concentration and lower pH significantly affected the tertiary structure of β-CN. Dynamic light scattering (DLS) results showed that the particle size of buffalo β-CN had a blueshift, and then a redshift within the pH range of 5.0-7.5, and it showed a redshift when the NaCl concentration was increased.

Valorization of Apple Peels through the Study of the Effects on the Amyloid Aggregation Process of κ-Casein

Waste valorization represents one of the main social challenges when promoting a circular economy and environmental sustainability. Here, we evaluated the effect of the polyphenols extracted from apple peels, normally disposed of as waste, on the amyloid aggregation process of κ-casein from bovine milk, a well-used amyloidogenic model system. The effect of the apple peel extract on protein aggregation was examined using a thioflavin T fluorescence assay, Congo red binding assay, circular dichroism, light scattering, and atomic force microscopy. We found that the phenolic extract from the peel of apples of the cultivar “Fuji”, cultivated in Sicily (Caltavuturo, Italy), inhibited κ-casein fibril formation in a dose-dependent way. In particular, we found that the extract significantly reduced the protein aggregation rate and inhibited the secondary structure reorganization that accompanies κ-casein amyloid formation. Protein-aggregated species resulting from the incubation of κ-casein in the presence of polyphenols under amyloid aggregation conditions were reduced in number and different in morphology.

A quantitative calcium phosphate nanocluster model of the casein micelle: the average size, size distribution and surface properties

Caseins (α_S1, α_S2, β and κ) are the main protein fraction of bovine milk. Together with nanoclusters of amorphous calcium phosphate (CaP) and divalent cations, they combine to form a polydisperse distribution of particles called casein micelles. A casein micelle model is proposed which is consistent with the way in which intrinsically disordered proteins interact through predominantly polar, short, linear, motifs. Using the model, an expression is derived for the size distribution of casein micelles formed when caseins bind to the CaP nanoclusters and the complexes further associate with each other and the remaining mixture of free caseins. The result is a refined coat-core model in which the core is formed mainly by the nanocluster complexes and the coat is formed exclusively by the free caseins. Example calculations of the size distribution and surface composition of an average bovine milk are compared with experiment. The average size, size distribution and surface composition of the micelles is shown to depend on the affinity of the nanocluster complexes for each other in competition with their affinity for free caseins, and on the concentrations of free caseins, calcium ions and other salts in the continuous phase.

Invited review: Heat stability of milk and concentrated milk: Past, present, and future research objectives

The ability of milk and concentrated milk to withstand a defined heat treatment without noticeable changes such as flocculation of protein is commonly denoted as heat stability. A heat treatment that exceeds the heat stability limit of milk or concentrated milk, which has a much lower heat stability, may result in undesired changes, such as separation of milk fat, grittiness, sediment formation, and phase separation. Most laboratory-scale batch heating methods were developed in the early 20th century to simulate commercial sterilization, and these methods have since been standardized. Heat stability studies have been motivated by different objectives during that time, addressing different processing issues and targets in the framework of available technology, legislation, and consumer demand. Although milk hygiene has improved during the last couple of decades, rendering milk less sensitive to coagulation, different standard methods suffered from poor comparability of results, even when comparing results for the same milk sample, indicating that unknown procedural steps affect heat stability. The prediction of heat stability of concentrated milk from the heat stability results of the corresponding unconcentrated milk for rapid quality testing purposes has been difficult, mainly due to different experimental conditions. The objective of this study is to review literature on heat stability, starting from studies in the early 20th century, to summarize the vast number of studies on compositional aspects of milk affecting heat stability, and to lead the way to the most recent work related to compositional changes in concentrates produced by membrane concentration and fractionation, respectively. Particular attention is paid to early and most recent developments and findings, such as the application of kinetic models to predict and limit protein aggregation to assess and describe heat stability as a temperature-time-total milk solids continuum.

Flammability Characteristics and Mechanical Properties of Casein Based Polymeric Composites

Even though casein has an intrinsic potential ability to act as a flame retardant (FR) additive, the research regarding the FR performance of casein filled polymeric composites has not been thoroughly conducted. In the present work, two commercial casein products, such as lactic casein 720 (LAC) and sodium casein 180 (SC), were chosen to investigate their effects on the performances of the polypropylene (PP) composites. The melt compounding and compression moulding processes were employed to fabricate these casein-based composites. Ammonium polyphosphate (APP) was also selected to explore its combined effects in conjunction with casein on the composite’s flammability. The cone calorimeter results showed that the addition of casein significantly reduced (66%) the peak heat release rate (PHRR) of the composite compared to that of neat PP. In particular, the combination of LAC and APP led to the formation of more compact and rigid char compared to that for SC based sample; hence, a further reduction (80%) in PHRR and self-extinguishment under a vertical burn test were accomplished. Moreover, the tensile modulus of the composite improved (23%) by the combined effects of LAC and APP. The overall research outcome has established the potential of casein as a natural protein FR reducing a polymer’s flammability.

Structural Biology of Calcium Phosphate Nanoclusters Sequestered by Phosphoproteins

Biofluids that contain stable calcium phosphate nanoclusters sequestered by phosphopeptides make it possible for soft and hard tissues to co-exist in the same organism with relative ease. The stability diagram of a solution of nanocluster complexes shows how the minimum concentration of phosphopeptide needed for stability increases with pH. In the stable region, amorphous calcium phosphate cannot precipitate. Nevertheless, if the solution is brought into contact with hydroxyapatite, the crystalline phase will grow at the expense of the nanocluster complexes. The physico-chemical principles governing the formation, composition, size, structure, and stability of the complexes are described. Examples are given of complexes formed by casein, osteopontin, and recombinant phosphopeptides. Application of these principles and properties to blood serum, milk, urine, and resting saliva is described to show that under physiological conditions they are in the stable region of their stability diagram and so cannot cause soft tissue calcification. Stimulated saliva, however, is in the metastable region, consistent with its role in tooth remineralization. Destabilization of biofluids, with consequential ill-effects, can occur when there is a failure of homeostasis, such as an increase in pH without a balancing increase in the concentration of sequestering phosphopeptides.

Disorder and cysteines in proteins: A design for orchestration of conformational see-saw and modulatory functions

Being responsible for more than 90% of cellular functions, protein molecules are workhorses in all the life forms. In order to cater for such a high demand, proteins have evolved to adopt diverse structures that allow them to perform myriad of functions. Beginning with the genetically directed amino acid sequence, the classical understanding of protein function involves adoption of hierarchically complex yet ordered structures. However, advances made over the last two decades have revealed that inasmuch as 50% of eukaryotic proteome exists as partially or fully disordered structures. Significance of such intrinsically disordered proteins (IDPs) is further realized from their ability to exhibit multifunctionality, a feature attributable to their conformational plasticity. Among the coded amino acids, cysteines are considered to be "order-promoting" due to their ability to form inter- or intramolecular disulfide bonds, which confer robust thermal stability to the protein structure in oxidizing conditions. The co-existence of order-promoting cysteines with disorder-promoting sequences seems counter-intuitive yet many proteins have evolved to contain such sequences. In this chapter, we review some of the known cysteine-containing protein domains categorized based on the number of cysteines they possess. We show that many protein domains contain disordered sequences interspersed with cysteines. We show that a positive correlation exists between the degree of cysteines and disorder within the sequences that flank them. Furthermore, based on the computational platform, IUPred2A, we show that cysteine-rich sequences display significant disorder in the reduced but not the oxidized form, increasing the potential for such sequences to function in a redox-sensitive manner. Overall, this chapter provides insights into an exquisite evolutionary design wherein disordered sequences with interspersed cysteines enable potential modulatory protein functions under stress and environmental conditions, which thus far remained largely inconspicuous.

Intrinsically disordered proteins of viruses: Involvement in the mechanism of cell regulation and pathogenesis

Intrinsically disordered proteins (IDPs) possess the property of inherent flexibility and can be distinguished from other proteins in terms of lack of any fixed structure. Such dynamic behavior of IDPs earned the name "Dancing Proteins." The exploration of these dancing proteins in viruses has just started and crucial details such as correlation of rapid evolution, high rate of mutation and accumulation of disordered contents in viral proteome at least understood partially. In order to gain a complete understanding of this correlation, there is a need to decipher the complexity of viral mediated cell hijacking and pathogenesis in the host organism. Further there is necessity to identify the specific patterns within viral and host IDPs such as aggregation; Molecular recognition features (MoRFs) and their association to virulence, host range and rate of evolution of viruses in order to tackle the viral-mediated diseases. The current book chapter summarizes the aforementioned details and suggests the novel opportunities for further research of IDPs senses in viruses.

Association of Enzymatically and Nonenzymatically Functionalized Caseins Analyzed by Size-Exclusion Chromatography and Light-Scattering Techniques

The influence of covalent protein modifications resulting from the Maillard reaction (glycation) of casein and lactose on the noncovalent association behavior of the protein was studied. Nonenzymatic cross-linking with methylglyoxal (MGO) and glutaraldehyde (GTA) as well as enzymatic cross-linking with microbial transglutaminase (mTG) was investigated in comparison. Molar mass, particle size, and conformational characteristics of nonmicellar casein associates as well as the extent of intraparticle protein cross-linking were examined utilizing size-exclusion chromatography (SEC) combined with UV detection and static and dynamic light scattering. Cross-linking resulted in the stabilization of a certain fraction of casein associates, with particle sizes of approximately 30 nm in radius of gyration (R_g), and promoted an incorporation of further casein molecules into those particles, yielding molar masses (M_w) of 1.0-1.2 × 10⁶ g/mol. When caseins were additionally conjugated with lactose during the early Maillard reaction, a further growth of the associates up to approximately 50 nm in R_g with a M_w of 2.1 × 10⁶ g/mol was observed. Furthermore, glycation reactions induced a transition from slightly elongated, random-coil structures toward more anisotropic conformations. Associates consisting of caseins cross-linked with GTA appeared to preserve the original particle conformation.

Characterization of Sample Loss Caused by Competitive Adsorption of Proteins in Vials Using Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis

Sample loss caused by competitive protein adsorption on solid surfaces from complex samples remains to be a major hurdle in sensitive analyses of proteins. No label-free techniques can easily quantify individual proteins adsorbed on irregular surfaces of Eppendorf vials or Falcon tubes, which are commonly used to contain complex biological samples. Multiplexed characterization of such adsorption by different proteins is technically challenging. Herein, we developed a direct protein analysis based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis for the characterization of sample loss occurred on the curved surface with limited area. Using this simple and easily accessible method, we discovered the effect of ethylenediaminetetraacetic acid on surface adsorption of different milk proteins, specifically an augmented loss of milk proteins in low-binding sample vials. In this study, we also identified severe biases of silver staining and established proteomics-based mapping of protein distribution in biological samples for absolute quantification of competitive protein adsorption on irregular surfaces.

A theoretical and experimental investigation of the effect of sodium dodecyl sulfate on the structural and conformational properties of bovine β-casein

A predicted three-dimensional structure of bovine β-casein was constructed using homology modeling with the aid of MODELLER and I-TASSER programs, with the validity and reliability of the models evaluated according to stereochemical qualities and small angle X-ray scattering. By comparing the results obtained from the two models using the CRYSOL program, an optimal model of the β-casein structure derived from I-TASSER was selected and used in subsequent molecular dynamics (MD) analysis. 300 ns MD simulations of β-casein in water and in the presence of different SDS concentrations at 300 K were performed. The results of the MD simulations indicated that SDS molecules played a dual role in modifying the conformation of β-casein at 300 K. Concentrations of SDS below its CMC (1 mM), at which only the monomer form of SDS was present, induced β-casein to lose its secondary structure by converting helices into random coils; however the conformation of the complex was still comparable with that of native β-casein. In the presence of 10 mM SDS (above its CMC), the helical content of β-casein was increased along with reduced random coils, and the structural rearrangement led to a more compact conformation. The latter change is likely related to the hydrophobic interactions that dominate the binding of the C-terminal region, along with the anchoring of sulfate groups of SDS on the positively charged N-terminal portion via electrostatic attraction. Hydrogen bonding supplemented the SDS-induced stabilization of β-casein. A correlated "necklace and bead" model, in which the micelles nucleate on the protein hydrophobic sites, was proposed for the structure of β-casein-SDS complexes.

Soft-Matter Approaches for Controlling Food Protein Interactions and Assembly

Animal- and plant-based proteins are present in a wide variety of raw and processed foods. They play an important role in determining the final structure of food matrices. Food proteins are diverse in terms of their biological origin, molecular structure, and supramolecular assembly. This diversity has led to segmented experimental studies that typically focus on one or two proteins but hinder a more general understanding of food protein structuring as a whole. In this review, we propose a unified view of how soft-matter physics can be used to control food protein assembly. We discuss physical models from polymer and colloidal science that best describe and predict the phase behavior of proteins. We explore the occurrence of phase transitions along two axes: increasing protein concentration and increasing molecular attraction. This review provides new perspectives on the link between the interactions, phase transitions, and assembly of proteins that can help in designing new food products and innovative food processing operations.

Micelas de caseína: dos monômeros à estrutura supramolecular

Resumo A importância primária das micelas de caseína reside no fato de que os processos empregados na transformação do leite em quaisquer de seus derivados dependem, direta ou indiretamente, de sua estabilidade ou de sua desestabilização controlada. Assim, o objetivo do presente trabalho é apresentar uma revisão atualizada sobre a organização estrutural das micelas de caseína. Em termos físico-químicos, as micelas de caseína podem ser definidas como agregados supramoleculares esféricos e porosos, altamente hidratados, carregados negativamente, com diâmetro médio de 200 nm, e que apresentam aproximadamente 104 cadeias polipeptídicas. Além de água, as micelas são constituídas por quatro tipos de caseínas, chamadas de αS1, αS2, β, e κ-caseínas, que estão unidas por meio de interações hidrofóbicas e eletrostáticas, e pela presença de minerais, sobretudo sais de fosfato de cálcio, os quais são os principais responsáveis pela manutenção da estrutura micelar. A estabilidade das micelas de caseína é atribuída à presença de uma camada externa difusa, formada basicamente por κ-caseína. Apesar de as propriedades coloidais das micelas de caseína serem conhecidas, ainda não há consenso sobre como as moléculas de caseína estão estruturadas em seu interior. Portanto, os principais modelos que descrevem a organização interna das micelas de caseína são apresentados na parte final do artigo.

Acid-Induced Gelation of Caseins Glycated with Lactose: Impact of Maillard Reaction-Based Glycoconjugation and Protein Cross-Linking

During food processing or storage, milk proteins can react with reducing sugars via the Maillard reaction (glycation), which may alter their techno-functional properties. The aim of this study was to investigate the relationship between molecular changes of casein occurring during different stages of the Maillard reaction and its acid-induced gelling properties. Therefore, sodium caseinate was heated in a dry state at 60 °C in the presence of lactose and analyzed for structural modifications by determining Amadori compounds (glycoconjugation) indirectly as furosine, the total lysine modification, and the extent of protein cross-linking. For techno-functional characterization, acid-induced gels were prepared by the addition of glucono-δ-lactone and evaluated by measuring pH kinetics during gel formation, gel strength, and water holding capacity. The time to reach pH 4.6 during the gelation process was significantly delayed with increasing extent of the Maillard reaction. Glycation with lactose also led to a significant increase in gel strength and water holding capacity. The increase in gel stability was rather independent from the amount of sugars covalently bound to the proteins during the early phase of the Maillard reaction but strongly correlated to the degree of protein polymerization. Small- and medium-sized casein oligomers, formed during advanced stages of the Maillard reaction, contributed considerably to the formation of stronger gels with higher water holding capacity, whereas a sharp increase in the relative amount of the polymer fraction observed during prolonged cross-linking processes caused a spontaneous destabilization of the gel network. Knowledge about structure-function relationships on a molecular level can provide useful information to control food texture by raw material quality.

Proteostasis and the Regulation of Intra- and Extracellular Protein Aggregation by ATP-Independent Molecular Chaperones: Lens α-Crystallins and Milk Caseins

Molecular chaperone proteins perform a diversity of roles inside and outside the cell. One of the most important is the stabilization of misfolding proteins to prevent their aggregation, a process that is potentially detrimental to cell viability. Diseases such as Alzheimer's, Parkinson's, and cataract are characterized by the accumulation of protein aggregates. In vivo, many proteins are metastable and therefore under mild destabilizing conditions have an inherent tendency to misfold, aggregate, and hence lose functionality. As a result, protein levels are tightly regulated inside and outside the cell. Protein homeostasis, or proteostasis, describes the network of biological pathways that ensures the proteome remains folded and functional. Proteostasis is a major factor in maintaining cell, tissue, and organismal viability. We have extensively investigated the structure and function of intra- and extracellular molecular chaperones that operate in an ATP-independent manner to stabilize proteins and prevent their misfolding and subsequent aggregation into amorphous particles or highly ordered amyloid fibrils. These types of chaperones are therefore crucial in maintaining proteostasis under normal and stress (e.g., elevated temperature) conditions. Despite their lack of sequence similarity, they exhibit many common features, i.e., extensive structural disorder, dynamism, malleability, heterogeneity, oligomerization, and similar mechanisms of chaperone action. In this Account, we concentrate on the chaperone roles of α-crystallins and caseins, the predominant proteins in the eye lens and milk, respectively. Intracellularly, the principal ATP-independent chaperones are the small heat-shock proteins (sHsps). In vivo, sHsps are the first line of defense in preventing intracellular protein aggregation. The lens proteins αA- and αB-crystallin are sHsps. They play a crucial role in maintaining solubility of the crystallins (including themselves) with age and hence in lens proteostasis and, ultimately, lens transparency. As there is little metabolic activity and no protein turnover in the lens, crystallins are very long lived proteins. Lens proteostasis is therefore very different to that in normal, metabolically active cells. Crystallins undergo extensive post-translational modification (PTM), including deamidation, racemization, phosphorylation, and truncation, which can alter their stability. Despite this, the lens remains transparent for tens of years, implying that lens proteostasis is intimately integrated with crystallin PTMs. Many PTMs do not significantly alter crystallin stability, solubility, and functionality, which thereby facilitates lens transparency. In the long term, however, extensive accumulation of crystallin PTMs leads to large-scale crystallin aggregation, lens opacification, and cataract formation. Extracellularly, various ATP-independent molecular chaperones exist that exhibit sHsp-like structural and functional features. For example, caseins, the major milk proteins, exhibit chaperone ability by inhibiting the amorphous and amyloid fibrillar aggregation of a diversity of destabilized proteins. Caseins maintain proteostasis within milk by preventing deleterious casein amyloid fibril formation via incorporation of thousands of individual caseins into an amorphous structure known as the casein micelle. Hundreds of nanoclusters of calcium phosphate are sequestered within each casein micelle through interactions with short, highly phosphorylated casein sequences. This results in a stable biofluid that contains a high concentration of potentially amyloidogenic caseins and concentrations of calcium and phosphate that can be far in excess of the solubility of calcium phosphate. Casein micelle formation therefore performs vital roles in neonatal nutrition and calcium homeostasis in the mammary gland.

To be disordered or not to be disordered: is that still a question for proteins in the cell?

There is ample evidence that many proteins or regions of proteins lack a well-defined folded structure under native-like conditions. These are called intrinsically disordered proteins (IDPs) or intrinsically disordered regions (IDRs). Whether this intrinsic disorder is also their main structural characteristic in living cells has been a matter of intense debate. The structural analysis of IDPs became an important challenge also because of their involvement in a plethora of human diseases, which made IDPs attractive targets for therapeutic development. Therefore, biophysical approaches are increasingly being employed to probe the structural and dynamical state of proteins, not only in isolation in a test tube, but also in a complex biological environment and even within intact cells. Here, we survey direct and indirect evidence that structural disorder is in fact the physiological state of many proteins in the proteome. The paradigmatic case of α-synuclein is used to illustrate the controversial nature of this topic.

Heteroprotein complex coacervation: A generic process

Proteins exhibit a rich diversity of functional, physico-chemical and biodegradable properties which makes them appealing for various applications in the food and non-food sectors. Such properties are attributed to their ability to interact and assemble into a diversity of supramolecular structures. The present review addresses the updated research progress in the recent field of complex coacervation made from mixtures of oppositely charged proteins (i.e. heteroprotein systems). First, we describe briefly the main proteins used for heteroprotein coacervation. Then, through some selected examples, we illustrate the particularity and specificity of each heteroprotein system and the requirements that drive optimal assembly into coacervates. Finally, possible and promising applications of heteroprotein coacervates are mentioned.

Milk Protein Polymer and Its Application in Environmentally Safe Adhesives

Milk proteins (caseins and whey proteins) are important protein sources for human nutrition; in addition, they possess important natural polymers. These protein molecules can be modified by physical, chemical, and/or enzymatic means. Casein is one of the oldest natural polymers, used for adhesives, dating back to thousands years ago. Research on milk-protein-based adhesives is still ongoing. This article deals with the chemistry and structure of milk protein polymers, and examples of uses in environmentally-safe adhesives. These are promising routes in the exploration of the broad application of milk proteins.