Towards recombinantly produced milk proteins: Physicochemical and emulsifying properties of engineered whey protein beta-lactoglobulin variants

Cysteines in β-lactoglobulin affects its interfacial adsorption and protein film stabilization

HYPOTHESIS

Disulfide bonds in proteins are strong chemical bonds forming the secondary and tertiary structure like in the dairy protein β-lactoglobulin. We hypothesize that the partial or complete removal of disulfide bonds affects the structural rearrangement of proteins caused by intra- and intermolecular interactions that in turn define the interfacial activity of proteins at oil/water interfaces. The experimental and numerical investigations contribute to the mechanistic understanding of the structure-function relationship, especially for the interfacial adsorption behavior of proteins.

EXPERIMENTAL AND NUMERICAL

Systematically, the 5 cysteines of β-lactoglobulin were recombinantly exchanged by alanine. First, the protein structure of the variants in bulk was analyzed with Fourier-transform-infrared-spectroscopy and molecular dynamic simulations. Second, the structural changes after adsorption to the interface have been also analyzed by molecular dynamic simulations. The adsorption behavior was investigated by pendant drop analysis and the interfacial film properties by dilatational rheology.

FINDINGS

The structural flexibility of β-lactoglobulin with no cysteines encourages its unfolding at the interface, and accelerates the interfacial protein film formation that results in more visco-elastic films in comparison to the reference.

New insights into protein stabilized emulsions captured via neutron and X-ray scattering: An approach with β-lactoglobulin at triacylglyceride-oil/water interfaces

HYPOTHESIS

To analyze protein stabilized emulsions, SAXS and SANS are emerging techniques capturing oil droplet radius, interfacial coverage and structure. Protein shape, thus protein structure change during interfacial adsorption with partial protein unfolding is detected via SAXS analysis at and below the monolayer concentration for proteins, known as critical interfacial concentration (CIC). SANS determines the same phenomena below and above the CIC, via contrast variation and coarse-grained modelling.

EXPERIMENTS

β-lactoglobulin concentration dependent SAXS experiments were performed focusing on molecular length scales to characterize protein shape in water, and interfacial structure in emulsions. Complementary SANS experiments with contrast variation via deuterated triacylglyceride-oil provided insight into oil droplet radius, interfacial coverage and structure via data analysis with scattering models and low-resolution shape reconstruction with the DENFERT model.

FINDINGS

SAXS and SANS experiments allowed to determine the interfacial structure below and above the CIC, as well as oil droplet radius and interfacial coverage. These findings were identified via Q-4 Porod scattering at low-Q, protein scattering at high Q, and a Q-2 scattering of the interface. Since SANS with accurate contrast variation highlights the interface in comparison to other techniques like FTIR, the presented results show a high impact to understand interfaces in emulsions.

Food proteins from yeast-based precision fermentation: Simple purification of recombinant β-lactoglobulin using polyphosphate

Proteins produced through precision fermentation are often purified through chromatographic methods. Faster and more cost-effective purification methods are desired for food application. Here, we present a simple method for purification of protein produced from yeast, using β-lactoglobulin secreted from Pichia pastoris as an example. The food-grade salt hexametaphosphate (HMP) was used to precipitate the protein at acidic pH, while the impurities (extracellular polysaccharides; mainly mannan) remained soluble. After re-solubilization of the protein-HMP complex by neutralization, excess HMP was selectively precipitated using calcium chloride. The protein content of the crude sample increased from 26 to 72 wt% (comparable to purification with anion exchange chromatography), containing only residual extracellular polysaccharides (9 wt%) and HMP (1 wt%). The established method had no significant impact on the structural and functional properties (i.e., ability to form emulsions) of the protein. The presented method shows potential for cost-effective purification of recombinant proteins produced through yeast-based expression systems.

Engineering artificial casein micelles for future food: Is casein phosphorylation necessary?

Industrial-scale production of recombinant proteins for food products may become economically feasible but correct post-translational modification of proteins by microbial expression systems remains a challenge. For efficient production of hybrid products from bovine casein and recombinant casein, it is therefore of interest to evaluate the necessity of casein post-translational phosphorylation for the preparation of hybrid casein micelles and study their rennet-induced coagulation. Our results show that dephosphorylated casein was hardly incorporated into artificial casein micelles but was capable of stabilising calcium phosphate nanoclusters with an increased size through adsorption on their surface. Thereby, dephosphorylated casein formed larger colloidal particles with a decreased hydration. Furthermore, the presence of increasing amounts of dephosphorylated casein resulted in increasingly poor rennet coagulation behaviour, where dephosphorylated casein disrupted the formation of a coherent gel network by native casein. These results emphasise that post-translational phosphorylation of casein is crucial for their assembly into micelles and thereby for the production of dairy products for which the casein micelle structure is a prerequisite, such as many cheese varieties and yoghurt. Therefore, phosphorylation of future recombinant casein is essential to allow its use in the production of animal-free dairy products.

Detection of the β-lactoglobulin genotype in zebu cattle (Gangatiri) milk using high-resolution accurate mass spectroscopy

We studied the genetic polymorphism of beta-lactoglobulin (β-Lg) whey protein in Gangatiri zebu cows for this Research Communication. The polymorphic nature of milk protein fractions and their association with milk production traits, composition and quality has attracted several efforts in evaluating the allelic distribution of protein locus as a potential dairy trait marker. Genetic variants of β-Lg have highly significant effects on casein number (B > A) and protein recovery (B > A) and also determine the yield of cheese dry matter (B > A). Molecular techniques of polyacrylamide gel electrophoresis and high-resolution accurate mass-spectroscopy were applied to characterize the β-Lg protein obtained from the Gangatiri breed milk. Sequence analysis of β-Lg showed the presence of variant B having UniProt database accession number P02754, coded on the PAEP gene. Our study can provide reference and guidance for the selection of superior milk (having β-LgB) from this indigenous breed that could potentially give a good yield of β-Lg for industrial applications.

Modification and oxidative degradation of β-lactoglobulin by UVB irradiation

Ultraviolet (UV) B irradiation induces protein modification, especially the conformational rearrangement of proteins, and is therefore promising as a non-thermal and non-chemical functionalization technique. Nevertheless, UVB irradiation introduces radicals and oxidizes side chains resulting in the loss of food quality. Thus, assessing the UVB irradiation-based functionalization of β-lactoglobulin (BLG) versus its oxidative degradation is of interest. UVB irradiation of up to 8 h was successfully applied to loosen the rigid folding of BLG and increase its flexibility. Thereby, the cysteine at position 121 and hydrophobic regions became surface-exposed as indicated by the increase in accessible thiol groups and increased surface hydrophobicity. Furthermore, we demonstrated the cleavage of the "outer" disulfide bond C⁶⁶-C¹⁶⁰ by LC-MS/MS after tryptic digestion of BLG. The 2-h-irradiated BLG showed adequate conformational rearrangement for protein functionalization while being minimally oxidized.

Unraveling the effects of low protein-phenol binding affinity on the structural properties of beta-lactoglobulin

Non-covalent interactions of phenolics with proteins cannot always be readily identified, often leading to contradictory results described in the literature. This results in uncertainties as to what extent phenolics can be added to protein solutions (for example for bioactivity studies) without affecting the protein structure. Here, we clarify which tea phenolics (epigallocatechin gallate (EGCG), epicatechin and gallic acid) interact with the whey protein β-lactoglobulin by combining various state-of-the-art-methods. STD-NMR revealed that all rings of EGCG can interact with native β-lactoglobulin, indicating multidentate binding, as confirmed by the small angle X-ray scattering experiments. For epicatechin, unspecific interactions were found only at higher protein:epicatechin molar ratios and only with ¹H NMR shift perturbation and FTIR. For gallic acid, none of the methods found evidence for an interaction with β-lactoglobulin. Thus, gallic acid and epicatechin can be added to native BLG, for example as antioxidants without causing modification within wide concentration ranges.

Trends and prospects in dairy protein replacement in yogurt and cheese

There is a growing demand for nutritional, functional, and eco-friendly dairy products, which has increased the need for research regarding alternative and sustainable protein sources. Plant-based, single-cell (SCP), and recombinant proteins are being explored as alternatives to dairy proteins. Plant-Based Proteins (PBPs) are commonly used to replace total dairy protein. However, PBPs are generally mixed with dairy proteins to improve their functional properties, which makes them dependent on animal protein sources. In contrast, single-Cell Proteins (SCPs) and recombinant dairy proteins are promising alternatives for dairy protein replacement since they provide nutritional components, essential amino acids, and high protein yield and can use industrial and agricultural waste as carbon sources. Although alternative protein sources offer numerous advantages over conventional dairy proteins, several technical and sensory challenges must be addressed to fully incorporate them into cheese and yogurt products. Future research can focus on improving the functional and sensory properties of alternative protein sources and developing new processing technologies to optimize their use in dairy products. This review highlights the current status of alternative dairy proteins in cheese and yogurt, their functional properties, and the challenges of their use in these products.

Unlocking the magic in mycelium: Using synthetic biology to optimize filamentous fungi for biomanufacturing and sustainability

Filamentous fungi drive carbon and nutrient cycling across our global ecosystems, through its interactions with growing and decaying flora and their constituent microbiomes. The remarkable metabolic diversity, secretion ability, and fiber-like mycelial structure that have evolved in filamentous fungi have been increasingly exploited in commercial operations. The industrial potential of mycelial fermentation ranges from the discovery and bioproduction of enzymes and bioactive compounds, the decarbonization of food and material production, to environmental remediation and enhanced agricultural production. Despite its fundamental impact in ecology and biotechnology, molds and mushrooms have not, to-date, significantly intersected with synthetic biology in ways comparable to other industrial cell factories (e.g. Escherichia coli,Saccharomyces cerevisiae, and Komagataella phaffii). In this review, we summarize a suite of synthetic biology and computational tools for the mining, engineering and optimization of filamentous fungi as a bioproduction chassis. A combination of methods across genetic engineering, mutagenesis, experimental evolution, and computational modeling can be used to address strain development bottlenecks in established and emerging industries. These include slow mycelium growth rate, low production yields, non-optimal growth in alternative feedstocks, and difficulties in downstream purification. In the scope of biomanufacturing, we then detail previous efforts in improving key bottlenecks by targeting protein processing and secretion pathways, hyphae morphogenesis, and transcriptional control. Bringing synthetic biology practices into the hidden world of molds and mushrooms will serve to expand the limited panel of host organisms that allow for commercially-feasible and environmentally-sustainable bioproduction of enzymes, chemicals, therapeutics, foods, and materials of the future.

Separation of α-Lactalbumin Enriched Fraction from Bovine Native Whey Concentrate by Combining Membrane and High-Pressure Processing

Whey exhibits interesting nutritional properties, but its high β-Lactoglobulin (β-Lg) content could be a concern in infant food applications. In this study, high-pressure processing (HPP) was assessed as a β-Lg removal strategy to generate an enriched α-Lactalbumin (α-La) fraction from bovine native whey concentrate. Different HPP treatment parameters were considered: initial pH (physiological and acidified), sample temperature (7-35 °C), pressure (0-600 MPa) and processing time (0-490 s). The conditions providing the best α-La yield and α-La purification degree balance (46.16% and 80.21%, respectively) were 4 min (600 MPa, 23 °C), despite the significant decrease of the surface hydrophobicity and the total thiol content indexes in the α-La-enriched fraction. Under our working conditions, the general effects of HPP on α-La and β-Lg agreed with results reported in other studies of cow milk or whey. Notwithstanding, our results also indicated that the use of native whey concentrate could improve the β-Lg precipitation degree and the α-La purification degree, in comparison to raw milk or whey. Future studies should include further characterization of the α-La-enriched fraction and the implementation of membrane concentration and HPP treatment to valorize cheese whey.

Determination of specific and non-specific protein-protein interactions for beta-lactoglobulin by analytical ultracentrifugation and membrane osmometry experiments

Protein-protein interactions are essential for the understanding of biological processes. Specific protein aggregation is an important aspect for many biological systems. In particular, electrostatic interactions play the key role for protein-protein interactions, as many amino acids have pH-dependent charge states. Moreover, protein dissociation is directly related to the solution pH, ionic strength, temperature and protein concentration. The subtle interplay between different specific and non-specific interactions is demonstrated for beta-lactoglobulin (BLG) with a focus on low salt concentrations, thus mimicking technically relevant processing conditions. BLG is a well-characterized model system, proven to attain its monomer-dimer equilibrium strongly dependent upon the pH of the solution. In this manuscript, we present a unique combination of analytical ultracentrifugation and membrane osmometry experiments, which quantifies specific and non-specific interactions, i.e. in terms of the dimer dissociation constants and the second osmotic virial coefficient, at pH 3 and 7 and sodium chloride concentrations of 10 mM and 100 mM. This provides direct insight to protein-protein interactions for a system with a concentration-dependent monomer-dimer equilibrium. Moreover, using a coarse-grained extended DLVO model in combination with molecular dynamics simulations, we quantify non-specific monomer-monomer, monomer-dimer and dimer-dimer interactions as well as the binding free energy of BLG dimerization from theoretical calculations. The experimentally determined interactions are shown to be mainly governed by electrostatic interactions and further agree with free energy calculations. Our experimental protocol aims to determine non-specific and specific interactions for a dynamically interacting system and provides an understanding of protein-protein interactions for BLG at low salt concentrations.

Fungal Cell Factories for Efficient and Sustainable Production of Proteins and Peptides

Filamentous fungi are a large and diverse taxonomically group of microorganisms found in all habitats worldwide. They grow as a network of cells called hyphae. Since filamentous fungi live in very diverse habitats, they produce different enzymes to degrade material for their living, for example hydrolytic enzymes to degrade various kinds of biomasses. Moreover, they produce defense proteins (antimicrobial peptides) and proteins for attaching surfaces (hydrophobins). Many of them are easy to cultivate in different known setups (submerged fermentation and solid-state fermentation) and their secretion of proteins and enzymes are often much larger than what is seen from yeast and bacteria. Therefore, filamentous fungi are in many industries the preferred production hosts of different proteins and enzymes. Edible fungi have traditionally been used as food, such as mushrooms or in fermented foods. New trends are to use edible fungi to produce myco-protein enriched foods. This review gives an overview of the different kinds of proteins, enzymes, and peptides produced by the most well-known fungi used as cell factories for different purposes and applications. Moreover, we describe some of the challenges that are important to consider when filamentous fungi are optimized as efficient cell factories.

Can recombinant milk proteins replace those produced by animals?

The consumption of animal proteins in general, and dairy proteins in particular, is associated with sustainability and animal welfare issues. Recombinant synthesis of milk proteins is therefore receiving increasing interest, with several studies showing synthesis of milk proteins using a wide range of expression systems. Achieving a high yield and purity is essential for economic production. Besides the synthesis, also the construction of the specific structure in which milk proteins are present in animal milks, casein micelles, is needed. Looking at the current state-of-the-art, the steps to produce recombinant dairy products are technically feasible, but whether it can be implemented at low cost, with the process being environmentally friendly, remains to be seen in the coming years.

Site Correlations, Capacitance, and Polarizability From Protein Protonation Fluctuations

We generalize the Kirkwood-Shumaker theory of protonation fluctuation for an anisotropic distribution of dissociable charges on a globular protein. The fluctuations of the total charge and the total dipole moment, in contrast to their average values, depend on the same proton occupancy correlator, thus exhibiting a similar dependence also on the solution pH. This has important consequences for the Kirkwood-Shumaker interaction and its dependence on the bathing solution conditions.

Structure and adsorption behavior of high hydrostatic pressure-treated β-lactoglobulin

HYPOTHESIS

High hydrostatic pressure treatment causes structural changes in interfacial-active β-lactoglobulin (β-lg). We hypothesized that the pressure-induced structural changes affect the intra- and intermolecular interactions which determine the interfacial activity of β-lg. The conducted experimental and numerical investigations could contribute to the mechanistic understanding of the adsorption behavior of proteins in food-related emulsions.

EXPERIMENTS

We treated β-lg in water at pH 7 with high hydrostatic pressures up to 600 MPa for 10 min at 20 °C. The secondary structure was characterized with Fourier-transform infrared spectroscopy (FTIR) and circular dichroism (CD), the surface hydrophobicity and charge with fluorescence-spectroscopy and ζ-potential, and the quaternary structure with membrane-osmometry, analytical ultracentrifugation (AUC) and mass spectrometry (MS). Experimental analyses were supported through molecular dynamic (MD) simulations. The adsorption behavior was investigated with pendant drop analysis.

FINDINGS

MD simulation revealed a pressure-induced molten globule state of β-lg, confirmed by an unfolding of β-sheets with FTIR, a stabilization of α-helices with CD and loss in tertiary structure induced by an increase in surface hydrophobicity. Membrane-osmometry, AUC and MS indicated the formation of non-covalently linked dimers that migrated slower through the water phase, adsorbed more quickly due to hydrophobic interactions with the oil, and lowered the interfacial tension more strongly than reference β-lg.

Isoelectric Point of Proteins at Hydrophobic Interfaces

Structural and colloidal stability of proteins at different surfaces and interfaces is of great importance in many fields including medical, pharmaceutical, or material science. Due to their flexibility, proteins tend to respond to their environmental conditions and can undergo structural and conformational changes. For instance, alterations in physiological factors such as temperature, ions concentration, or pH as well as the adsorption to an interface can initiate protein aggregation. Therefore, at different surfaces and interfaces the characterization of the structural and colloidal stability of proteins, which is mainly influenced by their electrostatic and hydrophobic interactions, is of fundamental importance. In this study, we utilized sum frequency generation (SFG) spectroscopy to assess the role of solution pH on the polarity and magnitude of the electric field within the hydration shell of selected model proteins adsorbed to a hydrophobic surface. We used polystyrene (PS) as a model hydrophobic surface and determined the isoelectric point (IEP) of four structurally different model proteins. Comparing the measured IEP of proteins at the PS/solution or air/solution interface with that determined in the bulk solution via zeta potential measurement, we found significant similarities between the IEP of surface adsorbed proteins and those in the bulk aqueous phase. The pH dependence behavior of proteins was correlated to their amino acid composition and degree of hydrophobicity.

Functionality of Ingredients and Additives in Plant-Based Meat Analogues

Meat analogue research and development focuses on the production of sustainable products that recreate conventional meat in its physical sensations (texture, appearance, taste, etc.) and nutritional aspects. Minced products, like burger patties and nuggets, muscle-type products, like chicken or steak-like cuts, and emulsion products, like Frankfurter and Mortadella type sausages, are the major categories of meat analogues. In this review, we discuss key ingredients for the production of these novel products, with special focus on protein sources, and underline the importance of ingredient functionality. Our observation is that structuring processes are optimized based on ingredients that were not originally designed for meat analogues applications. Therefore, mixing and blending different plant materials to obtain superior functionality is for now the common practice. We observed though that an alternative approach towards the use of ingredients such as flours, is gaining more interest. The emphasis, in this case, is on functionality towards use in meat analogues, rather than classical functionality such as purity and solubility. Another trend is the exploration of novel protein sources such as seaweed, algae and proteins produced via fermentation (cellular agriculture).

Alternatives to Meat and Dairy

The increasing size and affluence of the global population have led to a rising demand for high-protein foods such as dairy and meat. Because it will be impossible to supply sufficient protein to everyone solely with dairy and meat, we need to transition at least part of our diets toward protein foods that are more sustainable to produce. The best way to convince consumers to make this transition is to offer products that easily fit into their current habits and diets by mimicking the original foods. This review focuses on methods of creating an internal microstructure close to that of the animal-based originals. One can directly employ plant products, use intermediates such as cell factories, or grow cultured meat by using nutrients of plant origin. We discuss methods of creating high-quality alternatives to meat and dairy foods, describe their relative merits, and provide an outlook toward the future.