Mechanistic insight of photo-induced aggregation of chicken egg white lysozyme: the interplay between hydrophobic interactions and formation of intermolecular disulfide bonds

Protein nanoparticles as drug delivery systems for cancer theranostics

Protein-based nanoparticles have garnered significant attention in theranostic applications due to their superior biocompatibility, exceptional biodegradability and ease of functionality. Compared to other nanocarriers, protein-based nanoparticles offer additional advantages, including biofunctionality and precise molecular recognition abilities, which make them highly effective in navigating complex biological environments. Moreover, proteins can serve as powerful tools with self-assembling structures and reagents that enhance cell penetration. And their derivation from abundant renewable sources and ability to degrade into harmless amino acids further enhance their suitability for biomedical applications. However, protein-based nanoparticles have so far not realized their full potential. In this review, we summarize recent advances in the use of protein nanoparticles in tumor diagnosis and treatment and outline typical methods for preparing protein nanoparticles. The review of protein nanoparticles may provide useful new insights into the development of biomaterial fabrication.

Ester Bonds for Modulation of the Mechanical Properties of Protein Hydrogels

Hydrogels are soft materials constructed of physically or chemically crosslinked polymeric net-works with abundant water. The crosslinkers, as the mechanophores that bear and respond to mechanical forces, play a critical role in determining the mechanical properties of hydrogels. Here, we use a polyprotein as the crosslinker and mechanophore to form covalent polymer hydrogels in which the toughness and fatigue fracture are controlled by the mechanical unfolding of polyproteins. The protein Parvimonas sp. (ParV) is super stable and remains folded even at forces > 2 nN; however, it can unfold under loading forces of ~100 pN at basic pH values or low calcium concentrations due to destabilization of the protein structures. Through tuning the protein unfolding by pH and calcium concentrations, the hydrogel exhibits differences in modulus, strength, and anti-fatigue fracture. We found that due to the partially unfolding of ParV, the Young's modulus decreased at pH 9.0 or in the presence of EDTA (Ethylene Diamine Tetraacetic Acid), moreover, because partially unfolded ParV can be further completely unfolded due to the mechanically rupture of ester bond, leading to the observed hysteresis of the stretching and relaxation traces of the hydrogels, which is in line with single-molecule force spectroscopy experiments. These results display a new avenue for designing pH- or calcium-responsive hydrogels based on proteins and demonstrate the relationship between the mechanical properties of single molecules and macroscopic hydrogel networks.

A dual responsive robust human serum albumin-based nanocarrier for doxorubicin

Targeted drug therapy against cancer has been introduced as a smart strategy to combat the unwanted side effects due to systemic administration of chemotherapeutics. A human serum albumin (HSA)-based nanocarrier was fabricated with the aim to target reductive media and acidic pH of the tumor tissues. α-Lipoic acid (LA) was applied to increase the number of disulfide bonds in the nanocarrier to target higher glutathione concentrations present in tumor tissues and polyethylene glycol was used to target the acidic pH of tumors. UV illumination, ethanol desolvation, oxygen bubbling, and a mixture of redox buffers were employed to prepare doxorubicin-loaded HSA-LA nanoparticles. The nanocarrier was supposed to release the loaded doxorubicin in reductive and acidic pH media. Fourier-transform infrared spectroscopy and energy dispersive X-ray analysis indicated successful attachment of LA to HSA. The prepared nanoplatform presented improved doxorubicin loading efficiency and content and successfully released the loaded doxorubicin in the expected conditions. Protein corona study indicated that positively charged plasma proteins with molecular weights of nearly 80 kDa are absorbed to the surface of the nanoparticles. Furthermore, it showed desirable UV and storage stability, which implied its robustness and improved shelf life if applied in nanomedicine.

Evaluation of key aroma compounds and protein secondary structure in the roasted Tan mutton during the traditional charcoal process

The traditional charcoal technique was used to determine the changes in the key aroma compounds of Tan mutton during the roasting process. The results showed that the samples at the different roasting time were distinguished using GC-MS in combination with PLS-DA. A total of 26 volatile compounds were identified, among which 14 compounds, including (E)-2-octenal, 1-heptanol, hexanal, 1-hexanol, heptanal, 1-octen-3-ol, 1-pentanol, (E)-2-nonenal, octanal, 2-undecenal, nonanal, pentanal, 2-pentylfuran and 2-methypyrazine, were confirmed as key aroma compounds through the odor activity values (OAV) and aroma recombination experiments. The OAV and contribution rate of the 14 key aroma compounds were maintained at high levels, and nonanal had the highest OAV (322.34) and contribution rate (27.74%) in the samples after roasting for 10 min. The content of α-helix significantly decreased (P < 0.05), while the β-sheet content significantly increased (P < 0.05) during the roasting process. The content of random coils significantly increased in the samples roasted for 0-8 min (P < 0.05), and then no obvious change was observed. At the same time, β-turn content had no obvious change. Correlation analysis showed that the 14 key aroma compounds were all positively correlated with the content of α-helix and negatively correlated with the contents of β-sheet and random coil, and also positively correlated with the content of β-turn, except hexanal and 2-methypyrazine. The results are helpful to promoting the industrialization of roasted Tan mutton.

Exploring the Potentialities of Photoinduced Glycation to Steer Protein Functionalities: The Study Case of Freeze-Dried Egg White Proteins/Carbohydrates Mixtures

The capacity of UV-C light to induce glycation and modify functional properties of systems containing freeze-dried egg white proteins and carbohydrates with increasing molecular weight (i.e., glucose, maltose, trehalose and maltodextrin) was studied. Color changes induced by light exposure were taken as typical indicators of glycation. Samples were then analyzed for selected physical (critical concentration, particle size and viscosity), chemical (ovalbumin content) and technofunctional properties (gelling temperature and foaming capacity). The presence of sugars during exposure to UV-C light promoted intense browning and decreased ovalbumin content by circa 30%. Concomitantly, up to a 3-fold increase in critical concentration of the aqueous suspensions of the irradiated protein-carbohydrate powders and changes in particle size were detected. These modifications were consistent with the development of non-enzymatic browning reactions upon UV-C light irradiation. Photoinduced glycation was associated to a decrease in viscosity, a tendency to form gel at temperature lower by up to 8 °C and a better capacity of foam stabilization. The intensity of these changes seems to be affected by the nature of the carbohydrates reacting with proteins, with longer carbohydrates able to produce systems with higher foam stability capacity.

Stretchable hydrogels with low hysteresis and anti-fatigue fracture based on polyprotein cross-linkers

Hydrogel-based devices are widely used as flexible electronics, biosensors, soft robots, and intelligent human-machine interfaces. In these applications, high stretchability, low hysteresis, and anti-fatigue fracture are essential but can be rarely met in the same hydrogels simultaneously. Here, we demonstrate a hydrogel design using tandem-repeat proteins as the cross-linkers and random coiled polymers as the percolating network. Such a design allows the polyprotein cross-linkers only to experience considerable forces at the fracture zone and unfold to prevent crack propagation. Thus, we are able to decouple the hysteresis-toughness correlation and create hydrogels of high stretchability (~1100%), low hysteresis (< 5%), and high fracture toughness (~900 J m-2). Moreover, the hydrogels show a high fatigue threshold of ~126 J m-2 and can undergo 5000 load-unload cycles up to 500% strain without noticeable mechanical changes. Our study provides a general route to decouple network elasticity and local mechanical response in synthetic hydrogels.

Direct Ultraviolet Laser-Induced Reduction of Disulfide Bonds in Insulin and Vasopressin

Ultraviolet (UV) light has been shown to induce reduction of disulfide bonds in proteins in solution. The photoreduction is proposed to be a result of electron donation from excited Tyr or Trp residues. In this work, a powerful UV femtosecond laser was used to generate photoreduced products, while the hypothesis of Tyr/Trp mediation was studied with spectroscopy and mass spectrometry. With limited irradiation times of 3 min or less at 280 nm, the laser-induced reduction in arginine vasopressin and human insulin led to significant yields of ∼3% stable reduced product. The photogenerated thiols required acidic pH for stabilization, while neutral pH primarily caused scrambling and trisulfide formation. Interestingly, there was no direct evidence that Tyr/Trp mediation was a required criterion for the photoreduction of disulfide bonds. Intermolecular electron transfer remained a possibility for insulin but was ruled out for vasopressin. We propose that an additional mechanism should be increasingly considered in UV light-induced reduction of disulfide bonds in solution, in which a single UV photon is directly absorbed by the disulfide bond.

Characterization of Ultraviolet Photoreactions in Therapeutic Peptides by Femtosecond Laser Catalysis and Mass Spectrometry

Peptides and proteins have diverse ultraviolet (UV) photoreaction pathways that can be activated by the energy of the UV photons absorbed. Simple light sources such as lamps are conventionally used to study these photoreactions in solution. This work provides a proof of concept that femtosecond laser technology can function as a highly potent UV source in rapidly conducting UV photostability studies of peptides. Correspondingly, sufficient quantities of photoproducts were generated in 1 min or less, allowing for identification of known and new photomodifications in the therapeutic peptides somatostatin-14 and arginine vasopressin. Identical photoproducts were also generated with a conventional continuous source. The major modifications included N-formylkynurenine, a cross-link between Trp and Phe, a Tyr product with an NH3 loss, and disruption of an unstable disulfide bond into a complex mixture of a trisulfide bond and multiple scrambled dimeric products. In conclusion, femtosecond lasers are extremely useful to drive fast UV-induced reactions for high throughput screening of photostability and modifications in amino acid polymers.

Sn-Triggered Two-Dimensional Fast Protein Assembly with Emergent Functions

The discovery of a general strategy for organizing functional proteins into stable nanostructures with the desired dimension, shape, and function is an important focus in developing protein-based self-assembled materials, but the scalable synthesis of such materials and transfer to other substrates remain great challenges. We herein tackle this issue by creating a two-dimensional metal-protein hybrid nanofilm that is flexible and cost-effective with reliable self-recovery, stability, and multifunctionality. As it differs from traditional metal ions, we discover the capability of Sn²⁺ to initiate fast amyloid-like protein assembly (occurring in seconds) by effectively reducing the disulfide bonds of native globular proteins. The Sn²⁺-initiated lysozyme aggregation at the air/water interface leads to droplet flattening, a result never before reported in a protein system, which finally affords a multifunctional 2D Sn-doped hybrid lysozyme nanofilm with an ultralarge area (e.g., 0.2 m²) within a few minutes. The hybrid film is distinctive in its ease of coating on versatile material surfaces with endurable chemical and mechanical stability, optical transparency, and diverse end uses in antimicrobial and photo-/electrocatalytic scaffolds. Our approach provides not only insights into the effect of tin ions on macroscopic self-assembly of proteins but also a controllable and scalable synthesis of a potential biomimic framework for biomedical and biocatalytic applications.

A photo-inducible protein-inorganic nanoparticle assembly for active targeted tumour theranostics

The assembly of protein-inorganic nanoparticles is an important yet challenging approach that is utilized to develop functional materials in numerous areas, such as bio-catalysis, drug delivery, and biosensing. In this study, we report on a facile, photo-inducible self-assembly method to generate protein-inorganic hybrid nanoplatforms. More specifically, photo-treated disulfide bond rich proteins of lysozyme (LYS) were able to be used as host materials in order to encapsulate nanoparticles (i.e., as-synthesized hydrophobic NIR quantum dots (QDs)) and anti-cancer small molecule drugs (i.e., paclitaxel (PTX)), constructing functional theranostic protein-inorganic hybrid nanoparticles. The modification of the functional polymer of cRGD-PEG contributes to the active tumour targeting characteristic of this protein-inorganic nanocarrier. This novel PTX loaded protein-inorganic hybrid nanoplatform showed high tumour homing accumulation as well as effective tumour inhibition. We believe that this general approach represents a new direction for the development of a photo-induced assembly of protein-inorganic nanoparticles towards versatile applications in both materials science and biomedical fields.

An Environmentally Benign Antimicrobial Coating Based on a Protein Supramolecular Assembly

The use of antimicrobial materials, for example, silver nanoparticles, has been a cause for concern because they often exert an adverse effect on environmental and safety during their preparation and use. In this study, we report a class of green antimicrobial coating based on a supramolecular assembly of a protein extracted from daily food, without the addition of any other hazardous agents. It is found that a self-assembled nanofilm by mere hen egg white lysozyme has durable in vitro and in vivo broad-spectrum antimicrobial efficacy against Gram-positive/negative and fungi. Such enhanced antimicrobial capability over native lysozyme is attributed to a synergistic combination of positive charge and hydrophobic amino acid residues enriched on polymeric aggregates in the lysozyme nanofilm. Accompanied with high antimicrobial activity, this protein-based PTL material simultaneously exhibits the integration of multiple functions including antifouling, antibiofilm, blood compatibility, and low cytotoxicity due to the existence of surface hydration effect. Moreover, the bioinspired adhesion mediated by the amyloid structure contained in the nanofilm induces robust transfer and self-adhesion of the material onto virtually arbitrary substrates by a simple one-step aqueous coating or solvent-free printing in 1 min, thereby allowing an ultrafast route into practical implications for surface-functionalized commodity and biomedical devices. Our results demonstrate that the application of pure proteinaceous substance may afford a cost-effective green biomaterial that has high antimicrobial activity and low environmental impact.

Overcoming T. gondii infection and intracellular protein nanocapsules as biomaterials for ultrasonically controlled drug release

One of the pivotal matters of concern in intracellular drug delivery is the preparation of biomaterials containing drugs that are compatible with the host target.

2D Protein Supramolecular Nanofilm with Exceptionally Large Area and Emergent Functions

2D nanofilms assembled by pure protein with a macroscopic area and multiple functions can be directly formed at the air/water interface or at the solid surface at a timescale of several minutes. The multifunctionality of the nanofilm coating is demonstrated by both top-down and bottom-up micro-/nanoscale interfacial engineering, including surface modification, all-water-based photo/electron-beam lithography, and electroless deposition.

Direct measurement of the tryptophan-mediated photocleavage kinetics of a protein disulfide bond

Disulfide cleavage is one of the major causes underlying ultraviolet (UV) light-induced protein damage. While previous studies have provided strong evidence to support the notion that this process is mediated by photo-induced electron transfer from the excited state of an aromatic residue (e.g., tryptophan) to the disulfide bond, many mechanistic details are still lacking. For example, we do not know how quickly this process occurs in a protein environment. Herein, we design an experiment, which uses the unfolding kinetics of a protein as an observable, to directly assess the kinetics and mechanism of photo-induced disulfide cleavage. Our results show that this disulfide bond cleavage event takes place in ∼2 μs via a mechanism involving electron transfer from the triplet state of a tryptophan (Trp) residue to the disulfide bond. Furthermore, we find that one of the photoproducts of this reaction, a Trp-SR adduct, is formed locally, thus preventing the protein from re-cross-linking. Taken together, these findings suggest that a Trp-disulfide pair could be used as a photo-trigger to initiate protein folding dynamics and control the biological activities of disulfide-containing peptides.

A Superhydrophobic Surface Templated by Protein Self-Assembly and Emerging Application toward Protein Crystallization

A proteinaceous superhydrophobic material for facile protein crystallization is reported. The lysozyme phase transition is rationally manipulated to form a reliable superhydrophobic coating on virtually arbitrary material surfaces with good thermostability and mechanical robustness. Such a surface exhibits a fascinating capability to drive protein crystallization, and the protein crystal array can be facilitated in a large area at an ultralow protein concentration.

Lysozyme Photochemistry as a Function of Temperature. The Protective Effect of Nanoparticles on Lysozyme Photostability

The presence of aromatic residues and their close spatial proximity to disulphide bridges makes hen egg white lysozyme labile to UV excitation. UVB induced photo-oxidation of tryptophan and tyrosine residues leads to photochemical products, such as, kynurenine, N-formylkynurenine and dityrosine and to the disruption of disulphide bridges in proteins. We here report that lysozyme UV induced photochemistry is modulated by temperature, excitation power, illumination time, excitation wavelength and by the presence of plasmonic quencher surfaces, such as gold, and by the presence of natural fluorescence quenchers, such as hyaluronic acid and oleic acid. We show evidence that the photo-oxidation effects triggered by 295 nm at 20°C are reversible and non-reversible at 10°C, 25°C and 30°C. This paper provides evidence that the 295 nm damage threshold of lysozyme lies between 0.1 μW and 0.3 μW. Protein conformational changes induced by temperature and UV light have been detected upon monitoring changes in the fluorescence emission spectra of lysozyme tryptophan residues and SYPRO® Orange. Lysozyme has been conjugated onto gold nanoparticles, coated with hyaluronic acid and oleic acid (HAOA). Steady state and time resolved fluorescence studies of free and conjugated lysozyme onto HAOA gold nanoparticles reveals that the presence of the polymer decreased the rate of the observed photochemical reactions and induced a preference for short fluorescence decay lifetimes. Size and surface charge of the HAOA gold nanoparticles have been determined by dynamic light scattering and zeta potential measurements. TEM analysis of the particles confirms the presence of a gold core surrounded by a HAOA matrix. We conclude that HAOA gold nanoparticles may efficiently protect lysozyme from the photochemical effects of UVB light and this nanocarrier could be potentially applied to other proteins with clinical relevance. In addition, this study confirms that the temperature plays a critical role in the photochemical pathways a protein enters upon UV excitation.

Protein folding guides disulfide bond formation

The Anfinsen principle that the protein sequence uniquely determines its structure is based on experiments on oxidative refolding of a protein with disulfide bonds. The problem of how protein folding drives disulfide bond formation is poorly understood. Here, we have solved this long-standing problem by creating a general method for implementing the chemistry of disulfide bond formation and rupture in coarse-grained molecular simulations. As a case study, we investigate the oxidative folding of bovine pancreatic trypsin inhibitor (BPTI). After confirming the experimental findings that the multiple routes to the folded state contain a network of states dominated by native disulfides, we show that the entropically unfavorable native single disulfide [14-38] between Cys14 and Cys38 forms only after polypeptide chain collapse and complete structuring of the central core of the protein containing an antiparallel β-sheet. Subsequent assembly, resulting in native two-disulfide bonds and the folded state, involves substantial unfolding of the protein and transient population of nonnative structures. The rate of [14-38] formation increases as the β-sheet stability increases. The flux to the native state, through a network of kinetically connected native-like intermediates, changes dramatically by altering the redox conditions. Disulfide bond formation between Cys residues not present in the native state are relevant only on the time scale of collapse of BPTI. The finding that formation of specific collapsed native-like structures guides efficient folding is applicable to a broad class of single-domain proteins, including enzyme-catalyzed disulfide proteins.

Albumin antioxidant response to stress in diabetic nephropathy progression

Background

A new component of the protein antioxidant capacity, designated Response Surplus (RS), was recently described. A major feature of this component is the close relationship between protein antioxidant capacity and molecular structure. Oxidative stress is associated with renal dysfunction in patients with renal failure, and plasma albumin is the target of massive oxidation in nephrotic syndrome and diabetic nephropathy. The aim of the present study was to explore the albumin redox state and the RS component of human albumin isolated from diabetic patients with progressive renal damage.

Methods/Principal Findings

Serum aliquots were collected and albumin isolated from 125 diabetic patients divided into 5 groups according to their estimated glomerular filtration rate (GFR). In addition to clinical and biochemical variables, the albumin redox state, including antioxidant capacity, thiol group content, and RS component, were evaluated. The albumin antioxidant capacity and thiol group content were reciprocally related to the RS component in association with GFR reduction. The GFR decline and RS component were significantly negatively correlated (R = –0.83, p<0.0001). Age, creatinine, thiol groups, and antioxidant capacity were also significantly related to the GFR decline (R = –0.47, p<0.001; R = –0.68, p<0.0001; R = 0.44, p<0.001; and R = 0.72, p<0.0001).

Conclusion/Significance

The response of human albumin to stress in relation to the progression of diabetic renal disease was evaluated. The findings confirm that the albumin molecular structure is closely related to its redox state, and is a key factor in the progression of diabetes nephropathy.

Preventing disulfide bond formation weakens non-covalent forces among lysozyme aggregates

Nonnative disulfide bonds have been observed among protein aggregates in several diseases like amyotrophic lateral sclerosis, cataract and so on. The molecular mechanism by which formation of such bonds promotes protein aggregation is poorly understood. Here in this work we employ previously well characterized aggregation of hen eggwhite lysozyme (HEWL) at alkaline pH to dissect the molecular role of nonnative disulfide bonds on growth of HEWL aggregates. We employed time-resolved fluorescence anisotropy, atomic force microscopy and single-molecule force spectroscopy to quantify the size, morphology and non-covalent interaction forces among the aggregates, respectively. These measurements were performed under conditions when disulfide bond formation was allowed (control) and alternatively when it was prevented by alkylation of free thiols using iodoacetamide. Blocking disulfide bond formation affected growth but not growth kinetics of aggregates which were ∼50% reduced in volume, flatter in vertical dimension and non-fibrillar in comparison to control. Interestingly, single-molecule force spectroscopy data revealed that preventing disulfide bond formation weakened the non-covalent interaction forces among monomers in the aggregate by at least ten fold, thereby stalling their growth and yielding smaller aggregates in comparison to control. We conclude that while constrained protein chain dynamics in correctly disulfide bonded amyloidogenic proteins may protect them from venturing into partial folded conformations that can trigger entry into aggregation pathways, aberrant disulfide bonds in non-amyloidogenic proteins (like HEWL) on the other hand, may strengthen non-covalent intermolecular forces among monomers and promote their aggregation.

Protein-directed approaches to functional nanomaterials: a case study of lysozyme

Using lysozyme as a model, protein-directed approaches to functional nanomaterials were reviewed, making rational materials design possible in the future.

In vitro hyperglycemic condition facilitated the aggregation of lysozyme via the passage through a molten globule state

Hyperglycemic condition i.e. an increase in blood glucose concentration has been linked to bring about structural alterations in the native state of proteins. Glucose concentrations of 50 and 100 mM in vitro, which correspond to hyperglycemic condition, were tested to investigate their effect on lysozyme native structure. Incubating enzyme with 50 and 100 mM glucose for a period of 7 days, an intermediate state on day 4 and 3 was observed, respectively. The presence of intermediate state was characterized by a 22 % increase in the intrinsic fluorescence intensity with a red shift of 20 nm compared to the native state, a 5 % increase in ANS-fluorescence intensity relative to the native due to the surfacing of hydrophobic clusters and a sharp decrease in near-UV CD signal at around 284 and 291 nm. The state retains substantial native-like secondary structure. This partially unfolded intermediate state can be referred as 'molten globule', which finally tends to aggregate on day 6 and 4 with 50 and 100 mM glucose concentration, respectively, as a result of cross-linking between lysozyme molecules. The aggregates were confirmed by the presence of β-sheet structure as depicted by far-UV CD, an increase in ThT fluorescence as well as the fibrillar morphology shown by SEM. Moreover, advanced glycation end products were also accompanied as the emission peak was observed at 460 and 470 nm corresponding to the formation of pentosidine and malonaldehyde, respectively.

Amyloid fiber formation in human γD-Crystallin induced by UV-B photodamage

γD-Crystallin is an abundant structural protein of the lens that is found in native and modified forms in cataractous aggregates. We establish that UV-B irradiation of γD-Crystallin leads to structurally specific modifications and precipitation via two mechanisms: amorphous aggregates and amyloid fibers. UV-B radiation causes cleavage of the backbone, in large measure near the interdomain interface, where side chain oxidations are also concentrated. 2D IR spectroscopy and expressed protein ligation localize fiber formation exclusively to the C-terminal domain of γD-Crystallin. The native β-sandwich domains are not retained upon precipitation by either mechanism. The similarities between the amyloid forming pathways when induced by either UV-B radiation or low pH suggest that the propensity for the C-terminal β-sandwich domain to form amyloid β-sheets determines the misfolding pathway independent of the mechanism of denaturation.

One-step photo synthesis of protein-drug nanoassemblies for drug delivery

Upon controlled UV illumination, disulfide bonds in bovine α-lactalbumin (BLA) are selectively broken, leading to self-assembly of the BLA and doxorubicin (DOX) molecules into nanoparticles via hydrophobic interactions and intermolecular disulfide bonds. Such protein-drug nanoparticles have synergistic anticancer activity in vitro and tumor-homing specificity in vivo, which are of great potential for systemic drug delivery in cancer therapy.