Specific covalent immobilization of proteins through dityrosine cross-links

Tyrosine-Based Dual-Functional Interface for Trapping and On-Site Photo-Induced Covalent Immobilization of Proteins

Tyrosine, a simple and well-available natural amino acid, is featured by the small size of the compound that contains multiple reactive groups. This study developed an efficient bioconjugation strategy using tyrosine-based dual-functional interfaces. When tyrosine molecules are immobilized on the surface of a supporting material through amino groups, their carboxyl groups can function as an attracting trap due to their anionic nature at neutral pH and ability to chelate nickel(II) ions (Ni²⁺), allowing the capture and enrichment of cationic proteins and histidine (His)-tagged proteins on the surface. The trapped proteins can be further covalently immobilized on site through ruthenium-mediated photochemical cross-linking, which has been found to be highly efficient and can be completed within minutes. This strategy was successfully applied to two different material systems. We found that tyrosine-modified agarose beads had a binding capacity of the His-tagged enhanced green fluorescent protein comparable to that of commonly used nitrilotriacetic acid-based resins, and further covalent coupling via dityrosine cross-linking achieved a yield of 85% within 5 min, without compromising much on its fluorescence activity. On the surface of tyrosine-modified 316L stainless steel, lysozyme was captured through electrostatic interaction and further immobilized. The resultant surface exhibited remarkable antibacterial activity against both Staphylococcus aureus and Escherichia coli. Such a tyrosine-based capture-then-coupling method is featured by its simplicity, high coupling efficiency, and high utilization rate of target molecules, making it particularly suitable for the proteins that are highly priced or vulnerable to general immobilization chemistry.

Tyrosine-Based Cross-Linking of Peptide Antigens to Generate Nanoclusters with Enhanced Immunogenicity: Demonstration Using the Conserved M2e Peptide of Influenza A

A method of creating nanoclusters (NCs) from soluble peptide molecules is described utilizing an approach based on a tyrosine-tyrosine cross-linking reaction. A reactive tag comprising histidine and tyrosine residues was introduced at the termini of the peptide molecules. The cross-linking reaction led to the creation of dityrosine bonds within the tag, which allowed for the generation of peptide NCs. We show that it is essential for the reactive tag to be present at both the "N" and "C" termini of the peptide for cluster formation to occur. Additionally, the cross-linking reaction was systematically characterized to show the importance of reaction conditions on final cluster diameter, allowing us to generate NCs of various sizes. To demonstrate the immunogenic potential of the peptide clusters, we chose to study the conserved influenza peptide, M2e, as the antigen. M2e NCs were formulated using the cross-linking reaction. We show the ability of the clusters to generate protective immunity in a dose, size, and frequency dependent manner against a lethal influenza A challenge in BALB/c mice. Taken together, the data presented suggest this new cluster formation technique can generate highly immunogenic peptide NCs in a simple and controllable manner.

High-Strength Albumin Hydrogels With Hybrid Cross-Linking

Natural protein-based hydrogels possess excellent biocompatibility; however, most of them are weak or brittle. In the present work, high strength hybrid dual-crosslinking BSA gels (BSA DC gels), which have both chemical cross-linking and physical cross-linking, were fabricated by a facile photoreaction-heating process. BSA DC gels showed high transparency (light transmittance of ~90%) and high strength. At optimal conditions, BSA DC gel exhibited high compressive strength (σc,f) of 37.81 ± 2.61 MPa and tensile strength (σt,f) of 0.62 ± 0.078 MPa, showing it to be much stronger than physically cross-linked BSA gel (BSA PC gel) and chemically cross-linked BSA gel (BSA CC gel). More importantly, BSA DC gel displayed non-swelling properties while maintaining high strength in DI water, pH = 3.0, and pH = 10.0. Moreover, BSA DC gel also demonstrated large hysteresis, rapid self-recovery, and excellent fatigue resistance properties. It is believed that our BSA DC gel can potentially be applied in biomedical fields.

Study on the influence of oxidative stress on the fibrillization of fibrinogen

Human fibrinogen is an important coagulation factor as well as an independent predictor of coronary heart disease and stroke. Analysis of dysfibrinogens may provide useful information and help us to understand the molecular defects in fibrin polymerization. In the present study, we investigated the influence of oxidative stress of fibrinogen induced by H2O2 on the polymerization state of fibrin. UV absorbance spectroscopy, circular dichroism, ζ-potential, dynamic light scattering and steady shear viscosity were all employed to study the influence of oxidative stress on the molecular structure, the surface charges, and the size and shape of fibrinogen molecules. The fibrin morphology obtained was imaged and investigated using atomic force microscopy. The results demonstrated that the cross-linking, branching and height distribution of formed fibrin will be influenced by the oxidative stress of fibrinogen. This study presents new insights into the aggregation behaviour of fibrinogen and will be helpful to understand the formation mechanism of thrombosis under oxidative stress.

Radiation-synthesized protein-based drug carriers: Size-controlled BSA nanoparticles

Nanotechnology has broadened the options for the delivery of agents of biotechnological and clinical relevance. Currently, attention has been driven towards the development of protein-based nanocarriers due to high the biocompatibility and site-specific delivery. In this work we report radiation-synthesized bovine serum albumin nanoparticles as an attempt to overcome limitations of available albumin particles, as a novel route for the development of crosslinked protein nanocarriers for the administration of chemotherapic agents or radiopharmaceuticals. Albumin containing phosphate buffer solutions were irradiated using γ-irradiation at distinct cosolvent concentrations-ethanol or methanol. Nanoparticle size was followed by DLS and bityrosine crosslinking formation using fluorescence measurements and SDS-PAGE. In addition, computational experiments were performed to elucidate the mechanism and pathways for the nanoparticle formation. The synthesis of BSA nanoparticles using γ-irradiation in the presence of a cosolvent allowed the formation of the nanoparticles from 7 to 70 nm without the use of any chemical crosslinker as confirmed by SDS-PAGE and DLS analysis. The combination of cosolvent and γ-irradiation allowed a fine tuning with regard to protein size.

Designed Modular Proteins as Scaffolds To Stabilize Fluorescent Nanoclusters

Proteins have been used as templates to stabilize fluorescent metal nanoclusters thus obtaining stable fluorescent structures, and their fluorescent properties being modulated by the type of protein employed. Designed consensus tetratricopeptide repeat (CTPR) proteins are suited candidates as templates for the stabilization of metal nanoclusters due to their modular structural and functional properties. Here, we have studied the ability of CTPR proteins to stabilize fluorescent gold nanoclusters giving rise to designed functional hybrid nanostructures. First, we have investigated the influence of the number of CTPR units, as well as the presence of cysteine residues in the CTPR protein, on the fluorescent properties of the protein-stabilized gold nanoclusters. Synthetic protocols to retain the protein structure and function have been developed, since the structural and functional integrity of the protein template is critical for further applications. Finally, as a proof-of-concept, a CTPR module with specific binding capabilities has been used to stabilize gold nanoclusters with positive results. Remarkably, the protein-stabilized gold nanocluster obtained combines both the fluorescence properties of the nanoclusters and the functional properties of the protein. The fluorescence changes in nanoclusters fluorescence have been successfully used as a sensor to detect when the specific ligand was recognized by the CTPR module.

Identification of multiple dityrosine bonds in materials composed of the Drosophila protein Ultrabithorax

The recombinant protein Ultrabithorax (Ubx), a Drosophila melanogaster Hox transcription factor, self-assembles into biocompatible materials in vitro that are remarkably extensible and strong. Here, we demonstrate that the strength of Ubx materials is due to intermolecular dityrosine bonds. Ubx materials auto-fluoresce blue, a characteristic of dityrosine, and bind dityrosine-specific antibodies. Monitoring the fluorescence of reduced Ubx fibers upon oxygen exposure reveals biphasic bond formation kinetics. Two dityrosine bonds in Ubx were identified by site-directed mutagenesis followed by measurements of fiber fluorescent intensity. One bond is located between the N-terminus and the homeodomain (Y4/Y296 or Y12/Y293), and another bond is formed by Y167 and Y240. Fiber fluorescence closely correlates with fiber strength, demonstrating that these bonds are intermolecular. To our knowledge, this is the first identification of specific residues that participate in dityrosine bonds in protein-based materials. The percentage of Ubx molecules harboring both bonds can be decreased or increased by mutagenesis, providing an additional mechanism to control the mechanical properties of Ubx materials. Duplication of tyrosine-containing motifs in Ubx increases dityrosine content in Ubx fibers, suggesting these motifs could be inserted in other self-assembling proteins to strengthen the corresponding materials.

Robust, self-healing hydrogels synthesised from catechol rich polymers

Catechol rich polymers yield robust, self-healing hydrogels.

Phenothiazine-modified electrodes: a useful platform for protein adsorption study

Using glucose oxidase (GOx) as a target protein, we studied the adsorption of protein on the phenothiazine-modified electrodes and assessed the potential of using the electrodes in biochemical applications. Experiment results showed that thionine chloride (TC) and its structural analogues, such as toluidine blue and methylene blue, fluoresced under photochemical excitation after being immobilized on indium-doped tin oxide (ITO) electrodes fabricated using either diazotization-reduction or oxidative polymerization. The surface-bound phenothiazines exhibited substantial binding affinities to the protein. At a pH > 5, the adsorbate showed no sign of desorption even the electrodes were electrically biased with voltages between ±0.3 V vs SCE. Thus, emission decay occurred while GOx was injected over the electrodes, which was consistent with the observations made using conductive-mode atomic force microscopy (CM-AFM). Under a quiescent condition, the protein interacted with the immobilized TC via a pseudo-first-order kinetic mechanism. The reaction reached a maximum rate at a pH > 5, at which the rate constant was approximately 7 × 10(-8) L/(U s). Under this condition, the adsorption rate increased as the level of the protein increased, regardless of pH, revealing application potential for GOx quantitation. The adsorption rate, however, decreased with a decrease in pH if the pH < 5. We concluded that static interactions played a crucial role. By monitoring Fe(CN)6(3-/4-) taking place at the TC-modified electrodes in pH 7 solutions, we observed that the adsorption of GOx imposed impedance on Fe(CN)6(3-/4-). The resulting charge-transfer resistance (RCT) increased as the amount of the protein increased, leading to a conclusion that the protein could reach the maximum surface coverage when its concentrations were greater than 100 U/mL. The protein molecules were likely repel each other as approaching the TC sites. Despite this, they maintained the native bioactivity after being adsorbed, enabling the TC-modified electrodes to function as glucose sensors. Glucose concentrations between 1 and 60 mM could be detected. Long-term analysis, in addition, showed that the electrode responses to the analyte were consistent and reproducible. Phenothiazine-modified electrodes are evident as a useful tool for understanding the adsorption of protein on solid surfaces and quantifying proteins.

Oligomerization interface of RAGE receptor revealed by MS-monitored hydrogen deuterium exchange

Activation of the receptor for advanced glycation end products (RAGE) leads to a chronic proinflammatory signal, affecting patients with a variety of diseases. Potentially beneficial modification of RAGE activity requires understanding the signal transduction mechanism at the molecular level. The ligand binding domain is structurally uncoupled from the cytoplasmic domain, suggesting receptor oligomerization is a requirement for receptor activation. In this study, we used hydrogen-deuterium exchange and mass spectrometry to map structural differences between the monomeric and oligomeric forms of RAGE. Our results indicated the presence of a region shielded from exchange in the oligomeric form of RAGE and led to the identification of a new oligomerization interface localized at the linker region between domains C1 and C2. Based on this finding, a model of a RAGE dimer and higher oligomeric state was constructed.

Enzyme immobilization: an update

Compared to free enzymes in solution, immobilized enzymes are more robust and more resistant to environmental changes. More importantly, the heterogeneity of the immo-bilized enzyme systems allows an easy recovery of both enzymes and products, multiple re-use of enzymes, continuous operation of enzymatic processes, rapid termination of reactions, and greater variety of bioreactor designs. This paper is a review of the recent literatures on enzyme immobilization by various techniques, the need for immobilization and different applications in industry, covering the last two decades. The most recent papers, patents, and reviews on immobilization strategies and application are reviewed.

Catechol Redox Induced Formation of Metal Core-Polymer Shell Nanoparticles

A novel strategy was developed to synthesize polymer-coated metal nanoparticles (NPs) through reduction of metal cations with 3,4-dihydroxyphenylalanine (DOPA)-containing polyethylene glycol (PEG) polymers. Catechol redox chemistry was used to both synthesize metal NPs and simultaneously form a cross-linked shell of PEG polymers on their surfaces. DOPA reduced gold and silver cations into neutral metal atoms, producing reactive quinones that covalently cross-linked the PEG molecules around the surface of the NP. Importantly, these PEG-functionalized metal NPs were stable in physiological ionic strengths and under centrifugation, and hold broad appeal since they absorb and scatter light in aqueous solutions.

Cross-linking and immobilisation of different proteins with recombinant Verrucomicrobium spinosum tyrosinase

This paper reports on the cross-linking and immobilisation of various proteins by the recombinant tyrosinase from Verrucomicrobium spinosum (Vs-tyrosinase). In general it is found that Vs-tyrosinase can readily cross-link proteins with a low degree of complexity, such as casein, but that the enzyme cannot readily cross-link well folded protein substrates such as lysozyme, myoglobin, cytochrome c or Candida antarctica lipase B (CALB). However, the inclusion of phenolic compounds (phenol or caffeic acid) to reaction mixtures of these proteins can greatly enhance the levels of cross-linking. For example it is possible to prepare cross-linked aggregates of industrially applicable enzymes such as CALB by simply incubating it with Vs-tyrosinase and phenol. The resulting aggregates can be collected by centrifugation and retain high levels of activity and may find applications in biocatalysis.

Silk cocoon of Bombyx mori: proteins and posttranslational modifications--heavy phosphorylation and evidence for lysine-mediated cross links

Although silk is used to produce textiles and serves as a valuable biomaterial in medicine, information on silk proteins of the cocoon is limited. Scanning electron microscopy was applied to morphologically characterise the sample and the solubility of cocoon in lithium thiocyanate and 2-DE was carried out with multi-enzyme in-gel digestion followed by MS identification of silk-peptides. High-sequence coverage of the silk cocoon proteins fibroin light and heavy chain, sericins and fibrohexamerins was revealed and PTMs as heavy phosphorylation of silk fibroin heavy chain; lysine hydroxylation and Lys->allysine formation have been observed providing evidence for lysine-mediated cross linking of silk as found in collagens, which has not been reported so far. Tyrosine oxidation verified the presence of di-tyrosine cross links. A high degree of sequence conflicts probably representing single-nucleotide polymorphisms was observed. PTM and sequence conflicts may be modulating structure and physicochemical properties of silk.