Vmh2 hydrophobin as a tool for the development of “self-immobilizing” enzymes for biosensing

Three-Dimensional Electrochemical Sensors for Food Safety Applications

Considering the increasing concern for food safety, electrochemical methods for detecting specific ingredients in the food are currently the most efficient method due to their low cost, fast response signal, high sensitivity, and ease of use. The detection efficiency of electrochemical sensors is determined by the electrode materials' electrochemical characteristics. Among them, three-dimensional (3D) electrodes have unique advantages in electronic transfer, adsorption capacity and exposure of active sites for energy storage, novel materials, and electrochemical sensing. Therefore, this review begins by outlining the benefits and drawbacks of 3D electrodes compared to other materials before going into more detail about how 3D materials are synthesized. Next, different types of 3D electrodes are outlined together with common modification techniques for enhancing electrochemical performance. After this, a demonstration of 3D electrochemical sensors for food safety applications, such as detecting components, additives, emerging pollutants, and bacteria in food, was given. Finally, improvement measures and development directions of electrodes with 3D electrochemical sensors are discussed. We think that this review will help with the creation of new 3D electrodes and offer fresh perspectives on how to achieve extremely sensitive electrochemical detection in the area of food safety.

Electrochemical biosensor based on antibody-modified Au nanoparticles for rapid and sensitive analysis of influenza A virus

To cope with the easy transmissibility of the avian influenza A virus subtype H1N1, a biosensor was developed for rapid and highly sensitive electrochemical immunoassay. Based on the principle of specific binding between antibody and virus molecules, the active molecule-antibody-adapter structure was formed on the surface of an Au NP substrate electrode; it included a highly specific surface area and good electrochemical activity for selective amplification detection of the H1N1 virus. The electrochemical test results showed that the BSA/H1N1 Ab/Glu/Cys/Au NPs/CP electrode was used for the electrochemical detection of the H1N1 virus with a sensitivity of 92.1 µA (pg/mL)^-1 cm², LOD of 0.25 pg/ml, linear ranges of 0.25-5 pg/mL, and linearity of (R ² = 0.9846). A convenient H1N1 antibody-based electrochemical electrode for the molecular detection of the H1N1 virus will be of great use in the field of epidemic prevention and raw poultry protection.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s11581-023-04944-w.

Innovative surface bio-functionalization by fungal hydrophobins and their engineered variants

Research on innovative surface functionalization strategies to develop materials with high added value is particularly challenging since this process is a crucial step in a wide range of fields (i.e., biomedical, biosensing, and food packaging). Up to now, the main applied derivatization methods require hazardous and poorly biocompatible reagents, harsh conditions of temperature and pressure, and are time consuming and cost effective. The discovery of biomolecules able to adhere by non-covalent bonds on several surfaces paves the way for their employment as a replacement of chemical processes. A simple, fast, and environment-friendly method of achieving modification of chemically inert surfaces is offered by hydrophobins, small amphiphilic proteins produced by filamentous fungi. Due to their structural characteristics, they form stable protein layers at interfaces, serving as anchoring points that can strongly bind molecules of interest. In addition, genetic engineering techniques allow the production of hydrophobins fused to a wide spectrum of relevant proteins, providing further benefits in term of time and ease of the process. In fact, it is possible to bio-functionalize materials by simply dip-casting, or by direct deposition, rendering them exploitable, for example, in the development of biomedical and biosensing platforms.

Aspergillus Hydrophobins: Physicochemical Properties, Biochemical Properties, and Functions in Solid Polymer Degradation

Hydrophobins are small amphipathic proteins conserved in filamentous fungi. In this review, the properties and functions of Aspergillus hydrophobins are comprehensively discussed on the basis of recent findings. Multiple Aspergillus hydrophobins have been identified and categorized in conventional class I and two non-conventional classes. Some Aspergillus hydrophobins can be purified in a water phase without organic solvents. Class I hydrophobins of Aspergilli self-assemble to form amphipathic membranes. At the air–liquid interface, RolA of Aspergillus oryzae self-assembles via four stages, and its self-assembled films consist of two layers, a rodlet membrane facing air and rod-like structures facing liquid. The self-assembly depends mainly on hydrophobin conformation and solution pH. Cys4–Cys5 and Cys7–Cys8 loops, disulfide bonds, and conserved Cys residues of RodA-like hydrophobins are necessary for self-assembly at the interface and for adsorption to solid surfaces. AfRodA helps Aspergillus fumigatus to evade recognition by the host immune system. RodA-like hydrophobins recruit cutinases to promote the hydrolysis of aliphatic polyesters. This mechanism appears to be conserved in Aspergillus and other filamentous fungi, and may be beneficial for their growth. Aspergilli produce various small secreted proteins (SSPs) including hydrophobins, hydrophobic surface–binding proteins, and effector proteins. Aspergilli may use a wide variety of SSPs to decompose solid polymers.

A label-free lateral offset spliced coreless fiber MZI biosensor based on hydrophobin HGFI for TNF-α detection

A real-time label-free lateral offset spliced coreless fiber (CF) Mach-Zehnder interferometer (MZI) biosensor functionalized with hydrophobin Grifola frondosa I (HGFI) was proposed for the detection of cytokine tumour necrosis factor alpha (TNF-α). The nanolayer self-assembled on the optical fiber surfaces by HGFI rendered the immobilization of probe TNF-α antibody and recognition of antigen TNF-α. Trifluoroacetic acid was utilized to remove the HGFI layer from the glass surface, which was validated by field emission scanning electron microscopy (FESEM) and water contact angle (WCA). Results demonstrated that the processes of HGFI modification, antibody immobilization and specific antibody detection can be monitored in real time. The proposed biosensor exhibited good specificity, repeatability and low detection limit for TNF-α, extending its application in inflammation and disease monitoring.

A machine learning-enhanced biosensor for mercury detection based on an hydrophobin chimera

Marine waters are becoming contaminated by diverse pollutants at a fast rate, and detection of these water pollutants has become a major concern in recent years. Among these, mercury is considered the most toxic element for human health. At present, despite the commonly used methods for its detection are accurate, they often require sophisticated equipments, have relatively high costs, are demanding and time-consuming. Herein a novel solution to detect mercury (II) pollution in sea water is proposed, and an easy and portable detection method has been developed. Indeed, a hydrophobin based chimera able to both adhere to polystyrene multiwell plates and bind mercury (II) with a consequent fluorescent decrease was designed. The chimera was the recognition element in a fluorescence-based biosensor able to detect mercury (II) in the nM range. Indeed, this biosensor specifically measure Hg²⁺ concentration also in the presence of other metals, reaching a limit of detection of 0.4 nM in tap water and 0.3 nM in sea water. Moreover, the developed biosensor was coupled to machine learning methodologies with the big advantage of predicting mercury concentration levels without the use of classical reader devices, thus allowing in situ monitoring of sea pollution by non-skilled personnel.

Immobilization of Antibodies by Genetic Fusion to a Fungal Self-Assembling Adhesive Protein

Although antibody immobilization on solid surfaces is extensively used in several applications, including immunoassays, biosensors, and affinity chromatography, some issues are still challenging. Self-assembling protein layers can be used to coat easily different surfaces by direct deposition. A specific biofunctional layer can be formed using genetic engineering techniques to express fused proteins acting as self-immobilizing antibodies. In this study, fusion proteins combining the self-assembling adhesive properties of a fungal hydrophobin and the functionality of the single chain fragment variables (ScFvs) of two antibodies were produced. The chosen ScFvs are able to recognize marine toxins associated with algal blooms, saxitoxin, and domoic acid, which can bioaccumulate in shellfish and herbivorous fish causing food poisoning. ScFvs fused to hydrophobin Vmh2 from Pleurotus ostreatus were produced in Escherichia coli and recovered from the inclusion bodies. The two fusion proteins retained the functionality of both moieties, being able to adhere on magnetic beads and to recognize and bind the two neurotoxins, even with different performances. Our immobilization procedure is innovative and very easy to implement because it allows the direct functionalization of magnetic beads with ScFvs, without any surface modification. Two different detection principles, electrochemical and optical, were adopted, thus achieving a versatile platform suitable for different antigen detection methods. The sensitivity of the saxitoxin optical biosensor [limit of detection (LOD) 1.7 pg/ml] is comparable to the most sensitive saxitoxin immunosensors developed until now.

Enzyme-polymeric/inorganic metal oxide/hybrid nanoparticle bio-conjugates in the development of therapeutic and biosensing platforms

Background

Because enzymes can control several metabolic pathways and regulate the production of free radicals, their simultaneous use with nanoplatforms showing protective and combinational properties is of great interest in the development of therapeutic nano-based platforms. However, enzyme immobilization on nanomaterials is not straightforward due to the toxic and unpredictable properties of nanoparticles in medical practice.

Aim of review

In fact, because of the ability to load enzymes on nano-based supports and increase their renewability, scientific groups have been tempted to create potential therapeutic enzymes in this field. Therefore, this study not only pays attention to the therapeutic and diagnostic applications of diseases by enzyme-nanoparticle (NP) bio-conjugate (abbreviated as: ENB), but also considers the importance of nanoplatforms used based on their toxicity, ease of application and lack of significant adverse effects on loaded enzymes. In the following, based on the published reports, we explained that the immobilization of enzymes on polymers, inorganic metal oxide and hybrid compounds provide hopes for potential use of ENBs in medical activities. Then, the use of ENBs in bioassay activities such as paper-based or wearing biosensors and lab-on-chip/microfluidic biosensors were evaluated. Finally, this review addresses the current challenges and future perspective of ENBs in biomedical applications.

Key scientific concepts of review

This literature may provide useful information regarding the application of ENBs in biosensing and therapeutic platforms.

Plant Expression of Hydrophobin Fused K39 Antigen for Visceral Leishmaniasis Immunodiagnosis

Visceral leishmaniasis is a Neglected Tropical Disease of high mortality caused by the protozoan Leishmania infantum. Its transmission cycle is complex, and it has in the domestic dog its main reservoir. The diagnostic tests currently used rely on prokaryotic systems' proteins, but their low sensitivity increases the disease's burden. The plant transient expression of recombinant proteins allows the production of complex antigens. However, this system has limited competitiveness against the bacterial production of purified antigens. Thus, we have shown that the L. infantum K39 antigen's fusion to a hydrophobin allows its production for diagnostic tests without the need for intensive purification. The sera of naturally infected dogs specifically detect the semi-purified rK39-HFBI protein. The test validation against a panel of 158 clinical samples demonstrates the platform's viability, resulting in sensitivity and specificity of 90.7 and 97.5%, respectively. Thus, the use of semi-purified antigens fused to hydrophobins can become the standard platform for large-scale antigens production to expand diagnostic tests for other human and veterinary diseases worldwide.

Genetically Encodable Scaffolds for Optimizing Enzyme Function

Enzyme engineering is an indispensable tool in the field of synthetic biology, where enzymes are challenged to carry out novel or improved functions. Achieving these goals sometimes goes beyond modifying the primary sequence of the enzyme itself. The use of protein or nucleic acid scaffolds to enhance enzyme properties has been reported for applications such as microbial production of chemicals, biosensor development and bioremediation. Key advantages of using these assemblies include optimizing reaction conditions, improving metabolic flux and increasing enzyme stability. This review summarizes recent trends in utilizing genetically encodable scaffolds, developed in line with synthetic biology methodologies, to complement the purposeful deployment of enzymes. Current molecular tools for constructing these synthetic enzyme-scaffold systems are also highlighted.

Mesoporous Materials-Based Electrochemical Biosensors from Enzymatic to Nonenzymatic

Mesoporous materials have drawn more and more attention in the field of biosensors due to their high surface areas, large pore volumes, tunable pore sizes, as well as abundant frameworks. In this review, the progress on mesoporous materials-based biosensors from enzymatic to nonenzymatic are highlighted. First, recent advances on the application of mesoporous materials as supports to stabilize enzymes in enzymatic biosensing technology are summarized. Special emphasis is placed on the effect of pore size, pore structure, and surface functional groups of the support on the immobilization efficiency of enzymes and the biosensing performance. Then, the development of a nonenzymatic strategy that uses the intrinsic property of mesoporous materials (carbon, silica, metals, and composites) to mimic the behavior of enzymes for electrochemical sensing of some biomolecules is discussed. Finally, the challenges and perspective on the future development of biosensors based on mesoporous materials are proposed.

Self-assembling thermostable chimeras as new platform for arsenic biosensing

The correct immobilization and orientation of enzymes on nanosurfaces is a crucial step either for the realization of biosensors, as well as to guarantee the efficacy of the developed biomaterials. In this work we produced two versions of a chimeric protein, namely ArsC-Vmh2 and Vmh2-ArsC, which combined the self-assembling properties of Vmh2, a hydrophobin from Pleurotus ostreatus, with that of TtArsC, a thermophilic arsenate reductase from Thermus thermophilus; both chimeras were heterologously expressed in Escherichia coli and purified from inclusion bodies. They were characterized for their enzymatic capability to reduce As(V) into As(III), as well as for their immobilization properties on polystyrene and gold in comparison to the native TtArsC. The chimeric proteins immobilized on polystyrene can be reused up to three times and stored for 15 days with 50% of activity loss. Immobilization on gold electrodes showed that both chimeras follow a classic Langmuir isotherm model towards As(III) recognition, with an association constant (KAsIII) between As(III) and the immobilized enzyme, equal to 650 (± 100) L mol-1 for ArsC-Vmh2 and to 1200 (± 300) L mol-1 for Vmh2-ArsC. The results demonstrate that gold-immobilized ArsC-Vmh2 and Vmh2-ArsC can be exploited as electrochemical biosensors to detect As(III).

Effective adsorption of nisin on the surface of polystyrene using hydrophobin HGFI

Herein, a new method was demonstrated for effective immobilization of the antibacterial peptide nisin on Grifola frondosa hydrophobin (HGFI), without the need of any additional complex reaction. Hydrophobin can self-assemble as a monolayer to form continuous negative-charged surfaces with enhanced wettability and biocompatibility. Adding nisin solution to such hydrophobin surface created antibacterial surfaces. The quantification analysis revealed that more nisin could be adsorbed on the HGFI-coated than to control polystyrene surfaces at different pH values. This suggested that electronic attraction and wettability may play important roles in this process. The transmission electron microscopy, atomic force microscopy and fourier transform infrared (FTIR) analysis indicated the adsorption mode of nisin on the HGFI film, i.e., hydrophobins served as an adhesive layer for binding charged peptides to interfaces. The antibacterial activity of the treated surface was investigated via counting, a nucleic acid release test, scanning electron microscopy, and biofilm detection. These results indicated the excellent antibacterial activity of nisin adsorbed on the HGFI-coated surfaces. The activity retention of adsorbed nisin was demonstrated by immersing the modified substrates in a flowed liquid condition.

Development of anti-bacterial surfaces using a hydrophobin chimeric protein

The search for new approaches for developing antimicrobial surfaces is a challenge of great urgency to prevent and control microbial growth on surfaces. The strategy herein proposed relies on the design of a new, simple and general tool for creating antimicrobial surfaces, based on a hydrophobin chimeric protein which fuses the adhesive self-assembling class I hydrophobin Vmh2 from Pleurotus ostreatus to the human antimicrobial peptide LL-37. The recombinant LL37-Vmh2 protein displayed both the adhesive and the antimicrobic properties of its members, and when deposited on polystyrene surface, a positive effect due to the fusion was observed in terms of both efficacy and versatility of the coating. Indeed, the chimeric protein significantly enlarges the range of pathogens affected by Vmh2 layer rendering it able to inhibit three Gram-positive and two Gram-negative pathogens, selected among the renowned biofilm producer bacteria. Confocal Laser Scanning Microscopy analysis performed on Staphylococcus epidermidis biofilms formed on coated surfaces proved that, besides inhibiting biofilm formation, the LL37-Vmh2 coating also displayed biocidal activity, since dead cells were present in the biofilm layer. The reported results open new perspectives in various fields of application of LL37, and of antimicrobial peptides in general. LL37-Vmh2 increases the inventory of chimeric hydrophobins, further proving their effectiveness and versatility in surface functionalization.

From Graphite to Laccase Biofunctionalized Few-Layer Graphene: A "One Pot" Approach Using a Chimeric Enzyme

A chimeric enzyme based on the genetic fusion of a laccase with a hydrophobin domain was employed to functionalize few-layer graphene, previously exfoliated from graphite in the presence of the hydrophobin. The as-produced, biofunctionalized few-layer graphene was characterized by electrochemistry and Raman spectroscopy, and finally employed in the biosensing of phenols such as catechol and dopamine. This strategy paves the way for the functionalization of nanomaterials by hydrophobin domains of chimeric enzymes and their use in a variety of electrochemical applications.

Paper-Based Electrochemical Devices for the Pharmaceutical Field: State of the Art and Perspectives

The current international pharmaceutical scenario encompasses several steps in drug production, with complex and extremely long procedures. In the last few decades, scientific research has been trying to offer valid and reliable solutions to replace or support conventional techniques, in order to facilitate drug development procedures. These innovative approaches may have extremely positive effects in the production chain, supplying fast, and cost-effective quality as well as safety tests on active pharmaceutical ingredients (APIs) and their excipients. In this context, the exploitation of electrochemical paper-based analytical devices (ePADs) is still in its infancy, but is particularly promising in the detection of APIs and excipients in tablets, capsules, suppositories, and injections, as well as for pharmacokinetic bioanalysis in real samples.

Development of a biosensing platform based on a laccase-hydrophobin chimera

A simple and stable immobilization of a laccase from Pleurotus ostreatus was obtained through genetic fusion with a self-assembling and adhesive class I hydrophobin. The chimera protein was expressed in Pichia pastoris and secreted into the culture medium. The crude culture supernatant was directly used for coatings of polystyrene multi-well plates without additional treatments, a procedure that resulted in a less time-consuming and chemicals reduction. Furthermore, the gene fusion yielded a positive effect with respect to the wild-type recombinant enzyme in terms of both immobilization and stability. The multi-well plate with the immobilized chimera was used to develop an optical biosensor to monitor two phenolic compounds: L-DOPA ((S)-2-amino-3-(3,4-dihydroxyphenyl) propanoic acid) and caffeic acid (3-(3,4-dihydroxyphenyl)-2-propenoic acid); the estimation of which is a matter of interest in the pharmaceutics and food field. The method was based on the use of the analytes as competing inhibitors of the laccase-mediated ABTS oxidation. The main advantages of the developed biosensor are the ease of preparation, the use of small sample volumes, and the simultaneous analysis of multiple samples on a single platform.

Self-Assembling Protein-Polymer Bioconjugates for Surfaces with Antifouling Features and Low Nonspecific Binding

A new method is demonstrated for preparing antifouling and low nonspecific adsorption surfaces on poorly reactive hydrophobic substrates, without the need for energy-intensive or environmentally aggressive pretreatments. The surface-active protein hydrophobin was covalently modified with a controlled radical polymerization initiator and allowed to self-assemble as a monolayer on hydrophobic surfaces, followed by the preparation of antifouling surfaces by Cu(0)-mediated living radical polymerization of poly(ethylene glycol) methyl ether acrylate (PEGA) performed in situ. By taking advantage of hydrophobins to achieve at the same time the immobilization of protein A, this approach allowed to prepare surfaces for IgG1 binding featuring greatly reduced nonspecific adsorption. The success of the surface modification strategy was investigated by contact angle, XPS, and AFM characterization, while the antifouling performance and the reduction of nonspecific binding were confirmed by QCM-D measurements.

From Protein Features to Sensing Surfaces

Proteins play a major role in biosensors in which they provide catalytic activity and specificity in molecular recognition. However, the immobilization process is far from straightforward as it often affects the protein functionality. Extensive interaction of the protein with the surface or significant surface crowding can lead to changes in the mobility and conformation of the protein structure. This review will provide insights as to how an analysis of the physico-chemical features of the protein surface before the immobilization process can help to identify the optimal immobilization approach. Such an analysis can help to preserve the functionality of the protein when on a biosensor surface.

Applications of Functional Amyloids from Fungi: Surface Modification by Class I Hydrophobins

Class I hydrophobins produced from fungi are amongst the first proteins recognized as functional amyloids. They are amphiphilic proteins involved in the formation of aerial structures such as spores or fruiting bodies. They form chemically robust layers which can only be dissolved in strong acids. These layers adhere to different surfaces, changing their wettability, and allow the binding of other proteins. Herein, the modification of diverse types of surfaces with Class I hydrophobins is reported, highlighting the applications of the coated surfaces. Indeed, these coatings can be exploited in several fields, spanning from biomedical to industrial applications, which include biosensing and textile manufacturing.

Self-assembly of two hydrophobins from marine fungi affected by interaction with surfaces

Hydrophobins are amphiphilic fungal proteins endowed with peculiar characteristics, such as a high surface activity and an interface triggered self-assembly. Several applications of these proteins have been proposed in the food, cosmetics and biomedical fields. Moreover, their use as proteinaceous coatings can be effective for materials and nanomaterials applications. The discovery of novel hydrophobins with diverse properties may be advantageous from both the scientific and industrial points of view. Stressful environmental conditions of fungal growth may induce the production of proteins with peculiar features. Two Class I hydrophobins from fungi isolated from marine environment have been recently purified. Herein, their propensity to aggregate forming nanometric fibrillar structures has been compared, using different techniques, such as circular dichroism, dynamic light scattering and Thioflavin T fluorescence assay. Furthermore, TEM and AFM images indicate that the interaction of these proteins with specific surfaces, are crucial in the formation of amyloid fibrils and in the assembly morphologies. These self-assembling proteins show promising properties as bio-coating for different materials via a green process. Biotechnol. Bioeng. 2017;114: 2173-2186. © 2017 Wiley Periodicals, Inc.