Bioinspired enzymatic synthesis of silica nanocrystals provided by recombinant silicatein from the marine sponge Latrunculia oparinae

Biomimetic Sol-Gel Chemistry to Tailor Structure, Properties, and Functionality of Bionanocomposites by Biopolymers and Cells

Biosilica, synthesized annually only by diatoms, is almost 1000 times more abundant than industrial silica. Biosilicification occurs at a high rate, although the concentration of silicic acid in natural waters is ~100 μM. It occurs in neutral aqueous solutions, at ambient temperature, and under the control of proteins that determine the formation of hierarchically organized structures. Using diatoms as an example, the fundamental differences between biosilicification and traditional sol-gel technology, which is performed with the addition of acid/alkali, organic solvents and heating, have been identified. The conditions are harsh for the biomaterial, as they cause protein denaturation and cell death. Numerous attempts are being made to bring sol-gel technology closer to biomineralization processes. Biomimetic synthesis must be conducted at physiological pH, room temperature, and without the addition of organic solvents. To date, significant progress has been made in approaching these requirements. The review presents a critical analysis of the approaches proposed to date for the silicification of biomacromolecules and cells, the formation of bionanocomposites with controlled structure, porosity, and functionality determined by the biomaterial. They demonstrated the broad capabilities and prospects of biomimetic methods for creating optical and photonic materials, adsorbents, catalysts and biocatalysts, sensors and biosensors, and biomaterials for biomedicine.

Biosynthesis and Cytotoxic Properties of Ag, Au, and Bimetallic Nanoparticles Synthesized Using Lithospermum erythrorhizon Callus Culture Extract

The present study reports a green chemistry approach for the rapid and easy biological synthesis of silver (Ag), gold (Au), and bimetallic Ag/Au nanoparticles using the callus extract of Lithospermum erythrorhizon as a reducing and capping agent. The biosynthesized nanoparticles were characterized with ultraviolet-visible (UV-Vis) spectroscopy, X-ray diffraction (XRD) analysis, and transmission electron microscopy (TEM). Our results showed the formation of crystalline metal nanostructures of both spherical and non-spherical shape. Energy dispersive X-ray (EDX) spectroscopy showed the characteristic peaks in the silver and gold regions, confirming the presence of the corresponding elements in the monometallic particles and both elements in the bimetallic particles. Fourier-transform infrared (FTIR) spectroscopy affirmed the role of polysaccharides and polyphenols of the L. erythrorhizon extract as the major reducing and capping agents for metal ions. In addition, our results showed that the polysaccharide sample and the fraction containing secondary metabolites isolated from L. erythrorhizon were both able to produce large amounts of metallic nanoparticles. The biosynthesized nanoparticles demonstrated cytotoxicity against mouse neuroblastoma and embryonic fibroblast cells, which was considerably higher for Ag nanoparticles and for bimetallic Ag/Au nanoparticles containing a higher molar ratio of silver. However, fibroblast migration was not significantly affected by any of the nanoparticles tested. The obtained results provide a new example of the safe biological production of metallic nanoparticles, but further study is required to uncover the mechanism of their toxicity so that the biomedical potency can be assessed.

Fabrication of silica on chitin in ambient conditions using silicatein fused with a chitin-binding domain

High temperatures, harsh pH conditions, and toxic chemicals involved in the conventional synthesis and coating of silica limit the fabrication of new-generation hybrid materials immobilizing live cells and biomolecules such as enzymes and drugs. This hinders the application of inorganic-organic biohybrid materials in various fields, including bioelectronics, energy generation, and biomedicine. Silicatein, an enzyme found in siliceous sponges, catalyzes the polymerization of silica under mild conditions, that is, at room temperature and neutral pH. Silicatein was fused with a chitin-binding domain (ChBD) to selectively bind the fusion silicatein on the chitin material and with a small soluble tag called InakC, a hydrophilic protein from Pseudomonas syringae, to control the unfavorable aggregation of silicatein. The fusion silicatein was soluble in aqueous media and was successfully found to be adsorbed on the chitin material. The immobilized fusion silicatein acted as an interfacial catalyst to fabricate silica on chitin under ambient conditions. This technique can be used to fabricate inorganic-organic hybrid materials to immobilize biomolecules and can be applied to develop novel biocatalytic systems, biosensors, and tissue culture scaffolds.

Genetical Surface Display of Silicatein on Yarrowia lipolytica Confers Living and Renewable Biosilica-Yeast Hybrid Materials

In this work, a biological engineering-based biosilica-yeast hybrid material was developed. It was obtained by the aggregation of Yarrowia lipolytica through biosilicification catalyzed using genetically displayed silicatein on its cell surface. With orthosilicate or seawater as the substrate, the silicatein-displayed yeast could aggregate into flocs with a flocculation efficiency of nearly 100%. The resulting floc was found to be a sheetlike biosilica-yeast hybrid material formed by the biosilica-mediated immobilization of yeast cells via cross-linking and embedding, turning the original hydrophilicity of yeast cells into hydrophobicity. In addition, this material was characterized to be porous with an average pore diameter of approximately 10 μm and porosity of over 70%. Because of these properties, this hybrid material could achieve enhanced removal efficiencies for chromium ions and n-hexadecane, which were both above 99%, as compared to the free cells of Y. lipolytica in aqueous environments. Importantly, this hybrid material could be recultivated to generate new batches of yeast cells that maintain parallel properties to the first generation so that the same hybrid material could be reproduced with unchanged highly efficient removal of chromium and n-hexadecane to those of the first generation, demonstrating that this biosilica-yeast hybrid material was living and renewable. This work presented a novel way of harnessing silicatein and Y. lipolytica to achieve biological synthesis of a living inorganic-organic hybrid material that has potential to be applied in water treatment.

Silicatein expression in Haliclona indistincta (Phylum Porifera, Order Haplosclerida) at different developmental stages

Silicatein is the main protein responsible for the formation of spicules, tiny structures that constitute the silica skeleton of marine demosponges (Phylum Porifera). A unique innovation in Porifera that evolved from the cathepsin L family of proteins, it has been reported that two amino acids (S and H) are necessary to form the catalytic triad (SHN) to enable silica condensation. However, a diversity of silicatein sequence variants has since been reported with a variable pattern of presence/absence across sponge groups. Variants containing CHN or C/SQN at the active site appear more common in sponges from the Haplosclerida. Here, we report the expression levels of five silicatein variants through different developmental stages in the haplosclerid Haliclona indistincta. All five silicatein variants were expressed at low levels in the free-swimming larvae, which lack spicules and expression significantly increased at the two developmental phases in which spicules were visible. At these two phases, silicateins of CHN and C/SQN types were much more highly expressed than the SHN type indicating a possible ability of active sites with these alternative amino acids to condense silica and a more complex evolutionary story for spicule formation in marine demosponges than previously understood.

Evolution of the main skeleton-forming genes in sponges (phylum Porifera) with special focus on the marine Haplosclerida (class Demospongiae)

The skeletons of sponges (Phylum Porifera) are comprised of collagen, often embedded with small siliceous structures (spicules) arranged in various forms to provide strength and flexibility. The main proteins responsible for the formation of the spicules in demosponges are the silicateins, which are related to the cathepsins L of other animals. While the silicatein active site, necessary for the formation of biosilica crystals, is characterized by the amino acids SHN, different variants of the silicatein genes have been found, some that retain SHN at the active site and some that don't. As part of an effort to further understand skeleton formation in marine sponges of the order Haplosclerida, a search for all silicatein variants were made in Irish species representing the main clades of this large sponge group. For this task, transcriptomes were sequenced and de novo assembled from Haliclona oculata, H. simulans and H. indistincta. Silicatein genes were identified from these and all available genomes and transcriptomes from Porifera. These were analysed along with all complete silicateins from GenBank. Silicateins were only found in species belonging to the class Demospongiae but excluding Keratosa and Verongimorpha and there was significant duplication and diversity of these genes. Silicateins showing SHN at the active site were polyphyletic. Indeed silicatein sequences were divided into six major clades (CHNI, CHNII, CHNIII, SHNI, SHNII and C/SQN). In those clades where haplosclerids were well represented the silicatein phylogeny reflected previous ribosomal and mitochondrial topologies. The most basal silicatein clade (CHNI) contained sequences only from marine haplosclerids and freshwater sponges while one silicatein from H. indistincta was more related to cathepsins L (outgroup) than to the overall silicatein clade indicating the presence of an old silicatein or an intermediary form. This data could suggest that marine haplosclerids were one of the first groups of extant demosponges to acquire silicatein genes. Furthermore, we suggest that the paucity of spicule types in this group may be due to their single copy of SHNI variants, and the lack of a silintaphin gene.

Green synthesis of silver nanoparticles using transgenic Nicotiana tabacum callus culture expressing silicatein gene from marine sponge Latrunculia oparinae

In the present investigation, transgenic tobacco callus cultures and plants overexpressing the silicatein gene LoSilA1 from marine sponge Latrunculia oparinae were obtained and their bioreduction behaviour for the synthesis of silver nanoparticles (AgNPs) was studied. Synthesized nanoparticles were characterized using UV-visible spectroscopy, Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic flame electron microscopy (AFM) and nanoparticle tracking analysis (NTA). Our measurements showed that the reduction of silver nitrate produced spherical AgNPs with diameters in the range of 12-80 nm. The results of XRD analysis proved the crystal nature of the obtained AgNPs. FTIR analysis indicated that particles are reduced and stabilized in solution by the capping agent, which is likely to be proteins present in the callus extract. Interestingly, the reduction potential of LoSiLA1-transgenic callus line was increased three-fold compared with the empty vector-transformed calli. The synthesized AgNPs were found to exhibit strong antibacterial activity against Escherichia coli and Agrobacterium rhizogenes. The present study reports the first evidence for using genetic engineering for activation of the reduction potential of plant cells for synthesis of biocidal AgNPs.

Efficient silica synthesis from tetra(glycerol)orthosilicate with cathepsin- and silicatein-like proteins

Silicateins play a key role in biosynthesis of spicules in marine sponges; they are also capable to catalyze formation of amorphous silica in vitro. Silicateins are highly homologous to cathepsins L – a family of cysteine proteases. Molecular mechanisms of silicatein activity remain controversial. Here site-directed mutagenesis was used to clarify significance of selected residues in silica polymerization. A number of mutations were introduced into two sponge proteins – silicatein A1 and cathepsin L from Latrunculia oparinae, as well as into human cathepsin L. First direction was alanine scanning of the proposed catalytic residues. Also, reciprocal mutations were introduced at selected positions that differ between cathepsins L and silicateins. Surprisingly, all the wild type and mutant proteins were capable to catalyze amorphous silica formation with a water-soluble silica precursor tetra(glycerol)orthosilicate. Some mutants possessed several-fold enhanced silica-forming activity and can potentially be useful for nanomaterial synthesis applications. Our findings contradict to the previously suggested mechanisms of silicatein action via a catalytic triad analogous to that in cathepsins L. Instead, a surface-templated biosilification by silicateins and related proteins can be proposed.

A water-soluble precursor for efficient silica polymerization by silicateins

Silicateins, the spicule-forming proteins from marine demosponges capable to polymerize silica, are popular objects of biomineralization studies due to their ability to form particles varied in shape and composition under physiological conditions. Despite the occurrence of the many approaches to nanomaterial synthesis using silicateins, biochemical properties of this protein family are poorly characterized. The main reason for this is that tetraethyl orthosilicate (TEOS), the commonly used silica acid precursor, is almost insoluble in water and thus is poorly available for the protein. To solve this problem, we synthesized new water-soluble silica precursor, tetra(glycerol)orthosilicate (TGS), and characterized biochemical properties of the silicatein A1 from marine sponge Latrunculia oparinae. Compared to TEOS, TGS ensured much greater activity of silicatein and was less toxic for the mammalian cell culture. We evaluated optimum conditions for the enzyme - pH range, temperature and TGS concentration. We concluded that TGS is a useful silica acid precursor that can be used for silica particles synthesis and in vivo applications.