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

Abstract

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.

Arrested in Glass: Actin within Sophisticated Architectures of Biosilica in Sponges

“…This is in contrast to the mineralization-associated molecules such as silicateins, which are not branched, and are tightly bound in the biomineralized structure where they catalyze and template silica. [10] The diversity of forms of biosilica is exemplified by the extreme case of Monorhaphis sponges, which can produce meter-length spicules (Figures S26 and S27, Supporting Information). Our work provides an answer to one of the fundamental questions of biomaterials science, namely, how this "biological glass" grows at ambient temperatures (from -1.9°C to 24°C) to reach such great lengths.…”

“…[ 5,6 ] The inorganic chemistry of poriferan skeletons with respect to silica in Hexactinellida, Demospongiae, and Homoscleromorpha, or calcium carbonates in Calcarea, is well understood, but the role of the bio‐organic phases in skeletogenesis of sponges is still debated. [ 7–10 ] Silica‐based structures in sponges have several roles, including protection, support of the body form, maintenance of posture in flow, as well as anchoring to sandy and muddy bottoms. The main players in poriferan biosilicification and spiculogenesis are low molecular weight proteins (i.e., silicateins, cathepsins) [ 10–12 ] in demosponges, accompanied by glassin, [ 13 ] collagen, [ 14 ] and chitin [ 15,16 ] in glass sponges (Hexactinellida).…”

“…[ 7–10 ] Silica‐based structures in sponges have several roles, including protection, support of the body form, maintenance of posture in flow, as well as anchoring to sandy and muddy bottoms. The main players in poriferan biosilicification and spiculogenesis are low molecular weight proteins (i.e., silicateins, cathepsins) [ 10–12 ] in demosponges, accompanied by glassin, [ 13 ] collagen, [ 14 ] and chitin [ 15,16 ] in glass sponges (Hexactinellida). Though their presence during skeletal formation has been confirmed, there remain questions of pattern drivers that lead to the diversity of shape (more than 80 and 46 morphotypes in Demospongiae and Hexactinellida, respectively), [ 5,17 ] size (from micrometer‐ to up to 3 meter‐long), [ 6 ] network connectivity (monaxons, triaxons, tetraxons), [ 18,19 ] and superficial ornamentation.…”

“…This is in contrast to the mineralization‐associated molecules such as silicateins, which are not branched, and are tightly bound in the biomineralized structure where they catalyze and template silica. [ 10 ]…”

Calcite Nanotuned Chitinous Skeletons of Giant Ianthella basta Marine Demosponge

“…While evidence for the presence of the aforementioned biominerals in sponges skeletons is numerous and indisputable, debate still revolves around understanding the principles of biomineralization and corresponding mechanisms at the molecular level. Particular attention is paid to the identification of proteins (i.e., silicateins, glassin, cathepsins, collagens) [6][7][8][9] and polysaccharides (i.e., chitin) [10,11] that act as templates for the formation of silica-based mineral phases. The forehead sponge body is a soft and elastic organic matter resting on the mineral skeleton of a particular physical form.…”