Silicateins are

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

which catalyse the formation of biosilica from monomeric silicon compounds (such as

silicic acid In chemistry, a silicic acid () is any chemical compound containing the element silicon attached to oxide () and hydroxyl () groups, with the general formula or, equivalently, . Orthosilicic acid is a representative example. Silicic acids are ra ...

) extracted from the natural environment.Müller, W.E., Boreiko, A., Wang, X., Belikov, S.I., Wiens, M., Grebenjuk, V.A., Schloβmacher, U. and Schröder, H.C., 2007. Silicateins, the major biosilica forming enzymes present in demosponges: protein analysis and phylogenetic relationship. Gene, 395(1-2), pp.62-71. doi:10.1016/j.gene.2007.02.014 Environmental silicates are absorbed by specific biota, including

diatoms A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...

radiolaria The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ect ...

silicoflagellate The silicoflagellates (order Dictyochales) are a small group of unicellular photosynthetic protists, or algae, belonging to the supergroup of eukaryotes known as Stramenopiles. They behave as plankton and are present in oceanic waters. They a ...

s, and

siliceous sponge The siliceous sponges form a major clade of the phylum Porifera, consisting of classes Demospongiae (common sponges) and Hexactinellida (glass sponges). They are characterized by spicules made out of silicon dioxide, unlike calcareous sponges. ...

s; silicateins have so far only been found in

sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...

. Silicateins are homologous to the

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

cathepsin Cathepsins (Ancient Greek ''kata-'' "down" and ''hepsein'' "boil"; abbreviated CTS) are proteases (enzymes that degrade proteins) found in all animals as well as other organisms. There are approximately a dozen members of this family, which are d ...

.Otzen D. (2012). The role of proteins in biosilicification. Scientifica, 2012, 867562. doi:10.6064/2012/867562 In sponges, the silicatein enzymes reside in the axial filaments of the axial canals of the siliceous Sponge spicule, spicules. In contrast,

do not use silicateins but rather small specialised peptides called silaffins which attach long chain polyamines (LCPAs) to lysine groups. Free LCPAs can also cooperate with silaffins. Both silicateins and silaffins form higher-order structures which act both as structural templates (for exoskeletons) and mechanistic catalysts for the polycondensation reactions of silicon-compounds. The Venus' flower basket

is a well-known example of an organism that utilises silicatein. It is known for its remarkable ability to extract

from surrounding seawater, which is then converted into complex 3D silica structures at ambient temperatures underwater, something human engineering capabilities are unable to replicate without the use of high-temperature. Another example of silicatein-utilising organisms are the suberites, a genus of sea sponge in the family (biology), family Suberitidae. Suberites consist mostly of cells, in contrast with other ''Porifera'' (such as the class Hexactinellid, Hexactinellida, to which the Venus' flower basket belongs) which are syncytial.W. Muller, Review: How was metazoan threshold crossed? The hypothetical Urmetazoa. Comparative Biochemistry and Physiology Part A 129, 433 (2001). doi:10.1016/s1095-6433(00)00360-3 The extracellular matrix of siliceous Sponge spicule, spicules give suberites their structural foundation; these consist of bio-silica, a silicon dioxide polymer.W. Xiaohong et al., Evagination of Cells Controls Bio-Silica Formation and Maturation during Spicule Formation in Sponges. PLoS ONE 6, 1 (2011). doi:10.1371/journal.pone.0020523 These inorganic structures provide support for the animals.X. Wang et al., Silicateins, silicatein interactors and cellular interplay in sponge skeletogenesis: formation of glass fiber-like spicules. FEBS Journal 279, 1721 (2012) https:/doi.org10.1111/j.1742-4658.2012.08533.x Silica deposition begins intracellularly and is carried out by the enzyme silicatein. Silicateins are modulated by a group of proteins called silintaphins The process occurs in specialized cells known as sclerocytes. Lubomirskia baikalensis, also known as Lake Baikal sponge, has been studied to explore the gene family of silicateins and their role in the morphogenesis of these sponges.Belikov; Kaluzhnaya; Schröder; Müller; and Müller (2007)
Lake Baikal endemic sponge Lubomirskia baikalensis: structure and organization of the gene family of silicatein and its role in morphogenesis
Porifera Research: Biodiversity, Innovation and Sustainability, pp. 179-188

References

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