MASTER THESIS Biomimetic potential of sponge ... - IAP/TU Wien

Spiculogenesis
Overview
Spiculogenesis is a rather special case of biologically controlled biomineralization. It was
largely enigmatic how the highly ordered structure of sponge spicules originated, until Cha
and colleagues (1999) demonstrated in vitro that an enzyme directed the polymerization of
silica. The organic filament in the core of sponge spicules had long been known, yet only in
1998 it was found that it contained an enzyme, that has been termed silicatein (silica
protein) (Shimizu et al., 1998). The arising mechanism has been termed enzymatically
controlled and driven biomineralization (Morse, 1999) (Müller et al., 2007a). During this
catalytic process, the filament containing silicatein serves as a scaffold directing the
formation of the polysilicate (Müller et al., 2008b).
Subsequently more proteins that are involved in spiculogenesis have been identified.
Apart from identifying various sub-types of silicatein (silicatein α-γ) that are specific for
different species of demosponges (Müller et al., 2007c), the first silicatein of hexactinellids
has been reported (Müller et al., 2008a). This hexactinellid silicatein has a high structural
similarity to demospongial silicateins, corroborating the idea that the mechanisms involved
in early spiculogenesis of both classes of siliceous sponges are similar.
Another class of functional proteins has been identified in the axial filament of spicules
(Wiens et al., 2009) (Wiens et al., 2011). Silicatein interacting proteins (silintaphins) have
been reported to convey elasticity to the spicule during spiculogenesis and to be involved in
defining the morphology of spicules (Müller et al., 2009c).
A third class of proteins in the axial filaments are the catabolic (degrading) silicases, first
identified in the demosponge Suberites domuncula (Schröder et al., 2003). Silicases are
assumed to be involved in the deposition of silica, yet their exact functions remain elusive
(Wang et al., 2012b).
The underlying mechanism of the silicatein reaction has been revised since its first
description by Cha and colleagues (1999) (see above) and now accounts for the roles of
silintaphins, various forms of silicateins and silicases (Schröder et al., 2010) (cf. Figure 13f for
a reaction mechanism with a synthetic analogue of silicic acid).
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