The Role of Proteins in Biosilicification

Abstract

Although the use of silicon dioxide (silica) as a constituent of living organisms is mainly restricted to diatoms and sponges, the ways in which this process is controlled by nature continue to inspire and fascinate. Both diatoms and sponges carry out biosilificiation using an organic matrix but they adopt very different strategies. Diatoms use small and heavily modified peptides called silaffins, where the most characteristic feature is a modulation of charge by attaching long chain polyamines (LCPAs) to lysine groups. Free LCPAs can also cooperate with silaffins. Sponges use the enzyme silicatein which is homologous to the cysteine protease cathepsin. Both classes of proteins form higher-order structures which act both as structural templates and mechanistic catalysts for the polycondensation reaction. In both cases, additional proteins are continuously being discovered which modulate the process further. This paper concentrates on the role of these proteins in the biosilification process as well as in various applications, highlighting areas where focus on specific protein properties may provide further insight. The field of biosilification is a crossroads of different disciplines, where insight into the energetics and mechanisms of molecular self-assembly combine with fundamental biology, complex multicomponent colloidal systems, and an impressive array of potential technological applications.

Structure–function–property–design interplay in biopolymers: Spider silk

“…In biological organisms organic molecules exert remarkable level of control over the process of mineral formation, including the hierarchical organization and functional properties [97]. Through the use of recombinant DNA technology, a variety of novel materials can be engineered with properties not present in nature [81].…”

Multiscale design and synthesis of biomimetic gradient protein/biosilica composites for interfacial tissue engineering

“…For example, a variety of polycationic peptides e.g. silaffin that are able to precipitate silica nanospheres individually can be encapsulated in the silk substrates, leading to advanced functional materials with different mineral morphology and organization [31,89]. Depending on the peptide used for biomineralization and the external physical/chemical stimuli, different silica structures can be generated including sphere-like (with R5), fibrous structures (with p-(lys)1642) and platelike (with p-(lys)189) in vitro [90].…”

Response to Referee #1

“…Three classes of sponges in the phylum Porifera, Homoscleromorpha, Demospongiae and Hexactinellida, produce their spicules made of bio-silica (amorphous silica) through the incorporation and deposition of hydrated silica (SiO 2 · nH 2 O), a process referred to as biosilicification (e.g. Uriz, 2006;Otzen, 2012). The spicules may represent up to 70 %-90 % of the body (dry weight) depending on the species (e.g Sandford, 2003;Maldonado et al, 2012).…”