Recombinant Silicateins as Model Biocatalysts in Organosiloxane Chemistry

The family of silicatein enzymes from marine sponges (phylum
Porifera) is unique in nature for catalyzing the formation of inorganic
silica structures, which the organisms incorporate into their
skeleton. However, the synthesis of organosiloxanes catalyzed
by these enzymes has thus far remained largely unexplored. In
order to investigate the reactivity of these enzymes in relation
to this important class of compounds, their catalysis of Si-O bond
hydrolysis and condensation was investigated with a range of
model organosilanols and silyl ethers. The enzymes’ kinetic parameters
were obtained by a high throughput colorimetric assay
based on the hydrolysis of 4-nitrophenyl silyl ethers. These assays
showed unambiguous catalysis with k_cat/K_M values in the order
of 2–50 min⁻¹μM⁻¹. Condensation reactions were also demonstrated
by the generation of silyl ethers from their corresponding
silanols and alcohols. Notably, when presented with a substrate
bearing both aliphatic and aromatic hydroxy groups the enzyme
preferentially silylates the latter group, in clear contrast to nonenzymatic
silylations. Furthermore, the silicateins are able to catalyze
transetherifications, where the silyl group from one silyl
ether may be transferred to a recipient alcohol. Despite close
sequence homology to the protease cathepsin L, the silicateins
appear to exhibit no significant protease or esterase activity when
tested against analogous substrates. Overall, these results suggest
the silicateins are promising candidates for future elaboration into
efficient and selective biocatalysts for organosiloxane chemistry.