Recombinant silicateins as model biocatalysts in organosiloxane chemistry

Significance

Organosiloxanes are components in a huge variety of consumer products and play a major role in the synthesis of fine chemicals. However, their synthetic manipulation primarily relies on the use of chlorosilanes, which are energy-intensive to produce and environmentally undesirable. Synthetic routes that operate under ambient conditions and circumvent the need for chlorinated feedstocks would therefore offer a more sustainable route for producing this class of compounds. Here, a systematic survey is reported for the silicatein enzyme, which is able to catalyze the hydrolysis, condensation, and exchange of the silicon–oxygen bond in a variety of organosiloxanes under environmentally benign conditions. These results suggest that silicatein is a promising candidate for development of selective and efficient biocatalysts for organosiloxane chemistry.

Abstract

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. 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 on the order of 2–50 min ⁻¹ μM ⁻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 seem 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.