Journal of Molecular Catalysis B: Enzymatic 56 (2009) 24–28 Contents lists available at ScienceDirect Journal of Molecular Catalysis B: Enzymatic journal homepage: www.elsevier.com/locate/molcatb On the biocatalytic cleavage of silicon–oxygen bonds: A substrate structural approach to investigating the cleavage of protecting group silyl ethers by serine-triad hydrolases Andy Maraite a,1 , Marion B. Ansorge-Schumacher a,∗ , Benjamin Ganchegui b , Walter Leitner b , Gideon Grogan c a Department of Biotechnology, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany b Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany c Structural Biology Laboratory, Department of Chemistry, University of York, Heslington, York YO10 5YW, UK article info Article history: Received 19 April 2007 Received in revised form 8 April 2008 Accepted 10 April 2008 Available online 26 April 2008 Keywords: Enzymes Protecting groups Silylethers Tertiary acids Serine hydrolases abstract The biotransformation of compounds containing silicon has recently been a subject of much interest. In this study, a variety of commercially available serine hydrolases were tested for their ability to catalyse the hydrolysis of the silicon–ether bond in a variety of silyl ethers. The hydrolysis of trimethylethoxysi- lane in buffer was not found to be accelerated by the presence of trypsin, chymotrypsin, or a variety of other lipase and protease enzymes. Cleavage of a range of alternative silyl ether substrates, including a trimethylsilyl (TMS) ether, by these hydrolases was also not observed, but, interestingly, only two of the enzymes tested were able to cleave a t-butyl ,,-carboxylate that was approximately isosteric with the TMS-protected substrate. This suggests that the cleavage of Si–O bonds by serine hydrolases, such as the cathepsin homolog silicatein-, may be in part limited by steric effects, as the reactive centre in the substrate is always, by analogy to C-centred substrates, tertiary, and thus inherently sterically demanding regardless of the putative catalytic competence of the enzymes. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The biotransformation of compounds containing silicon has recently been a subject of much interest [1,2]. In addition to reports of biotransformations of organic substrates that contain silicon [3,4] and the resolution of Si-centred chiral molecules by lipases [5], the biocatalytic cleavage of the Si–O bond itself has been unearthed as an interesting biocatalytic reaction that is of emerging rele- vance. Much of this work has been prompted by an interest in the mechanisms by which siliaceous diatoms and marine sponges are able to sequester silicon from the marine environment and to bio- genetically catalyse the polymerisation of silicates to yield complex and beautiful microscopic architectures, the precise dimensions of which exceed the current capabilities of human engineering [6]. Morse and co-workers were able to demonstrate that an enzyme, silicatein-, isolated from the spicules of the marine sponge Tethya ∗ Corresponding author. Present address: Institute of Chemistry, Department of Enzyme Technology, TU Berlin, Straße des 17, Juni 124, 10623 Berlin, Germany. E-mail address: m.ansorge@chem.tu-berlin.de (M.B. Ansorge-Schumacher). 1 Present address: Institute of Chemistry, Department of Enzyme Technology, TU Berlin, Straße des 17, Juni 124, 10623 Berlin, Germany. aurantia was able to catalyse the polymerisation of silicates, in vitro, using the xenobiotic alkoxysilane tetraethoxysilane, as a substrate, via cleavage of the Si–O bond of the molecule [7]. Mutation exper- iments on the silicatein- of T. aurantia have demonstrated that Si–O bond cleavage by this enzyme is catalysed by a classical ser- ine hydrolase triad, that features in a protein whose sequence is somewhat related to that of mammalian cathepsins (Fig. 1) [8]. Other experiments have also demonstrated Si–O bond cleavage in microorganisms. Semprini and co-workers described the enrich- ment, in a chemostat, of a microbial consortium that was able to degrade tetraethoxysilane by apparent cleavage of the Si–O bond [9] and Fattakhova et al. demonstrated that the yeast Rhodotorula mucilaginosa produced a substrate-inducible esterase that catal- ysed the cleavage of the Si–O bond in a range of silatrane substrates [10,11]. Most recently, Bassindale et al. have shown that serine hydrolases such as trypsin and chymotrypsin appear to catalyse the formation of siloxane bonds at the active sites of the enzymes, yet the hydrolytic reaction, whilst accelerated in the presence of the protein, is promoted only by non-specific interactions with the enzyme in use [12]. Our interest was prompted by the possibility of using enzymes in the context of applied biocatalysis in organic synthesis. The use of silyl ether protecting groups in synthesis is widespread [13], 1381-1177/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.molcatb.2008.04.006