Abstract

Silicatein, a silica-synthesizing, catalytic triad hydrolase, was discovered in the silica spicules comprising the skeletons of certain marine sponges. Sequence similarity is closest to that of the mammalian cathepsin L, a catalytic triad hydrolase and protease. Genetic substitutions of residues in the catalytic triad, the predictive activities of polymeric and small-molecule analogs of the enzyme, and the wide range of structures accepted as substrates all support a reaction mechanism closely analogous to that established for the classical catalytic triad hydrolases. In this mechanism, hydrogen bonding of residues in the catalytic site is required to enhance nucleophilic attack and consequent hydrolysis of silicon alkoxide (and a wide range of other precursors), enabling subsequent polycondensation. Experimental and computational analyses revealed a novel pathway of self-assembly, in which the silicatein subunits first form a fractally patterned intermediate before entropic rearrangement to the hexagonally close-packed, macroscopic filament that serves both as the catalyst of silica synthesis in the sponge, and as a template guiding the deposition and emergent structure of the macroscopic silica filaments that form the sponge skeleton. Silicatein also proves capable of catalyzing the synthesis of organic silicones, metal oxides, metal phosphates, polylactides, and polymeric materials composed of organic metal compounds from their corresponding precursors, suggesting an evolutionary relaxation of structural substrate specificity that may have been necessary to accommodate the organic adducts of silicic acid suggested to comprise the natural precursor of the biogenic silica. Methods for purification, characterization, assay, and multiple uses of the enzyme are described.