Metal oxide formation by serine and cysteine proteases

Silicatein-α, a member of the cathepsin-L family of protease enzymes, has previously been identified as a mediator for the formation of metal oxides such as silica (K. Shimizu, J. Cha, G. D. Stucky and D. E. Morse, Proc. Natl. Acad. Sci. U. S. A., 1998, 95, 6234). The active site of silicatein-α, a serine-histidine-asparagine triad, prompted us to investigate metal oxide formation in protease enzymes with similar active sites. Three enzymes were explored: the cysteine proteases papain and TEV (tobacco etch virus protease), and the serine protease trypsin. It was observed that papain and trypsin were able to mediate the formation of titania at room temperature and neutral pH. Only papain was able to mediate the formation of silica, and TEV was unable to mediate the formation of either titania or silica. The metal oxide formation activities of papain and trypsin were inhibited by heat denaturation, suggesting a role of protein tertiary structure on condensation activity. These activities are discussed in terms of sequence homology to silicatein-α and active site accessibility.