The silicases are a family of enzymes that catalyze rearrangement of silicon-oxygen bonds and are found in some sea sponges.
The by-product of this reaction is silicic acid.
A zinc atom is held in place by three histidine residues. A fourth position on the zinc extracts a hydroxy group from water.
This reacts with an amorphous fragment of hydrated silica to break off silicic acid and leave zinc attached to the larger silica frament. Zn-O-H + Si-O-Si(OH)3 → Zn-O-Si + H-O-Si(OH)3 This is then hydrolysed: Zn-O-Si + H2O → Zn-OH + HO-Si
Silicases are related to a more common enzyme in animals, carbonic anhydrase. In the human body the carbonic anhydrase most similar to the sponge silicase is type II (CA-II).
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