Possible reaction mechanisms for cleavage of silyl ethers by SilE-R and SilE-S. A) Mechanism 1: nucleophile-covalent. H79 directly attacks the silicon while H8 transfers a proton to the silyl ether bond, releasing the alcohol moiety from the enzyme. The deprotonated H8 deprotonates a water molecule creating a hydroxyl, which is transferred to the silyl moiety releasing the silanol from the enzyme. M62 temporarily stabilises the positively charged histidines electrostatically. B) Mechanism 2: acid-base. H79 drives a nucleophilic attack on a water molecule forming a hydroxyl. This hydroxyl attacks the silyl moiety while H8 transfers a proton to the ether oxygen, cleaving the bond and releasing the newly formed silanol. To assist this, H8 forms a dyad with D97. M62 further enables this mechanism by transiently electrostatically stabilising the positive charge of H8. The proton from the original water molecule still bound to H79 is now transferred to the alcohol, which is also released from the enzyme.
