Chemistry Reference
In-Depth Information
Fig. 3.4
The hydrolysis of PTMS. The pseudo first order rate constant increases as the enzyme
concentration is increased. Legend: Pepsin-
black circles
, α-chymotrypsin-
red squares
, trypsin-
purple diamonds
. Error bars indicate the standard deviation of triplicate trials [
32
]
The observed enhanced rate of hydrolysis by pepsin can be explained in two
ways. Catalysis by pepsin is carried out by an aspartic acid dyad. One of the aspartic
acid residues exists as the acid form while the other exists as the carboxylate. Work-
ing in tandem these two amino acids polarize a water molecule making it an active
nucleophile. An acyl enzyme intermediate is not formed and if a similar mode of
action is in place here then the absence of a covalently bound PTMS in the active
site may serve to facilitate hydrolysis. Like pepsin, α-chymotrypsin favorably binds
the aromatic side chains of several amino acids. The observed differences may
potentially be attributed to the formation of an acyl-silane intermediate between
α-chymotrypsin and PTMS in a manner similar to that proposed for the silicatein-
mediated hydrolysis of TEOS. The enhanced catalytic activity of α-chymotrypsin
over trypsin may be attributed to the binding of PTMS in the
S1
pocket; the
S1
pocket of trypsin has not evolved to accept a hydrophobic phenyl ring.
3.5
Conclusions
Several enzymes have been shown to be catalytically active towards some alkoxysi-
lanes.
29
Si NMR spectroscopy was employed to elucidate the intermediates along
the reaction pathway of the hydrolysis of some of these alkoxysilanes. While not all
enzymes have a capacity for performing such chemistry, those that can must have an
