Chemistry 30A3 Fall 2011 Labs Dr. Troendle 57 - 7...

Chemical Biology 3OA3, September 2009 Page 57
7. I
NTERPRETING YOUR DATA
The following pieces of information will help you interpret your data.
(1) Active site serine
Esterases use a Ser residue as an active site nucleophile to form a covalent acyl-enzyme
intermediate, and release the first product, HO-R
2
(Scheme 2). In the second step, water acts as a
nucleophile to complete the reaction, releasing R
1
-COOH. Thus, we know that if a diester
substrate is cleaved at a particular site, then that ester group was located near the active site Ser
during catalysis.
R
1
O
R
2
O
Scheme 2.
HO
R
2
R
1
O
O
esterase HO esterase
R
1
COOH
H
2
O
esterase HO
(2) Hydrophobic substrates
Esterases tend to favour hydrophobic (non-polar) substrates. The hydrophobic
interactions between non-polar groups are relatively non-specific - the tightest binding (and
highest rate) is achieved with substrates that fill the active site to the greatest extent without
causing steric clashes. Substituents that are too large or too small will be worse substrates.
Substituents with polar functional groups may require you to consider electronic effects (electron