Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

30 Chapter 3
O
N
H
R
CO 2 H
acylase
H 2 N
R
CO 2 H
+
O
N
H
R
CO 2 H
C. cylandricea
lipase
+
OAc OH OAc
(±) (+) (−)
AcO
C. cylandricea
lipase
no reaction
Figure 3.2 Stereoselectivity in enzymatic hydrolysis reactions.
Enzymes are highly selective in the reactions that they catalyse. Since they
bind their substrates via a series of selective enzyme–substrate binding interactions
at a chiral active site, they are able to distinguish the most subtle
changes in substrate structure, and are able to distinguish between regioisomers
and between enantiomers, as shown in Figure 3.2. Finally, enzymes carry out
their reactions with near faultless precision: they are able to select a unique site
of action within the substrate, and carry out the enzymatic reaction stereospeciWcally,
as illustrated in Figure 3.3.
In this chapter we shall examine the factors that contribute to the remarkable
rate acceleration achieved in enzyme-catalysed reactions. Examples of enzyme
stereospeciWcity will be discussed in Chapter 4. It is worth at this point distinguishing
between selectivity, which is the ability of the enzyme to select a certain
substrate or functional group out of many; and speciWcity, which is a property of
CO 2 Me
pig liver
esterase
CO 2 H
CO 2 Me
CO 2 Me
pig liver esterase
AcO
OH
AcO
OAc
porcine pancreatic lipase
Figure 3.3 Stereoselectivity in enzymatic hydrolysis reactions.
HO
OAc