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Methods for Studying Enzymatic Reactions 61
then it will be chiral. One simple example shown in Figure 4.9 is that of the
amino acid l-alanine, which is a substrate for several pyridoxal 5 0 -phosphate
enzymes described in Chapter 9. Chiral centres can be designated as R or S
depending on the relative orientation of the four groups, according to the
Cahn–Ingold–Prelog rule (see Appendix 1). Thus l-alanine is more formally
written as 2S-alanine.
Enzyme-catalysed reactions are in general both stereoselective and stereospeciWc.
Stereoselectivity is the ability to select a single enantiomer of the
substrate in the presence of other isomers. StereospeciWcity is the ability to
catalyse the production of a single enantiomer of the product via a speciWc
reaction pathway. StereospeciWcity in enzyme catalysis arises from the fact that
catalysis is taking place in an enzyme active site in which the bound substrate is
held in a deWned orientation relative to the active site groups (unnatural
substrates which can adopt more than one orientation in an enzyme active
site may be processed with less stereospeciWcity). How do we elucidate the
stereochemical course of an enzymatic reaction
Generation of a chiral product
In many cases enzymes are able to generate a product containing one or more
chiral centres from achiral substrates – a process known as asymmetric induction.
For example, the aldolase enzyme fructose-1,6-bisphosphate aldolase
(see Chapter 7) catalyses the aldol condensation of achiral substrates dihydroxyacetone
phosphate and acetaldehyde to generate a chiral aldol product, as
shown in Figure 4.10.
If a chiral product is formed, then its absolute conWguration can be determined
in the following ways:
(1) by chemical conversion to a compound of established absolute conWguration,
and measurement of optical activity;
4
H
CH 3
3
H 2 N CO 2 H
1 2
S
L-alanine = 2S-alanine
Figure 4.9 A chiral molecule.
2− O 3 PO
O
OH
+
H
O
fructose-1,6-bisphosphate
aldolase
2− O 3 PO
O
OH
OH
Figure 4.10 Production of a chiral product.