Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

62 Chapter 4
(2) by treatment with another known chiral reagent to make a diastereomeric
derivative whose conWguration can be determined by methods such as
nuclear magnetic resonance (NMR) spectroscopy or X-ray crystallography;
(3) by treatment with an enzyme whose reaction proceeds with known stereospeciWcity.
Prochiral selectivity
Carbon centres which are surrounded by XXYZ groups are known as prochiral
centres. For example, the C-1 carbon atom of ethanol is prochiral since it is
attached to two hydrogens, one methyl group, and one hydroxyl group. This
is not a chiral centre, since there are two hydrogens attached, yet these two
protons can be distinguished by the enzyme alcohol dehydrogenase, which
oxidises ethanol to acetaldehyde as shown in Figure 4.11.
Prochiral hydrogens can be designated using a variation of the Cahn–
Ingold–Prelog rule. The convention is that the hydrogen to be assigned is
replaced by a deuterium atom (making the centre chiral), and the chirality of
the resulting centre determined using the Cahn–Ingold–Prelog rule (see Appendix
1). If the resulting chiral centre has conWguration R, then the hydrogen
atom replaced by deuterium is labelled proR. Conversely, if the deuteriumcontaining
centre is S, then the hydrogen atom is labelled proS. In the case of
alcohol dehydrogenase, the enzyme removes stereospeciWcally the proR hydrogen.
This was demonstrated by synthesising authentic samples of (1R- 2 H)- and
(1S- 2 H)-ethanol. Each sample was separately incubated with the enzyme and
the deuterium content of the product analysed in each case. In the case of the
1R substrate deuterium was removed by the enzyme, whereas with the 1S
substrate deuterium was retained in the product, as shown in Figure 4.12.
How does this enzyme achieve this remarkable selectivity If you imagine
that the ethanol molecule is Wxed in the plane of the page with the methyl group
pointing left and the hydroxyl group pointing right, as in Figure 4.12, then one
of the two hydrogens is pointing up out of the page, and the other is pointing
down into the page. Since we can visualise this situation in three dimensions, we
can easily distinguish between these two hydrogens. Thus, two hydrogen atoms
attached to a prochiral centre can be distinguished if they are held in a Wxed
orientation in a chiral environment. Enzyme active sites satisfy both these
criteria, since they are able to bind molecules in a deWned orientation using
H H
H
alcohol dehydrogenase
OH
NAD + NADH
Figure 4.11 Alcohol dehydrogenase reaction.
O