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Methods for Studying Enzymatic Reactions 63
4
H
H 3 C
2
3
D
OH
1
enzyme removes proR hydrogen
H S
Zn 2+
H R
H H
is R enantiomer
OH
H 3 C O
OH
NAD + O
H
H D
H
alcohol dehydrogenase
OH
NAD +
O
D H
D
alcohol dehydrogenase
Figure 4.12 Stereochemistry of alcohol dehydrogenase.
speciWc enzyme–substrate binding interactions, and of course the enzyme active
site is chiral.
Examination of the X-ray crystal structure of alcohol dehydrogenase reveals
that the C-1 oxygen substituent is bound by an active site Zn 2þ cofactor, and
the methyl group is bound in such a way that the proR hydrogen is pointing
directly at the NAD þ cofactor, as shown in Figure 4.13. The prochiral selectivity
can therefore easily be explained by the orientation adopted by the substrate
in the enzyme active site.
Figure 4.13 Active site of alcohol dehydrogenase (PDB Wle 1HLD), showing the positioning of a
bound pentaXuorobenzyl alcohol substrate. Zn 2þ cofactor and bound substrate shown in red,
NAD þ cofactor shown in black.