Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

218 Chapter 9
Arg 292 Asp 292
HN
H 2 N
H 2
N
O
−
O
H
O
− O + NH 2
O
NH 2 H O
NH 2
N
H 2 H
H 2 N
H 2
H
− N N Arg 386 H
N N
O
O
Arg 386
NH + NH +
2− O 3 PO
OH
2− O 3 PO
OH
+
N
H
Binding of L-aspartate to native enzyme
+
N
H
Binding of L-arginine to R292D mutant
Figure 9.11 Interaction of l-arginine substrate with Asp-292 of R292D mutant enzyme.
mutagenesis can be used to ‘engineer’ the substrate speciWcity of enzyme active
sites.
9.4 Reactions at the b- and g-positions of amino acids
There is a smaller group of enzymatic reactions that take place at the b- and
g-positions of a-amino acids which are also dependent upon PLP. These
reactions also make use of the PLP cofactor as an electron sink, but we shall
see that there are examples in this class in which PLP acts as a four-electron sink
rather than a two-electron sink. I shall illustrate one example of a reaction at
the b-position, threonine dehydratase, and one at the g-position, methionine
g-lyase (see Figure 9.12).
Threonine dehydratase catalyses the conversion of l-threonine into a-
ketobutyrate and ammonia. The enzymatic reaction starts from the aldimine
OH
H
CO −
2
NH 3
threonine
dehydratase
PLP
O
CO 2
−
+ NH 4
+
H
methionine
S
CO −
CO −
H 2
γ-lyase
2
3 C
+ H + NH + 2 O 4 + CH 3 SH
NH PLP
3
O
Figure 9.12 Reactions at the b- and g-positions of a-amino acids.