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Enzymatic Transformations of Amino Acids 219
adduct of PLP with l-threonine, which is deprotonated as above to generate the
familiar quinonoid species. In this case the hydroxyl substituent at the b-position
acts as a leaving group, presumably with acid catalysis, and an a,belimination
reaction ensues. Displacement of the imine linkage by the active
site lysine residue releases the enamine equivalent of a-ketobutyrate. Hydrolysis
of the enamine generates a-ketobutyrate and ammonia, and regenerates the
PLP cofactor. The mechanism is depicted in Figure 9.13.
Methionine g-lyase catalyses the conversion of l-methionine into
a-ketobutyrate, ammonia and methanethiol (a particularly smelly enzyme to
work with!). The mechanism of this enzyme starts from the aldimine adduct of
PLP with l-methionine, which is deprotonated to generate the quinonoid
intermediate. However, at this point a second deprotonation takes place at
the b-position, utilising the adjacent iminium species as a second electron sink
to stabilise the b-carbanion. Elimination of the g-substituent can then take
place, followed by reprotonation at the g-position, to generate the enamine
intermediate seen above. The enamine equivalent of a-ketobutyrate is released,
which after hydrolysis generates a-ketobutyrate and ammonia. The mechanism
is depicted in Figure 9.14.
There are a number of other enzymes in this class, most of which also
employ a second deprotonation step, eVectively utilising the PLP-amino acid
iminium salt as a four-electron sink. The cellular role of these enzymes is often
for the degradation of the respective amino acids and recycling of their nitrogen
content.
OH
− BEnz
H
CO −
2
OH H BEnz
CO 2
−
CO 2
−
NH +
NH + NH +
OH
2− OH
O 3 PO 2− O 3 PO 2− O 3 PO
+
N
+
N
H
N
H
H
displacement
by Lys-NH 2
OH
NH 3
CO −
2
+
H 2 O
O
Figure 9.13 Mechanism for threonine dehydratase.
NH
+ 2
CO 2
−
NH 2
CO 2
−