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Enzymatic Addition/Elimination Reactions 203
HO
O
P OH
OH
−H +
+H +
HO
O
P
O −
OH
−H +
+H +
HO
O
P
O −
O −
−H +
+H +
− O
O
P
O −
O −
pK 1 = 2.1 pK 2 = 7.2 pK 3 = 12.3
Figure 8.15 pK a values for phosphate anions.
ester would be singly deprotonated, thus for departure of a phosphate leaving
group the pK a of its conjugate acid (H 2 PO 4 ) would be 7.2 – thus phosphate is a
much better leaving group than a hydroxide ion.
Two important examples of the elimination of phosphate are shown in
Figure 8.16. The Wrst is the formation of isopentenyl pyrophosphate by an
eliminative decarboxylation. The enzyme catalysing this reaction has been
puriWed from baker’s yeast, and has been shown to be strongly inhibited by a
tertiary amine substrate analogue (K i ¼ 0:8 mm). At neutral pH the tertiary
amine will be protonated, thus the potent inhibition could be explained by
this analogue mimicking a tertiary carbonium ion intermediate in the mechanism
of this enzyme, as shown in Figure 8.17.
Chorismate synthase catalyses the 1,4-elimination of phosphate from
5-enolpyruvyl-shikimate-3-phosphate (EPSP) to give chorismic acid (see Figure
8.1). The stereochemistry of the enzymatic elimination in E. coli has been shown
to be anti-. The highly unusual feature of this enzymatic reaction is that the
enzyme requires reduced Xavin as a cofactor, although the overall reaction
2− O 3 PO
H 3 C
− O 2 C
CO 2
−
OH
OPO 3
2−
H R
O
OPP
CO 2
−
+ HOPO 2−
3 + CO
OPP
2
CO −
2
chorismate
synthase
+ HOPO 2−
3
FMNH 2
O CO −
2
OH
Figure 8.16 Enzymatic eliminations of phosphate. FMNH 2 , Xavir mononuclectide (reduced).
Inhibitor
H 3 C H
N
− O O
K i 0.8 µM
OPP
Transition state
PO 2−
3 − O
H 3 C
OPP
− O O
Figure 8.17 Transition state inhibitor for mevalonate pyrophosphate decarboxylase.