Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Isomerases 233
Cys 139
Cys 139
S
− S
H
H +
OPP
OPP
H H S R
H HR
S
O O
Glu 207
analogue of
carbonium ion
Cys 139
−
intermediate
S
N +
H OPP
OPP
H
K i 1.4 x 10 −11 M
O O
Figure 10.10 Mechanism for isopentenyl pyrophosphate isomerase.
Glu 207
1,3-antarafacial hydrogen shift, since the hydrogen is re-inserted on the opposite
face of the molecule.
The antarafacial stereochemistry of hydrogen transfer suggests the involvement
of two active site groups in the mechanism. This was conWrmed by
subsequent chemical modiWcation studies. Site-directed mutagenesis experiments
have implicated Cys-139 as an acidic group which protonates the
4,5-double bond, generating a tertiary carbonium ion intermediate. StereospeciWc
deprotonation is then carried out by Glu-207, as shown in Figure 10.10.
The existence of the carbocation intermediate is supported by the potent inhibition
of this enzyme (K i 1:4 10 11 m) by a tertiary amine substrate analogue.
At neutral pH this amine is protonated, and the ammonium cation mimics the
carbocation intermediate which is bound tightly by the enzyme.
10.5 CASE STUDY: Chorismate mutase
Pericyclic reactions are commonly used in organic synthesis; the Diels–Alder
reaction being an important synthetic reaction for the stereoselective synthesis
of cyclohexane rings. However, only one enzyme-catalysed pericyclic reaction
has been well characterised, which is the reaction catalysed by chorismate
mutase.
Chorismate mutase catalyses the Claisen rearrangement of chorismic acid
into prephenic acid. This reaction is important for plants and micro-organisms,