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234 Chapter 10
O
H
NH 3
CO 2
−
CO 2
−
CO 2
−
chorismate
mutase
− O 2 C
O
CO 2
−
prephenate
dehydratase
aminotransferase
PMP PLP
O CO −
2
OH
chorismic acid
OH
prephenic acid
prephenate
dehydrogenase
NAD +
O
CO 2
−
aminotransferase
H
NH 3
CO 2
−
PMP
PLP
OH
OH
Figure 10.11 Reaction catalysed by chorismate mutase.
which utilise prephenic acid as a precursor to the amino acids l-phenylalanine
and l-tyrosine (see Figure 10.11). This reaction occurs non-enzymatically at a
signiWcant rate, contributing to the singular instability of the important metabolite
chorismic acid. However, the reaction is accelerated 10 6 -fold by the
enzyme chorismate mutase.
The stereochemistry of the enzyme-catalysed and uncatalysed reactions has
been investigated, revealing that both reactions proceed through a chair-like
transition state involving the trans-diaxial conformer of the substrate, as shown
in Figure 10.12. Tritium substitution at C-5 or C-9 gives no secondary kinetic
isotope eVect on the enzyme-catalysed reaction, whereas such isotope eVects are
observed on the uncatalysed reaction. This suggests that the rate-determining
step of the enzyme-catalysed reaction is substrate binding, hence isotope eVects
on subsequent steps would not be observed. This situation, in which there is a
high preference for catalytic processing of a bound substrate, rather than
dissociation, is termed a ‘high commitment to catalysis’.
How does the enzyme achieve the 10 6 -fold rate acceleration If the mechanisms
of the uncatalysed and enzyme-catalysed reactions are similar, does the
– O 2 C
9
– O 2 C
– O 2 C
CO 2
–
O
CO 2
–
O
CO – O
2 CO –
2
HO
O
5
– O 2 C
di-equatorial conformer
OH
di-axial conformer
OH
OH
Figure 10.12 Mechanism for chorismate mutase.