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10 Isomerases
10.1 Introduction
The Wnal group of enzymatic transformations that we shall meet are the
isomerisation reactions: the interconversion of two isomeric forms of a molecule.
The interconversion of enantiomers is catalysed by racemase enzymes, as
discussed in Section 9.2. In this chapter we shall meet a second family of
racemases that require no cofactors. We shall also meet enzymatic proton
transfer reactions which involve the interconversion of tautomeric forms of
ketones, and the interconversion of positional isomers of allylic compounds.
These isomerisation reactions are summarised in Figure 10.1.
10.2 Cofactor-independent racemases and epimerases
In Section 9.2 we met the family of a-amino acid racemase enzymes which
utilise the coenzyme pyridoxal 5 0 -phosphate (PLP). In addition there is a family
of racemase and epimerase enzymes which require no cofactors at all.
A separate section is being devoted to these enzymes because of a single
important mechanistic issue: how do these enzymes achieve the deprotonation
of the a-proton of an a-amino acid In the PLP-dependent racemases the
formation of an imine linkage with the a-amino group dramatically increases
the acidity of the a-proton. If these enzymes contain no PLP then no such
assistance is possible, so they must have some alternative way of carrying out
this deprotonation. This question is of wider signiWcance, since there is evidence
for enzymatic deprotonation adjacent to carboxylic acids in other enzymes such
as the Xavin-dependent amino acid oxidase enzymes (see Section 6.3).
Examples of cofactor-independent a-amino acid racemases are glutamate
racemase from Lactobacillus fermenti and aspartate racemase from Streptococ-
Racemases, epimerases
Allylic isomerases X = C
Keto/enol tautomerases X = O
H
X
H
X
Y
Z
X
X
H
Y
Z
H
Pyrophosphate isomerases X = OPP
X
X
Figure 10.1 Summary of enzyme-catalysed isomerisation reactions.
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