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230 Chapter 10
‘low-barrier’
+ hydrogen bond
Lys 166 NH
Lys 166 NH 3
2
O
O
Glu 317 Glu 317
H O H O
O – H O
+
HO O– NH 3 Lys 164
N + H
+
O–
HN NH +
HN
NH 3 Lys 164
Mg 2+ His
Mg 2+
297
His 297
+
Lys 166 NH 3
O
Glu 317
O H O
H
+
HO O– NH 3 Lys 164
HN N
Mg 2+
Figure 10.5 Mechanism for mandelate racemase.
His 297
HO
proposed that Glu-317 forms a strong ‘low barrier’ hydrogen bond with the
substrate carboxylate which stabilises the formation of the a-carbanion, as
shown in Figure 10.5. It may be that similar methods of stabilisation are
employed in other cases of enzymatic deprotonation adjacent to carboxylate
groups.
10.3 Keto–enol tautomerases
The enolisation of ketones is a well-known reaction in organic chemistry that is
utilised as an intermediate process in many enzyme-catalysed reactions, notably
the aldolases encountered in Section 7.2. In most cases enols and enolate anions
are thermodynamically unstable species that are not isolable. However, in a few
cases enol tautomers of ketones are suYciently stabilised to be isolable, and
there are several enzymes which catalyse the interconversion of keto and enol
tautomers.
One simple example is that of phenylpyruvate tautomerase, which catalyses
the interconversion of phenylpyruvic acid with its stabilised enol form, as
shown in Figure 10.6. The enzyme catalyses the stereospeciWc exchange of the
proR hydrogen with 2 H 2 O via the E enol isomer, using acid/base active site
chemistry.