Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymes are Wonderful Catalysts 43
Table 3.1 Amino acid side chains used for nucleophilic catalysis in enzymatic reactions.
Amino acid Side chain Examples
Serine 2CH 2 2OH Serine proteases, esterases, lipases
Threonine 2CH(CH 3 )2OH Phosphotransferases
Cysteine 2CH 2 2SH Cysteine proteases, acyl transferases
Aspartate, glutamate 2(CH 2 ) n 2CO 2 H Epoxide hydrolase, haloalkane dehalogenase
Lysine 2(CH 2 ) 4 2NH 2 Acetoacetate decarboxylase, class I aldolases
Histidine imidazole2NH Phosphotransferases
Tyrosine Ar2OH DNA topoisomerases
Ar, aromatic group.
treatment of enzyme with substrate and sodium borohydride leads to irreversible
enzyme inactivation, via in situ reduction of the enzyme-bound imine
intermediate by borohydride. Treatment of enzyme with borohydride alone
gives no inactivation. Second, incubation of acetoacetate labelled with 18 Oat
the ketone position leads to the rapid exchange of 18 O label out of this position,
consistent with reversible imine formation. As mentioned above, the pK a of this
lysine group is abnormally low at 5.9, which is suYciently low for it to act as a
nucleophile at pH 7.
The other nitrogen nucleophile available to enzymes is the versatile
imidazole ring of histidine. This group is more often used for acid/base chemistry,
but is occasionally used as a nucleophile in, for example, phosphotransfer
reactions. Finally, enzymes have oxygen nucleophiles available in the form of
Lys
NH 2
acetoacetate
O O
decarboxylase
O
O−
+ CO 2
Lys
Lys
Lys
+ +
NH O
NH
NH
O−
H
EnzA
Figure 3.17 Mechanism for acetoacetate decarboxylase.