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OH
R
O
Zn
2+
O
O
O
Ph
Glu 270
H 2 N
(a) Base-catalysed (illustrated for thermolysin)
O
O
O
Glu 143 Glu
O
143
O
H O H
O
H
R R N
H
N
H
O O
Ph
HO
Zn
2+
Zn
2+
O
Ph
Glu 143
O
OH
R
O
Zn
2+
(b) Nucleophilic (illustrated for carboxypeptidase A)
O
O
H
O
O
O
Glu 270 H Glu 270 R H Glu
O R N
N
270 R
O −
O
O −
O
O
O
Zn
O
2+
Ph
Zn
2+
AEnz
Ph
Zn
2+
O
H
-AEnz
H
Figure 5.14 Possible base-catalysed (a) and nucleophilic (b) mechanisms for thermolysin and carboxypeptidase A. R, peptide chain.