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174 Chapter 7
CO 2
−
vitamin K-dependent
carboxylase
O
− Ca 2+
O
O −
O
N
H
O
vitamin K, CO 2 , O 2
N
H
O
OH
reduced vitamin K
O
OH
3
carboxylase
CO 2 , O 2
prothrombin
O
O
R
reductase
O
reductase
NADH
NADH
Figure 7.22 The vitamin K-dependent carboxylase reaction.
O
R
vitamin K is converted stoichiometrically into vitamin K epoxide, which is
recycled via separate reductase enzymes back to reduced vitamin K.
A major clue to the role of vitamin K in the enzymatic reaction came from a
chemical model reaction shown in Figure 7.23. In the presence of molecular
oxygen and a crown ether, a naphthol analogue of reduced vitamin K was
O − K +
CO 2 Et
O
CO 2 Et
+
CO 2 Et
O 2
18-crown-6
THF
OH
O
+
O
O 2
O
O −
O
EtO
O O − O O
O −
O
O
H
CO 2 Et
Figure 7.23 Model reaction for role of vitamin K.