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Enzymatic Carbon–Carbon Bond Formation 173
2−
CH 2 OPO 3
O
OH
HO
2−
CH 2 OPO 3
ribulose-1,5-bisphosphate
2−
CH 2 OPO 3
HO
CO −
2
Rubisco
+
Mg 2+ , Cu 2+
CO −
2
CO 2 HO
2−
CH 2 OPO 3
2 phosphoglycerate
Figure 7.20 Reaction catalysed by ribulose-1,5-bisphosphate carboxylase.
the second molecule of 3-phosphoglycerate. The proposed mechanism is shown
in Figure 7.21.
EnzB −
H
2−
CH 2 OPO 3
O
OH
H AEnz
OH
2−
CH 2 OPO 3
O
C
O
2−
CH 2 OPO 3
OH − AEnz
O H
OH
2−
CH 2 OPO 3
Enz
− O 2 C
H O
H
B −
2−
CH 2 OPO 3
OH
O
OH
2−
CH 2 OPO 3
− O2 C
CH 2 OPO 3
2−
OH
3 H
3 H 2 O
− O 2 C
HO
2−
CH 2 OPO 3
− OH
O
OH
2−
CH 2 OPO 3
− O 2 C
HO
2−
CH 2 OPO 3
OH
O −
OH
2−
CH 2 OPO 3
Figure 7.21 Mechanism for the ribulose-1,5-bisphosphate carboxylase reaction.
7.7 Vitamin K-dependent carboxylase
One other carboxylase enzyme worthy of note is the vitamin K-dependent
carboxylase responsible for the activation of prothrombin via carboxylation
of a series of glutamate amino acid residues. This step is involved in the
calcium-dependent activation of platelets during the blood clotting response.
Vitamin K is a fat-soluble naphthoquinone which can exist in either oxidised
(quinone form) or reduced (hydroquinone form) states. The carboxylase
enzyme is also an integral membrane protein which has only recently been
puriWed. The carboxylation of prothrombin requires reduced vitamin K, carbon
dioxide, oxygen and a carboxylation substrate (see Figure 7.22). Reduced