Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

172 Chapter 7
O
HO O − O
ATP
ADP
2−
HO OPO 3
O
O −
HN NH
N NH
H H
H H
Enz
S
S
O
O O
− O C
O
CoAS
O − CoAS
Enz
− O
CoAS
O
N
H
O
O
NH
H
Enz
S
H
− B Enz
Figure 7.18 Mechanism for biotin-dependent carboxylation.
− O 2 C
O
H
T D
pyruvate
carboxylase
biotin, ATP, CO 2
− O2 C
O
−
CO 2
T D
NADH
malate
dehydrogenase
− O 2 C
CO 2
−
fumarase
− O2 C
OH
CO 2
−
D
3 H 2 O
T D
Figure 7.19 Stereochemistry of the pyruvate carboxylase reaction.
The reaction catalysed by Rubisco is the carboxylation and concomitant
fragmentation of ribulose-1,5-bisphosphate to generate two molecules of
3-phosphoglycerate (see Figure 7.20). This is a step on the Calvin cycle of
green plants, which transforms 3-phosphoglycerate via a series of aldolase
and transketolase enzymes once again into ribulose-1,5-bisphosphate. Thus
each cycle incorporates one equivalent of carbon dioxide into cellular carbon.
The enzymatic reaction has been shown to commence by deprotonation at
C-3 of ribulose-1,5-bisphosphate, and enolisation of the C-2 ketone, since
[3- 3 H]-ribulose-1,5-bisphosphate rapidly exchanges 3 H with solvent in the presence
of enzyme. The resulting enediol intermediate reacts with carbon dioxide
to generate a carboxylated intermediate. Carbon–carbon bond cleavage is
thought to occur by hydration of the C-3 ketone, followed by fragmentation
of the C-2,C-3 bond to generate a carbanion product, which protonates to give