Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Carbon–Carbon Bond Formation 189
(5) Suggest a mechanism for the TPP-dependent transketolase reaction
illustrated in Figure 7.25.
(6) Pyruvate oxidase catalyses the oxidative decarboxylation of pyruvate to
acetate (not acetyl CoA). The enzyme requires TPP and oxidised Xavin as
cofactors, and utilises oxygen as an electron acceptor (i.e. it is converted to
hydrogen peroxide). Suggest a mechanism.
O
CO 2
−
pyruvate
O
oxidase
+ CO
TPP, FAD,O O −
2
2
(7) The conversion of geranyl pyrophosphate into bornyl pyrophosphate is
catalysed by bornyl pyrophosphate synthetase. Write a mechanism for the
enzymatic reaction. When the enzyme was incubated with geranyl pyrophosphate
speciWcally labelled with 18 O attached to C-1, the product was found
to contain 18 O only in the oxygen attached to carbon, and not in any of the
other six oxygens. Comment on this remarkable result.
O − O − O − O −
O
O −
P O P
O O
bornyl PP
synthetase
Mg 2+
O P O P
O
O
O −
(8) Kaurene synthetase catalyses the cyclisation of geranylgeranyl pyrophosphate
(GGPP) to kaurene. An intermediate in the reaction is thought to
be copalyl pyrophosphate. Suggest mechanisms for conversion of GGPP
into copalyl PP and thence to kaurene. How would you attempt to prove
that copalyl pyrophosphate is an intermediate in this reaction
OPP
GGPP
(C 20 )
H
H
H
H
copalyl PP
kaurene