Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

168 Chapter 7
onto the acetyl-thioester intermediate, with decarboxylation of the malonyl
group, by a ketosynthase (KS) activity. This reaction, producing a
b-keto-thioester, is similar to that of the Claisen enzymes above, except that
decarboxylation occurs at the same point.
The question which then arises is whether decarboxylation occurs before,
after, or at the same time as carbon–carbon bond formation StereospeciWc
labelling studies shown in Figure 7.15 have demonstrated that this reaction
occurs with inversion of conWguration at C-2 of the malonyl unit and with little
or no hydrogen exchange at C-2. This implies that carbon–carbon bond formation
and decarboxylation are, in fact, concerted.
In the case of fatty acid biosynthesis, the new b-keto-thioester is reduced to
a b-hydroxy-thioester, eliminated to give an a,b-unsaturated thioester, and
then further reduced to give a two-carbon-extended acyl chain, as shown in
Figure 7.14. In the case of polyketide biosynthesis, each two-carbon unit can
be processed as either the b-keto-thioester, the b-hydroxy-thioester, the
a,b-unsaturated thioester or as the fully reduced thioester. Assembly of each
polyketide is therefore controlled by the arrangement of processing enzyme
activitites on the polyketide synthase multi-enzyme complex. How is this done
Information regarding the molecular structure and organisation of polyketide
synthases is now emerging from the cloning and sequencing of genes which
encode these enzymes. The genes responsible for the biosynthesis of the
polyketide antibiotic erythromycin have been identiWed and their nucleotide
sequences determined. They encode three huge multi-functional polypeptides of
size 300–500 kDa, illustrated in Figure 7.16. The enzyme activities responsible
for processing of the growing polyketide chain have been identiWed by amino
acid sequence alignments, and are found sequentially along the polypeptide
chains. Remarkably, the arrangement of processing enzyme activities on the
polyketide synthases matches the order of chemical steps required for biosynthesis
of the polyketide precursor. It, therefore, appears that these multienzyme
complexes function as molecular production lines built up of ‘modules’
of enzyme activities.
H
D T
O
SCoA
acetyl CoA
carboxylase
biotin
CO 2
ATP
− O2 C
D T
O
SCoA
fatty acid
synthase
RCO-S-ACP
− O2 C
O
D T
O
R
S-ACP
S-Enz
R
O
T D
ketosynthase
ketoreductase
O
S-ACP
NADPH
R
T
O
S-ACP
enoyl
reductase
NADPH
R
T
O
S-ACP
dehydratase
R
H OH O
T D
S-ACP
Figure 7.15 Stereochemistry of fatty acid biosynthesis.