Chemistry Reference
In-Depth Information
8.2 CHEMISTRY
Classical feeding experiments with both stable and radioactive isotopic labels
(7) enabled the biosynthetic origin of the polyethers to be elucidated and for
a general stereochemical model to be proposed (1). More recent work on this
class of compounds has focused on a genetic approach, and unusual and inter-
esting genes specific to polyether biosynthesis have been isolated from these
clusters.
8.2.1 Monensin
Monensins A
2
and B
3
are polyether ionophores produced by
Streptomyces
cinnamonensis
that differ only in the sidechain at C16 (ethyl/methyl). Monensin
acts as a specific ionophore to dissipate ionic gradients across cell membranes and
is used widely in veterinary medicine and as a food additive in animal husbandry
(7). Antimalarial activity has also been reported (8, 9). Monensin is the best
studied of the polyether ionophore antibiotics, and it was the first to have its
gene cluster sequenced (2).
Early feeding studies established that monensin A is biosynthesized from a
classical polyketide pathway and is derived from five acetate, seven propionate,
and one butyrate (for monensin B, an additional propionate unit replaces the
butyrate) (7). Four of the nine oxygen atoms are derived from molecular oxy-
gen, with the remaining five deriving from the corresponding carboxylic acid
precursors (Fig. 8.2a). Based on these initial experiments, it was proposed that
the monensin PKS produced a linear
E,E,E-
triene precursor
24
that was oxidized
and cyclized to give the final structure (1). Alternative proposals using
Z,Z,Z
-
and
E
,
Z,Z
-trienes have been made (2).
Publication of the gene cluster for monensin (Table 8.2) (2, 13-17) showed
that the PKS comprised 12 modules in eight contiguous open reading frames
consistent with the production of the linear triene premonensin
25
. The loading
module contains an N-terminal KSQ domain that functions as a malonylCoA
decarboxylase to generate starter units
in situ
. The monensin PKS does not con-
tain an integrated C-terminal thioesterase domain at the end of module 12; instead
an unusual 121 amino acid extension, rich in glycine, asparagines, and glutamine,
was found (13).
MonCII
, which was originally assigned as an epoxide cyclase,
has been shown to hydrolyze monensinyl NAC thioester
21
(Fig. 8.2c) as well as
two other model substrates (14). Deletion of
monCII
gave a mutant that produced
none or only trace amounts of monensin, which is consistent with its role as a
chain-terminating TE. Complementation of
monCII
on a plasmid restored mon-
ensin production. Moreover, cell-free extracts from the
monCII
mutant treated
with KOH gave significant amounts of monensin that resulted from the hydroly-
sis of a monensinyl ester or thioester present in the cell-free extract. Two ORFs,
monAIX
and
monAX
, have been shown to function as Type II thioesterases. Dele-
tion of
monAIX
and/or
monAX
resulted in a modest drop in the monensin titer,
which is consistent with their editing Type II TE role (14).
