Chemistry Reference
In-Depth Information
Isotopic feeding experiments using
13
C-labeled acetate indicated the folding
pattern shown in Fig. 6.2b for the naphthopyrone (
39
). Thus, both the isocoumarin
(
38
) and the naphthopyrone (
39
) must result from the cyclization of the common
intermediate (
40
). This finding suggests that the WAS chain extension compo-
nent produces a heptaketide and catalyzes the cyclization and aromatization of
the first ring. The C-terminal domain must therefore catalyze a second (Claisen)
cyclization reaction to form (
42
). The involvement of conserved serine and his-
tidine residues suggests involvement of the CLC-bound intermediate (
41
) shown
in Fig. 6.2b. Thus, the TE domain has been renamed as Claisen Cyclase (CLC).
These domains also occur in the known NSAS and THNS proteins in which the
same chemistry must occur to provide the observed products.
6.4.4 C-MeT Domains
Few NR PKS are known to possess
C
-methylation domains, although
many known fungal nonreduced polyketides are
C
-methylated, such as
3,5-dimethylorsellinic acid. A small group of NR PKS have been identified in
genome sequences (5), which feature a
C
-MeT domain located after the ACP
(Fig. 6.2c). The first correlation between a gene sequence and a compound came
from citrinin (
3
), in which the PKS involved in citrinin biosynthesis in
Monascus
ruber
was reported (11). Here, the
C
-MeT domain must be programmed
because it acts twice during polyketide biosynthesis when a probable methylated
diketide starter unit is extended. It is not yet clear whether the
C
-MeT domain
acts during extension, after chain extension but before aromatization, or after
aromatization. 1,3-Dihydroxyaromatics are known to tautomerize easily to keto
forms, and it is conceivable that it could act as the nucleophile for the reaction
with SAM.
6.4.5 R Domains
Reductases are currently rare as part of the processing component of NR PKS.
Evidence for the role of these reductase domains has been obtained by the isola-
tion of the PKS responsible for the formation of the tetraketide component found
in xenovulene A (
22
). The PKS gene (MOS) was found to have SAT, KS, AT,
C-MeT, and R domains (Fig. 6.2c), and heterologous expression of the gene in
A. oryzae
resulted (9) in a high yield of 3-methylorcinaldehyde (
43
). Although
not described in the literature, sequence analysis of the citrinin PKS sequence
discussed above shows that it also possesses a C-terminal thiolester reductase
domain.
Similar domains are known from NRPS systems in which reductase domains
are sometimes used as chain release mechanisms, which release an aldehyde
or primary alcohol. In the case of MOS and citrinin biosynthesis the reductive
release mechanism makes good sense as this provides the products with C-1 at
the correct oxidation state (Fig. 6.2c).
