Chemistry Reference
In-Depth Information
length of sequence with no known function. This domain is followed by a KR
domain, and the PKS often terminates with an ACP. No domain similar to the
PT domain of the NR PKS or the core domain of the PR PKS seems to exist, and
there is no N-terminal SAT domain as found in the NR PKS. As with the NRs and
PR PKSs, many HR PKS genes are known from the numerous fungal genome
sequences, but as yet few gene sequences have been linked to the production of
known compounds. However, of the few cases in which both gene and chemical
product are known, some progress has been made in understanding function and
programming.
6.6.1 The Lovastatin Polyketide Synthases
Lovastatin (
10
) (also known as mevinolin) is produced by
A. terreus
. The related
compound compactin (mevastatin) (
11
), produced by
Penicillium citrinum
is
identical to lovastatin apart from the C-12 methyl group absent in compactin.
Isotopic feeding experiments have shown that two polyketide chains are required:
a nonaketide and a methylated diketide. The requirement for two polyketide syn-
thases is evident in the gene clusters associated with biosynthesis of (
10
)and
(
11
) where two PKS genes are found (4),
lovB
and
lovF
, which encode LNKS
and LDKS in the case of lovastatin (Fig. 6.3b).
LNKS formally should synthesize a fully elaborated nonaketide such as (
47
),
which could undergo a biological Diels-Alder reaction to form dihydromonacolin
L(
48
)—which is the observed first PKS-free intermediate. Note that it is possible
that the Diels Alder reaction actually occurs at the more activated hexaketide
stage. When
lovB
was expressed in the heterologous fungal host
A. nidulans
(4) however, the polyunsaturated compounds (
45
)and(
46
) were isolated. These
pyrones are related to the expected nonaketide (e.g., methylation has occurred at
the correct position), but it is evident that reductions, specifically enoyl reductions
and later keto reductions, have not occurred correctly and that chain extension
has terminated prematurely (Fig. 6.3a), which is becauase the NADPH binding
site of the ER domain of LNKS is impared. A separate ER encoding gene,
lovC
,
occurs downstream from
lovB
, and Kennedy et al. (4) showed that in coexpression
experiments the lovC protein could complement the missing ER domain and the
expected dihydromonacolin L (
48
) was produced (Fig. 6.3b). It is evident that
the lovC protein must interact with LNKS and control programming by ensuring
enoyl reduction at the correct positions and allowing complete chain extension.
It now seems that lovC type proteins are a common feature of HR PKS systems
in fungi.
In the bacterial modular PKS systems, usually a C-terminal TE domain is
involved in off-loading the product—either to another PKS or to solution (see
Chapter 7). LNKS seems to possess part of an NRPS condensation (C) domain
immediately downstream of the ACP, and this domain has been proposed to be
involved in product release, presumably by either activating water as a nucle-
ophile or the C-5 hydoxyl rather than the nitrogen of an aminothiolester as
activated by most NRPS C domains. LDKS is closely related to LNKS, but its ER
