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In-Depth Information
a large PKS (curacin and jamaicamide), and as an embedded domain (pederin
and onnamide). Although most clusters published to date are mixed PKS/NRPS
systems with
in trans
ATs and tandem ACPs at the site of HMGS modifica-
tion, exceptions exist for each example (difficidin is PKS only, curacin and
jamaicamide contain embedded ATs, and bryostatin and myxovirescin do not
contain tandem ACPs at the site of HMGS modification).
As HMGS enzyme cassettes have been identified and functionally character-
ized only recently, many mechanistic details as well as the key protein - protein
interactions needed to orchestrate communication among the polypeptide com-
ponents remain unclear. Details on how the individual proteins are brought to
the correct place in the pathway to perform their functions are still unknown
for most pathways. In the case of the PksX/bacillaene pathway, some intrigu-
ing microscopy performed on
B. subtilis
suggests that the bacillaene proteins
are clustered into a huge mega-enzyme factory inside the bacterial cell (66).
Whether this organization extends (or is limited) to the other members of HMGS
cassette containing pathways remains to be observed. Additionally, in some path-
ways, key enzymes have yet to be identified. Two lingering questions include, 1)
Which AT domain loads the discrete ACP in the embedded AT systems typified
by the curacin and jamaicamide pathways? and 2) where are the missing domains
located in the incomplete cassettes? Despite these remaining issues, the stage is
now set for these unique suites of enzymes to be included and applied in the
growing metabolic engineering/combinatorial biosynthesis toolbox.
The goal of this review has been to highlight a series of novel systems for cre-
ating chemical diversity in polyketide natural product biosynthesis. This review
includes the mechanistic basis for introduction of
trans
or
cis
double bonds
within linear or macrocyclic compounds, or assembly of the rare terminal alkene
in select secondary metabolites such as curacin and tautomycetin. Similarly, intro-
duction of methyl groups to create branch points or gem dimethyl functionality
can occur by several processes that have been dissected in several systems over
the past few years. Finally, one of the most intriguing new methods for introduc-
tion of diverse branching functionality involves the HMGS-containing enzymes
that are being identified in a growing number of PKS and mixed NRPS-PKS
pathways. The rapidly increasing knowledge and mechanistic understanding of
these complex metabolic systems will provide growing opportunities to engineer
chemical diversity using rational approaches.
7.4 ACKNOWLEDGMENTS
Research on modular type I polyketide synthases in the Sherman laboratory
is supported generously by grants from the National Institutes of Health
(GM076477, CA108874, TW007404), the Hans and Ella McCollum Vahlteich
Research Fund at the University of Michigan College of Pharmacy, and the
H.W. Vahlteich Professorship in Medicinal Chemistry. J.D.K. is supported by
an NRSA postdoctoral fellowship (GM075641) from the NIH.
