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(Last
modification: 19. Feb. 2008)
Pyronsynthase aus Gerbera hybrida
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und Erklärungen über die Umwandlung einer CHS in eine Pyronsynthase:
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Publikationen
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Eckermann, S., Schröder, G., Schmidt, J.,
Strack, D., Edrada, R.A., Helariutta, Y., Elomaa, P., Kotilainen, M.,
Kilpeläinen, I., Proksch, P., Teeri, T.H. and Schröder, J.:
New pathway to polyketides in plants.
Nature (London) 396, 387-390 (1998).
The
repertoire of secondary metabolism (involving the production of compounds
not essential for growth) in the plant kingdom is enormous, but the genetic
and functional basis for this diversity is hard to analyse as many of the
biosynthetic enzymes are unknown. We have now identified a key enzyme in the
ornamental plant Gerbera hybrida (Asteraceae) that
participates in the biosynthesis of compounds that contribute to insect and
pathogen resistance. Plants transformed with an antisense construct of
gchs2, a complementary DNA encoding a previously unknown function,
completely lack the pyrone derivatives gerberin and parasorboside. The
recombinant plant protein catalyses the principal reaction in the
biosynthesis of these derivatives: GCHS2 is a polyketide synthase that uses
acetyl-CoA and two condensation reactions with malonyl-CoA to form the
pyrone backbone of the natural products. The enzyme also accepts benzoyl-CoA
to synthesize the backbone of substances that have become of interest as
inhibitors of the HIV-1 protease. GCHS2 is related to chalcone synthase
(CHS) and its properties define a new class of function in the protein
superfamily. It appears that CHS-related enzymes are involved in the
biosynthesis of a much larger range of plant products than was previously
realized.
Sonderdruckanfrage
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Jez,
J.M., Austin, M.B., Ferrer, J.-L., Bowman, M.E., Schröder, J. and Noel,
J.P.:
Structural control of polyketide formation in plant-specific polyketide
synthases.
Chemistry & Biology 7, 919-930 (2000).
Background:
Polyketide synthases (PKSs) generate molecular diversity by utilizing
different starter molecules and by controlling the final length of the
polyketide. Although exploitation of this mechanistic variability has
produced novel polyketides, the structural foundation of this versatility is
unclear. Plant-specific PKSs are essential for the biosynthesis of
anti-microbial phytoalexins, anthocyanin pigments, and inducers of
Rhizobium nodulation genes. 2-Pyrone synthase (2-PS) and chalcone
synthase (CHS) are plant-specific PKSs that exhibit 74% amino acid identity.
2-PS forms the triketide methylpyrone from an acetyl-CoA starter molecule
and two malonyl-CoAs. CHS forms the tetraketide chalcone using a p-coumaroyl-CoA
starter molecule and three malonyl-CoAs. Our goal was to elucidate the
molecular basis of starter molecule selectivity and control of polyketide
length in this class of PKS.
Results: The 2.05 Å resolution crystal structure of 2-PS complexed
with the reaction intermediate acetoacetyl-CoA was determined by molecular
replacement. 2-PS and CHS share a common three-dimensional fold, a set of
conserved catalytic residues, and similar CoA binding sites. However, the
active site cavity in 2-PS is approximately one-third the size of that in
CHS. Of the twenty-eight residues lining the 2-PS initiation/elongation
cavity, four positions are different in CHS. Mutations at three of these
positions in CHS (T197L, G256L, and S338I) each altered product formation.
Generation of a CHS triple mutant (T197L/G256L/S338I) yielded an enzyme that
was functionally identical to 2-PS.
Conclusions: Structural and functional characterization of 2-PS
together with generation of a CHS mutant with an initiation/elongation
cavity analogous to 2-PS demonstrates that cavity volume governs the choice
of starter molecule and controls the final length of the polyketide. These
results provide a structural basis for control of polyketide length in other
PKSs, and suggest strategies for further increasing the scope of polyketide
biosynthetic diversity.
Sonderdruckanfrage
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