(Last modification:
26. Feb. 2008) So sieht der Künstler die Evolution in vitro
Titelbild in:
Chemistry & Biology, Vol. 7, Ausgabe 12, Dezember 2000
Dies ist eine künstlerische Darstellung der Evolution der Pyronsynthase in Gerbera hybrida. Die funktionell wichtigsten Änderungen in einem CHS-Vorfahr sind Austausche dreier Aminosäuren in der aktiven Tasche. Das Ergebnis ist eine Verengung, die nur kleinere Substrate erlaubt (Acetyl-CoA anstatt 4-Coumaroyl-CoA!) und nur zwei Kondensations-Reaktionen: Sonst wird das Produkt zu gross!
Links zu:
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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.
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