1. Enzyme Structure & Mechanism
Peanut STS
Alfalfa CHS
Highly homologous overall structures.

2. Reactions Catalyzed by CHS and STS

3. Hydrolysis occurs ONLY during the Cyclization of STS
Different Cyclization in STS and CHS:
Hydrolysis occurs ONLY during the Cyclization of STS
No thioesterase-like motif was found in STS.
How is the hydrolysis controlled in STS?

4. (Nucleophilic) Water Molecule at the Active Sites of STSs
Crystal structure of stilbene synthase from Arachis hypogaea.
Shomura, Y, Torayama, I, Suh, D-Y, Kita, A, Sankawa, U and Miki, K
(2005) Proteins 60: 803

5. Proposed “Aldol Switch” Hydrogen Bond Network
at the STS Active Site
STS Active Site
3.0 Å
3.0 Å
2.8 Å
(3.6 Å)
2.8 Å
(3.5 Å)
2.7 Å
2.7 Å
(3.8 Å)
2.7 Å
2.7 Å
2.7 Å
putative
nucleophilic
water /
hydroxide ion
An aldol switch discovered in stilbene synthases mediates cyclization specificity
of type III polyketide synthases. Austin et al. Noel, JP (2004) Chem. Biol. 11:1179

6. Reaction Sequence in the STS Cyclization: Hydrolysis Occurs First.② ①
CTAA