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Published on: March 25, 2020
Structural Basis of Starter-Substrate Selectivity Governed by a Single Residue in Orcinol Synthase
Yu Nakashima1, Saw Yu Yu Hnin1, Subin Kim1
1Institute of Natural Medicine, University of Toyama, 2630 Sugitani, Toyama 930-0194, Japan.
Chemical & Pharmaceutical Bulletin
|July 30, 2026
Summary
Orcinol synthase (RdORS) prefers short-chain substrates due to a small catalytic cavity. A specific tryptophan residue (Trp357) controls this selectivity, and its mutation expands the cavity, altering substrate preference.
Area of Science:
- Biochemistry
- Structural Biology
- Plant Metabolism
Background:
- Orcinol synthase (RdORS) from Rhododendron dauricum is a type III polyketide synthase.
- It participates in the biosynthesis of orsellinic acid-derived compounds.
- RdORS exhibits distinct substrate selectivity compared to related enzymes like Cannabis sativa tetraketide synthase (CsTKS).
Purpose of the Study:
- To elucidate the structural basis for RdORS's substrate selectivity.
- To understand how RdORS preferentially utilizes short-chain starter substrates.
Main Methods:
- X-ray crystallography to determine the structure of RdORS.
- Site-directed mutagenesis to create RdORS variants (e.g., Trp357S).
- In vitro enzymatic assays to characterize substrate utilization and product formation.
Main Results:
- The crystal structure of RdORS revealed a smaller catalytic cavity than CsTKS, attributed to a bulky Tryptophan 357 (Trp357) residue.
- Wild-type RdORS efficiently used acetyl- and butyryl-CoAs but showed reduced activity with longer chain substrates.
- The Trp357S mutation expanded the catalytic cavity, enabling RdORS to accept medium-chain acyl-CoAs up to decanoyl-CoA.
Conclusions:
- Tryptophan 357 is a critical determinant of RdORS's substrate specificity for short-chain starter molecules.
- Structural modifications, specifically cavity size, directly influence the enzyme's substrate preference.
- This study provides direct structural evidence for the mechanism behind RdORS's distinct substrate selectivity.
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