An oxidosqualene cyclase makes numerous products by diverse mechanisms: a challenge to prevailing concepts of triterpene biosynthesis.
The genome of the model plant Arabidopsis thaliana encodes 13 oxidosqualene cyclases, 9 of which have been characterized by heterologous expression in yeast. Here we describe another cyclase, baruol synthase (BARS1), which makes baruol (90%) and 22 minor products (0.02-3% each). This represents as many triterpenes as have been reported for all other Arabidopsis cyclases combined. By accessing an extraordinary repertoire of mechanistic pathways, BARS1 makes numerous skeletal types and deprotonates the carbocation intermediates at 14 different sites around rings A, B, C, D, and E. This undercurrent of structural and mechanistic diversity in a superficially accurate enzyme is incompatible with prevailing concepts of triterpene biosynthesis, which posit tight control over the mechanistic pathway through cation-pi interactions, with a single proton acceptor in a hydrophobic active site. Our findings suggest that mechanistic diversity is the default for triterpene biosynthesis and that product accuracy results from exclusion of alternative pathways.
Lodeiro
Silvia
S
Department of Chemistry, Rice University, Houston, Texas 77005, USA.
Xiong
Quanbo
Q
Wilson
William K
WK
Kolesnikova
Mariya D
MD
Onak
Carl S
CS
Matsuda
Seiichi P T
SP
D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000378:metabolism; D019751:Intramolecular Transferases / Q000737:chemistry / Q000378:metabolism*; D015394:Molecular Structure; D009699:N-Glycosyl Hydrolases / Q000737:chemistry / Q000378:metabolism; D014315:Triterpenes / Q000737:chemistry / Q000378:metabolism*
D029681:Arabidopsis Proteins; D014315:Triterpenes; D009699:N-Glycosyl Hydrolases; C505663:PEN2 protein, Arabidopsis; D019751:Intramolecular Transferases; C021924:lanosterol synthase