Mechanism and stereochemistry of enzymatic cyclization of 24,30-Bisnor-2,3-oxidosqualene by recombinant beta-amyrin synthase.
Recombinant beta-amyrin synthase from Pisum sativum converted 24,30-bisnor-2,3-oxidosqualene into a 3:1:0.2 mixture of 29,30-bisnor-beta-amyrin, 29,30-bisnorgermanicol, and 29,30-bisnor-delta-amyrin. Further, enzyme reactions with [23-13C]- and [23,23-2H]-labeled isotopomers demonstrated that the cyclization did not proceed through formation of a lupanyl primary cation with a five-membered E-ring, but an electrophilic addition of the tetracyclic C-18 cation on to the terminal double bond directly generated a thermodynamically favored pentacyclic secondary cation with a less-strained six-membered E-ring. Interestingly, the formation of the three regioisomers suggested that the absence of the terminal methyl groups resulted in a structural perturbation in the folding conformation of the E-ring of the oleanyl cation at the active site of the enzyme.
Abe
Ikuro
I
School of Pharmaceutical Sciences and the COE 21 Program, University of Shizuoka, Shizuoka 422-8526, Japan. abei@ys7.u-shizuoka-ken.ac.jp
Sakano
Yuichi
Y
Sodeyama
Megumi
M
Tanaka
Hideya
H
Noguchi
Hiroshi
H
Shibuya
Masaaki
M
Ebizuka
Yutaka
Y
D003500:Cyclization; D019751:Intramolecular Transferases / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D013185:Squalene / Q000031:analogs & derivatives / Q000737:chemistry* / Q000378:metabolism*; D013237:Stereoisomerism
C488772:24,30-bisnor-2,3-oxidosqualene; D013185:Squalene; D019751:Intramolecular Transferases; C039924:2,3-oxidosqualene-beta-amyrin-cyclase