title: β-Amyrin Biosynthesis: Promiscuity for Steric Bulk at Position 23 in the Oxidosqualene Substrate and the Significance of Hydrophobic Interaction between the Methyl Group at Position 30 and the Binding Site.
abstract: To examine how the sterics at the 23 position of (3S)-2,3-oxidosqualene 1 influence the polycyclization cascade in β-amyrin biosynthesis, substrate analogues substituted with an ethyl group (10, 11), a hydrogen atom (12, 13), or a propyl residue (14) at the 23 position were incubated with β-amyrin synthase. The bulkier ethyl group was accepted as a substrate, leading to formation of the β-amyrin skeleton (42, 43) without truncation of the multiple cyclization reactions. Analogue 13, possessing a hydrogen atom and an ethyl group at the 23E and 23Z positions, respectively, was also converted into the β-amyrin skeleton 45. However, the analogue lacking an ethyl group at the 23Z position (12) underwent almost no conversion, strongly indicating that an alkyl group must exist at the Z position. The cyclization of the analogue with a propyl substituent at the Z position (14) was poor. Analogue 15 possessing CH2OH at the 23E position afforded a new compound 47 in a high yield as a result of trapping of the final oleanyl cation. Conversely, 16 with 23Z-CH2OH afforded novel compounds 48-50 in low yields, which resulted from the intermediary dammarenyl and baccharenyl cations. Therefore, the hydrophobic interaction between the 23Z-alkyl group and its binding site (possibly via CH/π interaction) is critical for adopting the correct chair-chair-chair-boat-boat conformation and for the full cyclization cascade.
authors.affiliation: Department of Applied Biological Chemistry, Faculty of Agriculture and Graduate School of Science and Technology, Niigata University , Ikarashi 2-8050, Nishi-ku, Niigata, Japan 950-2181.
authors.forename: Ikki
authors.initials: I
authors.lastname: Kaneko
authors.affiliation: Department of Applied Biological Chemistry, Faculty of Agriculture and Graduate School of Science and Technology, Niigata University , Ikarashi 2-8050, Nishi-ku, Niigata, Japan 950-2181.
authors.forename: Tsutomu
authors.initials: T
authors.lastname: Hoshino
mesh_terms: D001665:Binding Sites; D003500:Cyclization; D006859:Hydrogen / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D019751:Intramolecular Transferases; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015394:Molecular Structure; D009828:Oleanolic Acid / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D013185:Squalene / Q000031:analogs & derivatives* / Q000737:chemistry; D013237:Stereoisomerism; D013379:Substrate Specificity; D014315:Triterpenes / Q000737:chemistry
chemical_list: C002821:2,3-oxidosqualene; D014315:Triterpenes; C520393:oxidosqualene; D009828:Oleanolic Acid; D013185:Squalene; D006859:Hydrogen; D019751:Intramolecular Transferases; C039924:2,3-oxidosqualene-beta-amyrin-cyclase; C036380:beta-amyrin; D010100:Oxygen