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Strictosidine synthase () a key enzyme in alkaloid biosynthesis. It catalyses the condensation of tryptamine with secologanin to form strictosidine: :tryptamine + secologanin 3-alpha(S)-strictosidine + H2O Thus, the two substrates of this enzyme are tryptamine and secologanin, whereas its two products are 3-alpha(S)-strictosidine and H2O. Since the condensation of tryptamine and secologanin is the first committed step in alkaloid synthesis, strictosidine synthase plays a fundamental role for the great majority of the indole-alkaloid pathways. This enzyme belongs to the family of lyases, specifically amine lyases, which cleave carbon-nitrogen bonds. It can be isolated from several alkaloid-producing plants from the ''Apocynaceae'' family (e.g. ''Catharanthus roseus'', ''Voacanga africana''). The systematic name of this enzyme class is 3-alpha(S)-strictosidine tryptamine-lyase (secologanin-forming). Other names in common use include strictosidine synthetase, STR, and 3-alpha(S)-strictosidine tryptamine-lyase. Originally isolated from the plant ''Rauvolfia serpentina'', a medicinal plant widely used in Indian folk medicine, this enzyme participates in terpenoid biosynthesis and indole and ipecac alkaloid biosynthesis, both of which produce many compounds with significant physiological and medicinal properties. == Mechanism of catalysis == According to structural studies of strictosidine synthase from ''Rauvolfia serpentina'', tryptamine is located at the bottom of the pocket, where Glu 309 forms a hydrogen bond with the substrate’s primary amine group. The residues Phe 226 and Tyr 151, which lie parallel to the tryptamine’s indole ring, further stabilize its binding by fixing tryptamine in a sandwich structure through pi-bond interactions. Upon substrate binding, secologanin's position is located at the pocket’s entrance, where the positively charged residues His 307 and His 277 bind with secologanin's glucose moiety. A Schiff base forms between secologanin's aldehyde-group and tryptamine’s amine group, from which Glu309 deprotonates tryptamine’s carbon 2. This allows for strictosidine’s formation under the subsequent ring closure via electrophilic substitution, as shown in the image to the right.〔 Strictosidine synthase facilitates 3-alpha(S)-strictosidine formation by acting as a scaffold to increase local concentrations of tryptamine, secologanin, and acid catalysts. Its binding pocket also properly orients the iminium intermediate during cyclization to disastereoselectively produce its alkaloid products. Unlike the mechanisms behind the formation of several Pictet-Spengler compounds, a spiroindolenine intermediate containing a five-membered ring does not form during strictosidine synthesis. Theoretical calculations indicated that a direct interconversion from the iminium to a six-membered ring is several orders of magnitude faster than the spiroindolenine. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Strictosidine synthase」の詳細全文を読む スポンサード リンク
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