Strictosidine synthase (EC 4.3.3.2) a key enzyme in alkaloid biosynthesis. It catalyses the condensation of tryptamine with secologanin to form strictosidine:

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.

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.

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