ALKALOID BIOSYNTHESIS IN PLANTS: Biochemistry, Cell ... - UFV

34 FACCHINI
Figure 1 Reactions catalyzed by enzymes involved in monoterpenoid indole alkaloid
biosynthesis for which the corresponding genes have been cloned. TDC, tryptophan decarboxylase;
STR, strictosidine synthase; SGD, strictosidine β-D-glucosidase; T16H, tabersonine
16-hydroxylase; D4H, desacetoxyvindoline 4-hydroxylase; DAT, deacetylvindoline
4-O-acetyltransferase.
Tryptamine and secologanin are condensed by strictosidine synthase (STR) to
form strictosidine, the common precursor to all TIAs (Figure 1). STR cDNAs have
been isolated from Rauvolfia serpentina (84) and C. roseus (104, 132). The enzyme
is encoded by a single gene in C. roseus, indicating that the multiple STR isoforms
reported previously result from posttranslational modification of a single precursor
(132). Strictosidine is deglucosylated by strictosidine β-D-glucosidase (SGD),
which has been purified from C. roseus cell cultures (92). The native enzyme
exhibits a high molecular mass, suggesting that it exists as an aggregate composed
of multiple 63-kDa subunits. Digestion of SGD with trypsin causes the complex to
disintegrate, solubilizing the enzyme without loss of activity. SGD is encoded by a
single gene in C. roseus that shares ∼60% homology with other plant glucosidases
(55). Deglucosylated strictosidine is converted via several unstable intermediates
to 4,21-dehydrogeissoschizine. Although several TIAs are produced from 4,21dehydrogeissoschizine,
few of the enzymes involved have been isolated (108).
For example, none of the enzymes leading to catharanthine has been described.