Berberine bridge-forming enzyme, from

Figure 2.8 Production of alkaloids by enzymatic conversion of amino acids in plant cell, yeast, and E. coli by transfer of plant biosynthetic enzyme genes into these microorganisms, from [63]. MAO overexpressed mono-amine oxidase from E. coli, NCS norcoculaurine synthase, MTs specific O- and N-methyltransferases, BBE berberine bridge-forming enzyme, CYP80G2 diphenyl ring bridging enzyme.

Berberine or protoberberine alkaloids [such as stylopine (79)] may be converted into alkaloids of the protopine type (60) (Fig. 32.24) (Geissman and Crout, 1969 Hartmann, 1991). Highly specific microsomal cytochrome P-450-de-pendent enzymes from cells of Eschscholtzia californica (and other species of the Papaveraceae and Fumariaceae) introduce two methylenedioxy bridges into (5)-scoulerine (72) to form (5)-stylopine (79), which is, in turn, an important precursor of the protopine, phthalideisoquinoline, and benzophenanthridine groups (Hartmann, 1991). 

The formation of the methylenedioxy bridge in Berberis has been found to be caused by the demethylating activity of a peroxidase (POD) found within the vesicle. It was also found that the cytochrome P450-requiring enzyme (canadine synthase) from microsomes of Berberis, Thalictrum and Coptis species formed the methylene bridge in (S)-tetrahydrocolumbamine (Ikezawa et al, 2003), but not in the quaternary alkaloid columbamine (Galneder et al, 1988 Zenk, 1995). Because of the substrate specificity of canadine s)mthase, the berberine pathway is considered to be that presented in Fig. 2.5 (Rueffer and Zenk, 1994). Columbamine, once proposed as an alternative route to berberine, is however converted to palmatine by a specific methyltransferase first isolated from Berberis wilsoniae cell cultures (Rueffer and Zenk, 1985 Ikezawa et al, 2003). 

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