Biosynthesis of the N-benzoyl phenylisoserinoyl side chain of the anticancer drug Taxol starts with the conversion of 2S-␣-phenylalanine to 3R-␤-phenylalanine by phenylalanine aminomutase (PAM). A gene cloning approach was based on the assumption that PAM would resemble the well known plant enzyme phenylalanine ammonia lyase. A phenylalanine ammonia lyase-like sequence acquired from a Taxus cuspidata cDNA library was expressed functionally in Escherichia coli and confirmed as the target aminomutase that is virtually identical to the recombinant enzyme and clone from Taxus The final stages of Taxol biosynthesis (see Fig. 1A) in yew species involve the assembly and attachment to C-13 of the taxane core of the N-benzoyl phenylisoserinoyl side chain, which is an important pharmacophoric descriptor of this anticancer drug (1, 2). In the current practice of Taxol production, this side chain is attached by chemical semisynthesis to baccatin III, which is derived from 10-deacetylbaccatin III, a Taxus (yew) metabolite that is much more readily available than Taxol itself (3-5). In the biosynthetic pathway, five steps are involved in the construction of the side chain. The first step is considered to be the conversion of 2S-␣-phenylalanine to 3R-␤-phenylalanine by an aminomutase that catalyzes an intramolecular migration of the amino group and a partial internal transfer of the pro-3S hydrogen (6, 7). This step is seemingly followed by the ligase-mediated activation to the corresponding CoA ester and then the transfer of ␤-phenylalanoyl to the C-13 hydroxyl of baccatin III. The resulting intermediate (designated ␤-phenylalanoyl baccatin III or N-debenzoyl-2Ј-deoxytaxol) then likely undergoes cytochrome P450-mediated hydroxylation at the side chain 2Ј-position to generate the isoserinoyl moiety and final N-benzoylation of this side chain (8) to complete the biosynthesis of Taxol. cDNAs encoding the two transferases involved in C-13 side chain assembly have been described previously (8,9).The relative abundance of ␤-phenylalanine and side chaindeficient late pathway metabolites such as baccatin III and 10-deacetylbaccatin III in vivo (10) suggests that either the CoA ligase for ␤-phenylalanine or the CoA ester-dependent ␤-phenylalanoyltransferase (9), both of which function downstream of the aminomutase, may be rate limiting in side chain assembly and, thus, in Taxol biosynthesis. Because the phenylalanine aminomutase (PAM) 1 catalyzes the first step of the side chain assembly process and shares its primary metabolite substrate, phenylalanine, with several competing, non-taxoid phenylpropanoid pathway enzymes in plants (11-13), it is therefore an important target for genetic engineering in yew or derived cell cultures to increase Taxol production yields. PAM is also of interest enzymologically because the reaction that is catalyzed is unusual, and, in addition to the adenosylcobalamin-dependent leucine 2,3-aminomutase from Andrographis paniculata and potato tubers (14 -16), it is the only other aminomutase of plant origin d...

Chalcone synthase and stilbene synthase are plant-specific polyketide synthases. They catalyze three common consecutive decarboxylative condensations and specific cyclization reactions. They are highly homologous to each other, and are likely to fall into a family of polyketide synthases along with acridone synthase and bibenzyl synthase. Two cDNA clones (named HmC and HmS), both of which show high homology to the known chalcone synthases, were obtained from leaves of Hydrangea macrophylla var. thunbergii. They were expressed in Escherichia coli in order to determine their enzyme functions. Detection of chalcone formation clearly indicated that HmC encoded chalcone synthase, while HmS protein catalyzed the formation of neither chalcone nor stilbene. However, a novel pyrone, a lactonization product of a linear tetraketide was detected in reaction products of HmS protein. This proves that HmS encodes a novel polyketide synthase that catalyzes only chain elongation without cyclization.

Chalcone synthase (CHS) and stilbene synthase (STS) are related plant polyketide synthases belonging to the CHS superfamily. CHS and STS catalyze common condensation reactions of p-coumaroyl-CoA and three C 2 -units from malonylCoA but different cyclization reactions to produce naringenin chalcone and resveratrol, respectively. Using purified Pueraria lobata CHS and Arachis hypogaea STS overexpressed in Escherichia coli, bisnoryangonin (BNY, the derailed lactone after two condensations) and p-coumaroyltriacetic acid lactone (the derailed lactone after three condensations) were detected from the reaction products. More importantly, we found a crossreaction between CHS and STS, i.e. resveratrol production by CHS (2.7^4.2% of naringenin) and naringenin production by STS (1.4^2.3% of resveratrol), possibly due to the conformational flexibility of their active sites.z 1999 Federation of European Biochemical Societies.

cDNA clones for chalcone synthase (CHS) of Pueraria lobata cultured cells were isolated by screening the cDNA library using CHS cDNA of Phaseolus vulgaris as a probe. Analysis of nucleotide sequences of the cloned cDNA revealed a 1170-bp open reading frame that encoded a 390-amino acid polypeptide with an Mr of 43,000. The full-length cDNA was cloned into the expression vector pT7-7. CHS activity was found in the crude extracts of transformed E. coli after induction and two protein bands of ca. 43 and 34 kd were hybridized with anti-persley CHS antiserum.