Purification, cloning, and functional expression of phenylalanine aminomutase: The first committed step in Taxol side-chain biosynthesis

The conversion of α-phenylalanine to β-phenylalanine is the first committed step in the biosynthesis of the C-13 side chain of Taxol. Thus, the novel enzyme responsible for this step, phenylalanine aminomutase (PAM), is of considerable interest for studies of Taxol biosynthesis and represents a potential target for genetic engineering. A method is described for purifying PAM from Taxus chinensis cell cultures. The purified enzyme has a Km of 1.1 mM, a Vmax of 110.1 μm/min/mg protein, a pH optimum of 7.5-8.0, and a denatured molecular weight of about 80 kDa. Peptide sequences derived from the purified protein were used to design and synthesize degenerate primers enabling the PCR synthesis of the PAM cDNA. The PAM cDNA encodes a protein of 687 amino acid residues with a deduced molecular weight of 75.3 kDa. The PAM cDNA was cloned and expressed in Escherichia coli, and PAM activity was demonstrated. As a gene symbol for the PAM enzyme, pam is proposed. Protein sequence alignments of PAM, phenylalanine ammonia-lyase (PAL), and histidine ammonia-lyase (HAL) sequences exhibit significant similarity providing insight into potential active site residues of PAM.