Cloning and Expression of a Phloretin Hydrolase Gene from Eubacterium ramulus and Characterization of the Recombinant Enzyme

Phloretin hydrolase catalyzes the hydrolytic C-C cleavage of phloretin to phloroglucinol and 3-(4-hydroxyphenyl)propionic acid during flavonoid degradation in Eubacterium ramulus. The gene encoding the enzyme was cloned by screening a gene library for hydrolase activity. The insert of a clone conferring phloretin hydrolase activity was sequenced. Sequence analysis revealed an open reading frame of 822 bp (phy), a putative promoter region, and a terminating stem-loop structure. The deduced amino acid sequence of phy showed similarities to a putative protein of the 2,4-diacetylphloroglucinol biosynthetic operon from Pseudomonas fluorescens. The phloretin hydrolase was heterologously expressed in Escherichia coli and purified. The molecular mass of the native enzyme was approximately 55 kDa as determined by gel filtration. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the deduced amino acid sequence of phy indicated molecular masses of 30 and 30.8 kDa, respectively, suggesting that the enzyme is a homodimer. The recombinant phloretin hydrolase catalyzed the hydrolysis of phloretin to equimolar amounts of phloroglucinol and 3-(4-hydroxyphenyl)propionic acid. The optimal temperature and pH of the catalyzed reaction mixture were 37°C and 7.0, respectively. The K_m for phloretin was 13 ± 3 μM and the k_cat was 10 ± 2 s⁻¹. The enzyme did not transform phloretin-2′-glucoside (phloridzin), neohesperidin dihydrochalcone, 1,3-diphenyl-1,3-propandione, or trans-1,3-diphenyl-2,3-epoxy-propan-1-one. The catalytic activity of the phloretin hydrolase was reduced by N-bromosuccinimide, o-phenanthroline, N-ethylmaleimide, and CuCl₂ to 3, 20, 35, and 85%, respectively. Phloroglucinol and 3-(4-hydroxyphenyl)propionic acid reduced the activity to 54 and 70%, respectively.