Correlation between structure of the lactones and substance specificity in enzyme-catalyzed polymerization for the synthesis of polyesters

Small-size (4-membered) and medium-size (5-, 6-, and 7-membered) unsubstituted lactones as well as unsubstituted macrolides (12 and 13 membered) were subjected to the ring-opening polymerization using the extracellular PHB depolymerase from Alcaligenes faecalis T1 (PhaZ_Afa). The characteristic reactivities of the lactones were discussed based on a tertiary structure model of the active site of the PhaZ_Afa. With respect to the ring-size of the lactones, the 4-membered β-propiolactone and 6-membered δ-valerolactone (δ-VL) showed the highest polymerization activity, and δ-VL seemed to be the upper size limit for the molecular recognition of the narrow active site cleft of PhaZ_Afa. On the other hand, ε-caprolactone, 11-undecanolide, and 12-dodecanolide, which showed excellent polymerization activities by lipases, were scarcely polymerized by PhaZ_Afa. This was ascribed to the difference in the recognition sites between PhaZ_Afa and lipase. In addition, the effect of the substrate-binding domain of PhaZ_Afa and the enantioselective ring-opening polymerization of (R,S)-β-butyrolactone ((R,S)-β-BL) were studied. The substrate-binding domain lacking PhaZ_Afa showed higher reactivities than PhaZ_Afa for the polymerization of the lactones and that a significant enantioselectivity was observed at the early stage of the polymerization of (R,S)-β-BL to produce the (R )-enriched optically active poly(3-hydroxybutyrate).