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 (PhaZAfa). The characteristic reactivities of the lactones were discussed based on a tertiary structure model of the active site of the PhaZAfa. 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 PhaZAfa. On the other hand, ε-caprolactone, 11-undecanolide, and 12-dodecanolide, which showed excellent polymerization activities by lipases, were scarcely polymerized by PhaZAfa. This was ascribed to the difference in the recognition sites between PhaZAfa and lipase. In addition, the effect of the substrate-binding domain of PhaZAfa and the enantioselective ring-opening polymerization of (R,S)-β-butyrolactone ((R,S)-β-BL) were studied. The substrate-binding domain lacking PhaZAfa showed higher reactivities than PhaZAfa 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).
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