Proteinase K-Conjugated Liposomes as Stable Catalysts for Depolymerization of Poly( l -lactic acid) Plastics

Controlling enzymatic depolymerization of poly(l-lactic acid) (PLLA) contributes to ensure the sustainability of PLLA-based biodegradable plastics, although an approach to use functionally modified enzymes remains at an early stage. In the present work, poly(ethylene glycol) (PEG)-tethered liposomes covalently conjugated with proteinase K (proK) were prepared and applied to catalyze the degradation of PLLA films as well as native β-galactosidase and heat-denatured bovine serum albumin. The activity of a dilute liposome-conjugated proK dispersion ([proK] = 0.17 μM) was stably available at pH = 8.0 and 25–45 °C in a polypropylene (PP) container, whereas free proK at the same enzyme concentration could not function due to the adsorption to PP. Liposome-conjugated proK showed consistently lower catalytic activity than free proK at the initial phase of the above degradation reactions partly because of steric hindrance caused by lipid membranes. Nevertheless, liposome-conjugated proK catalyzed the depolymerization of PLLA films at 25 °C and the initial pH of 8.0. The adsorptive interaction between proK and PLLA, which was indicated to be the cause of the deactivation of free proK, was weakened for liposome-conjugated proK because of the hydrophilic environment provided by PEG-tethered lipid membranes. The proK-to-lipids ratio and the properties of PEG chains were indicated to determine the balance between the activity and stability of liposome-conjugated proK in the prolonged PLLA depolymerization. Furthermore, the difference in intrinsic interaction of liposome-conjugated proK with PLLA or PP may be utilized to facilitate PLLA depolymerization in the presence of PP.