Kinetic analysis of the enzymatic degradation behavior of polyhydroxyalkanoate (PHA) based on its solid-state structure

Polyhydroxyalkanoate (PHA) is a semicrystalline thermoplastic with biodegradable and biocompatible properties and is expected to be an environmentally friendly material. In this study, the enzymatic degradation rates of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), which belong to PHAs, were quantified using PHB depolymerase, and the degradation behavior was analyzed from the viewpoint of kinetics. The objective of this research was to quantitatively evaluate the relationship between the solid structure and the enzymatic degradation rate. The enzymatic degradation behavior of PHB and PHBH was analyzed by the adsorption-reaction model. The results showed that the degradation rate decreased with increasing crystallization temperature and that the enzyme adsorption decreased and the degradation rate increased with increasing second monomer unit ratios. The enzymatic degradation mechanism was clarified based on the kinetic parameters and especially focused on the reaction resistance; specifically, the degradation proceeded in series from the surface of the stacked crystalline and amorphous phases. Furthermore, the degradation of the crystalline phase was rate-limiting, and the reaction resistance of the crystalline phase increased proportionally with the lamellar thickness. The kinetic analysis performed in this study enabled the quantitative evaluation of the relationship between the solid structure and degradation rate.