Abstract: 

The importance of biopolymers increases continuously and they are applied in all areas of life, but especially in packaging and biomedicine [1]. In many of the applications, controlled degradation is needed, the time in compost or the disappearance of an implant have optimum duration. However, to control the rate of degradation, its kinetics must be known and described quantitatively. The goal of our research was to study and model the enzymatic degradation of aliphatic polyesters used in the largest quantities in packaging and in medical applications.

The biopolymers studied were PLA, PHB and PCL. Various methods were used to follow degradation kinetics, the measurement of weight change, UV-Vis spectroscopy, mass spectrometry, and chromatography (HPLC). Enzymes were either dissolved in the degradation media or incorporated into the polymer. The enzymatic degradation of bio-polyesters proceeds in two steps: the adsorption of the enzyme on the surface of the polymer forming an enzyme/substrate complex, and the subsequent cleavage of ester bonds. The enzyme was modified by point mutagenesis to deactivate its active centre and hinder catalysis. Measurements were carried out with the active and the inactive enzyme and the two steps, adsorption and degradation were separated. The results clearly showed that adsorption is not instantaneous and can be the rate determining step. The analysis of the degradation medium showed that our active enzyme (Bacillus Megaterium) initiates only the formation of the monomer. Based on the Michelis-Menten model developed for homogeneous reactions, a completely new model was proposed, which takes into account the heterogeneous nature of degradation [2]. The agreement between measurements and the prediction of the model was good. Deviation was observed at longer times caused by the denaturation of the enzyme. The analysis of the degradation process showed that one of the rate determining factors is the size of the degrading surface. Contact surface can be increased if the enzyme is incorporated into the polymer and various approaches were used to achieve this. The enzyme was introduced into electrospun PLA fibres in the form of emulsion. An enzyme was adsorbed on the surface of halloysite nanotubes and introduced into PCL films. The degradation rate of the films accelerated proportionally to the amount of enzyme added. Besides increasing contact surface the method has the additional advantage that it might stabilize the enzyme, decrease its rate of denaturation.

 

References

1. Gandini, A., Lacerda, T.M., Progr. Polym. Sci. 48, 1-39 (2015)

2. Polyák, P. et al., Int. J. Biol. Macromol. 112, 156-162 (2018)