Polymeric substrates, enzymatic degradation

The rates of adsorption and chain scission are affected by physicochemical properties of the substrate, such as the molecular weight, chemical composition, crystallinity, and surface area, and also by the inherent characteristics of the enzyme which can be measured in terms of its activity, stability, concentration, amino acid composition, and conformation. Moreover, environmental conditions such as pH and temperature also influence the activity of enzymes. The presence of stabilizers, activators, or inhibitors released from the polymer during the degradation process or additives that are leached out may also affect enzyme activity. Chemical modification of biopolymers may also affect the rate of enzymatic resorption since, depending on the degree of chemical modification, it may prevent the enzyme from recognizing the polymeric substrate. The rate of enzymatic resorption is limited by an enzyme saturation point. Beyond this enzyme concentration, no further increase in the rate of resorption is observed even when more enzyme is added. 

Oligomeric PET model compounds have been widely used to study enzymatic hydrolysis of PET, since their degradation is faster and easier to analyze compared to a polymeric substrate. Diethyl terephthalate (DET), diethyl p-phthalate (DP), h/i(benzoyloxyethyl) terephthalate (PET trimer), ethylene glycol dibenzyl ester (BEB), and hA-(p-methylbenzoic acid)-ethylene glycol ester (PET dimer) have been employed (Eig. 2). Their degradation by PET hydrolases from Fusarium solani [27, 38], T. insolens [1, 5, 48, 101], P. mendocina [25, 49], T. fusca [26, 27, 38], Burkholderia cepacia [26, 27, 38], Aspergillus oryzae [103], Bacillus spp. [87], and porcine liver esterase [109, 110] has been reported, and the corresponding hydrolysis products have been partially characterized (Table 1). 

Each of the enzymatically resorbable biopolymers has certain specific enzymes that can attack the chemical bonds of the polymeric material, leading to its degradation and resorption. Different enzymes have different actions. Some enzymes change the substrate through a free radical mechanism, while others follow an alternative chemical route. Typical examples are biological oxidation and biological hydrolysis. 

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