## 4 Discussion
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Polymer** & \(\mathbf{T_{g}}\) & \(\mathbf{T_{cc}}\) & \(\mathbf{T_{m}}\) & \(\Delta\mathbf{H_{cc}}\) & \(\Delta\mathbf{H_{m}}\) & **Crystallinity \({}^{*}\)** \\  & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (J/g) & (J/g) & **(\%)** \\ \hline PET & & & & & & \\ (untreated) & 83 & 127 & 242 & 9.0 & 35.9 & 19.3 \\ PET & & & & & & \\ (treated) & - & 110 & 247 & 2.9 & 32.4 & 21.1 \\ \hline \hline \end{tabular} \({}^{*}\)The crystallinity was calculated using Equation (1).  
\end{table} 
Table 6: Properties of the PET powder untreated and treated with the extracellular medium containing the cutinase-like enzyme. source of ester-active enzymes, and some lipases and esterases were cloned and characterized (Table 7). The first characterized lipase for this genus was named LipSM54 and was described as cold-active, solvent-tolerant, and alkaline [38]. A thermostable lipase, LipSm [39], and a thermostable-alkaline-stable esterase [40] were also cloned and characterized. The gene sequences of the cutinase-like enzyme purified in this work is unknown. However, it differs from the cloned ester-active enzymes in terms of the molecular weight and optimum temperatures. In addition, the cloned enzymes were not tested against the natural or synthetic polyesters. The purified cutinase-like enzyme from _S. maltophilia_ PRS8 produced TPA, MHET and BHE from the PET flakes. A similar product profile was also found after the treatment of PET with PETases [17] and cutinases [33]. Some metal ions, surfactants, and organic solvents could be used as additives to increase the enzyme activity. Some metal ions could stabilize the enzyme structure, and the surfactants and organic solvents could facilitate the interaction with the hydrophobic polymers. The cutinase-like enzyme activity was slightly increased by the various metal ions (mono and divalent cationic) except for Cd2+, Co2+, and Hg\({}^{2+}\), which strongly inhibited the activity. The activity limitation in the presence of Cd\({}^{2+}\) and Hg\({}^{2+}\) revealed the presence of a gathering of critical vicinal sulfhydryl [42]. However, Cu\({}^{2+}\) and Ni\({}^{2+}\) enhanced the relative activity [43]. Regarding the surfactants, nonionic surfactants were reported to have an important role in promoting cutinase activity [44]. Tween and Triton-100 showed a positive effect, but the SDS and CTAB strongly inhibited the enzyme activity. The enzyme was stable in the presence of the various solvents except for ethanol and butanol, which might be due to a change in the structure of the bound substrate or the active site polarity [45]. The DSC measurements of the PET powders revealed an increased crystallinity after the treatment with the extracellular fraction of _S. maltophilia_ PRS, which suggested that the enzymatic degradation of the PET powders primarily occurred at the amorphous region, as reported in the literature [46]. Interestingly, it was previously reported that the enzymatic degradation of the PET flakes using _ls_PETase from _Ideonella sakaiensis_ resulted in a slightly decreased crystallinity, which was attributed to the formation of the dimers after the degradation that might have disrupted the crystallinity [47]. In our result, no dimer (melting or crystallization) peak was observed using DSC, which was consistent with the increased crystallinity after the enzymatic degradation by _S. maltophilia_ PRS8. Thus, the cutinase-like enzyme activity seemed restricted to the amorphous part. Finally, the _S. maltophilia_ PRS8 extracellular medium containing the cutinase-like enzyme showed a PET-depolymerizing activity comparable to _ls_PETase and significantly higher than the thermostable HiCut (HiC) from _Humicola insolens_. In a practical sense, these results indicated that the _S. maltophilia_ PRS8 extracellular medium produced under optimal conditions could be used directly in the PET degradation processes, with the advantage of skipping the enzyme purification processes and costs.