聚對苯二甲酸乙二酯(PET)為廣泛使用的一類塑膠材料包裝，如食品和飲料包裝、纖維製品及容器等，不論是在海洋中還是土壤中都會殘留，甚至導致水質汙染，由於聚酯高分子在自然環境中非常不易完全被降解，大量被使用的PET為現今社會必須解決的環保議題。使用生物降解技術將 PET 分解成其結構單元對環境相對友善，可以減少塑膠廢物並回收石油基原料對苯二甲酸(TPA) 和乙二醇(EG)，進而使 PET 生產和回收形成循環。
大腸桿菌（Escherichia coli）具有快速繁殖且成本低廉，被廣泛使用於生產觸酶， mPETase (mature PETase) 酵素能有效分解PET，當在E.coli BL21(DE3) 宿主細胞中表達，會形成包涵體而失去活性，本論文因此使用pET-21b系統以E.coli Rosetta gami B (DE3)為宿主，在胞內可成功表達可溶mPETase蛋白，純化後可獲得比活性為14.78 U/mg。
葉枝堆肥角酯酶(Leaf-branch compost cutinase, LCC) 具有降解PET的活性，且有非常高的熱穩定性，本論文表達其突變體LCCYCCG (F243Y/D238C/S283C/Y127G)，使用pET-26b系統在E.coli BL21 (DE3)宿主中表達，在20 °C及25 °C下皆可從胞外獲得此蛋白，在胞內亦可獲得大量可溶蛋白。將此基因轉至pET-21b在E.coli BL21 (DE3)及Rosetta gami B (DE3)中表達，皆可在胞內與胞外獲得此蛋白，從胞內外純化獲得的比活性分別為15.17 U/mg及 15.43 U/mg。除了良好的熱穩定性，在50 °C及70 °C更具有較高PET解聚活性。
而聚酯水解酶(PHL7)是從堆肥宏基因組中分離出之聚酯水解酶，使用pET-26b在E.coli BL21 (DE3)宿主表達，在胞外也可得到高純度的蛋白，純化後獲得的比活性為19.35 U/mg，而從胞內純化獲得的比活性則為8.39 U/mg。
PET在酶水解過程中會釋放出對苯二甲酸(TPA)、單-(2-羥乙基) 對苯二甲酸酯 (MHET)及雙-(2-羥乙基) 對苯二甲酸酯 (BHET)，而BHET、MHET等產物已被確定會抑制PET 水解酶的活性。而來自I. sakaiensis的對苯二甲酸單羥乙酯水解酶(MHETase)則可水解MHET及BHET，因此同時使用MHETase可以促進PET的有效分解。mPETase 、LCCYCCG及PHL7三種酵素其水解PET能力與降解的PET其結晶度極為相關，因此本論文使用結晶度不同的PET膜材、板材、寶特瓶及纖維進行降解研究，發現MHETase能有效地在30oC下提升mPETase解聚PET板材，而LCCYCCG及PHL7則在70 oC 下才能實現最佳降解能力，由於MHETase耐溫性不佳，因此在第二天降溫至30 oC下加入MHETase參與降解反應，仍然可發現MHETase明顯提升了LCCYCCG及PHL7降解低結晶PET板材的程度。
解聚率分析結果顯示mPETase對PET寶特瓶的水解能力高於LCCYCCG及PHL7 1.5倍，添加MHETase後更是高於LCCYCCG及PHL7 3.5倍。 mPETase與 LCCYCCG則對PET板材有最高的解聚率，分別為0.461 %及7.952 %，添加MHETase後最高解聚率分別為0.850 %及9.217 %。PHL7則是對PET纖維解聚率為最高4.496 %，添加MHETase後最高解聚率可達為5.525 %。

Polyethylene terephthalate (PET) is one of widely used plastics for packaging, such as food and beverage containers, fiber products, and bottles. Due to its poor degradation capability in the natural environment, the excessive use of PET has become a pressing environmental issue. Decompose PET via various biological process is an environmental-friendly method to recycle petroleum-based raw materials of PET such as terephthalic acid (TPA) and ethylene glycol (EG).
Escherichia coli (E.coli) is a rapid growing and cost-effective bacterium widely used for enzymes production. The mPETase (mature PETase) enzyme expressed from E. coli has been shown can effectively degrade PET. However, expression in E. coli BL21(DE3) as a host cell, the mPETase forms inclusion bodies and loses its activity. Therefore, this study utilized the pET-21b as expression vector in E. coli Rosetta gami B (DE3) host cell, that successfully allowed the intracellular expression of soluble mPETase protein. Leaf-branch compost cutinase (LCC) also possesses activity in degrading PET and exhibits exceptional high thermal stability. In this study, the variant LCCYCCG (F243Y/D238C/S283C/Y127G) was expressed in E.coli BL21 (DE3) using pET-26b as vector. The expressed LCC could be obtained from the extracellular fraction when cultured at 20 °C and 25 °C. A significant amount of soluble LCC was also obtained intracellularly. In addition to its excellent thermal stability, LCC exhibited higher PET depolymerization activity at 50 °C and 70 °C. Polyester hydrolase (PHL7) is a polyester hydrolase isolated from compost metagenomes. It was expressed using the pET-26b vector in E. coli BL21 (DE3), and high-purity protein could also be obtained from the extracellular fraction.
During the enzymatic hydrolysis process of PET, terephthalic acid (TPA), mono-(2-hydroxyethyl) terephthalate (MHET), and bis-(2-hydroxyethyl) terephthalate (BHET) were released as hydrolysis products. However, BHET, MHET, and other by-products have been found to inhibit the activity of PET hydrolytic enzymes. On the other hand, the terephthalic acid mono-hydroxyethyl ester hydrolase (MHETase) derived from I. sakaiensis can hydrolyze both MHET and BHET, thereby facilitating the efficient degradation of PET in an enzymatic hydrolysis process.
The hydrolytic activity of mPETase, LCCYCCG, and PHL7 enzymes towards PET is highly correlated with the crystallinity of the substrate PET. Therefore, PET films, plates, bottles, and fibers of different crystallinities were employed to be degraded by mPETase, LCCYCCG, and PHL7, respectively. It was found that MHETase can effectively enhance the depolymerization of PET plates by mPETase at 30 °C. However, LCCYCCG and PHL7 exhibited optimal degradation activity at 70°C. Due to the poor thermal stability of MHETase, it was always added to the degradation reaction on the second day when reaction temperature was lowered to 30 °C. Remarkably, it was observed that MHETase enhanced the degradation of low-crystallinity PET plates when LCCYCCG and PHL7 were employed.
The PET depolymerization ratio showed that the hydrolysis ability of mPETase toward PET bottles was 1.5 times higher than that of LCCYCCG and PHL7. In addition, with the coordinated hydrolysis of MHETase and mPETase, the depolymerization ratio was increased by 3.5 times compared with LCCYCCG and PHL7 alone. PET plate as a low crystallinity substrate, mPETase and LCCYCCG demonstrated the highest depolymerization ratio of 0.461 % and 7.952 %, respectively. With addition of MHETase, the highest depolymerization ratio increased to 0.850 % for mPETase and 9.217 % for LCCYCCG, respectively. PHL7 exhibits a maximum depolymerization ratio of 4.496% for PET fibers. However, with the addition of MHETase, the maximum depolymerization ratio can reach as high as 5.525%.

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