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The cutinase-like enzyme from the thermophile Saccharomonospora viridis AHK190, Cut190, is a good candidate to depolymerize polyethylene terephthalate (PET) efficiently. We previously developed a mutant of Cut190 (S226P/R228S), which we designated as Cut190* that has both increased activity and stability and solved its crystal structure. Recently, we showed that mutation of D250C/E296C on one of the Ca2+ -binding sites resulted in a higher thermal stability while retaining its polyesterase activity. In this study, we solved the crystal structures of Cut190* mutants, Q138A/D250C-E296C/Q123H/N202H, designated as Cut190*SS, and its inactive S176A mutant, Cut190*SS_S176A, at high resolution. The overall structures were similar to those of Cut190* and Cut190*S176A reported previously. As expected, Cys250 and Cys296 were closely located to form a disulfide bond, which would assuredly contribute to increase the stability. Isothermal titration calorimetry experiments and 3D Reference Interaction Site Model calculations showed that the metal-binding properties of the Cut190*SS series were different from those of the Cut190* series. However, our results show that binding of Ca2+ to the weak binding site, site 1, would be retained, enabling Cut190*SS to keep its ability to use Ca2+ to accelerate the conformational change from the closed (inactive) to the open (active) form. While increasing the thermal stability, Cut190*SS could still express its enzymatic function. Even after incubation at 70°C, which corresponds to the glass transition temperature of PET, the enzyme retained its activity well, implying a high applicability for industrial PET depolymerization using Cut190*SS.

A mutant of cutinase-like enzyme from Saccharomonospora viridis AHK190, Cut190_S226P/R228S, designated as Cut190*, possesses high-thermal stability and has high polyethylene terephthalate (PET)-degrading activity. The functional characterizations of PET-degrading enzymes are generally conducted using accessible substrates such as poly(butylene succinate-co-adipate) (PBSA) and p-nitrophenyl butyrate (pNPB), even though their structures are different from that of PET. Bis(2-hydroxyethyl) terephthalate (BHET) is a component of PET, and the structure is similar to that of PET compared to PBSA and pNPB. Therefore, the analysis using BHET as the substrate is important to evaluate effective PET degradation. In this study, we analyzed the enzymatic activity of Cut190* using BHET under various conditions including Ca2+ concentration and pressure. Although terephthalate was supposed to be the final product, the intermediate product, mono(2-hydroxyethyl) terephthalate (MHET), was the only product generated, possibly due to the low binding affinity of MHET. The Cut190* activity towards BHET was observed even in the absence of Ca2+ and increased with increasing Ca2+ concentration, which is similar to its activity towards pNPB, but different from its activity towards PBSA. The difference can be attributed to the size of substrates. We also analyzed the activities of Cut190* and another high-activity mutant, Cut190*Q138A/D250C-E296C, under high pressures up to 400 MPa. BHET was non-enzymatically hydrolyzed under high pressure at 37 ℃. Enzyme activities were maintained under high pressures, and degradation of BHET in the presence of enzyme was higher than that in the absence of enzyme. Furthermore, the structural analysis of Cut190* under high pressure using Fourier transform infrared spectroscopy showed that most of the enzyme molecules were populated in the native structure below 400 MPa. These results indicate that BHET can contribute to the effective functional analysis of PET-degrading enzyme, and the combination of enzyme and pressure can lead to eco-friendly PET degradation.

A cutinase from Saccharomonospora viridis AHK190, Cut190, can hydrolyze polyethylene terephthalate and has a unique feature that the activity and stability are regulated by Ca2+ binding. Our recent structural and functional analyses showed three Ca2+ binding sites and their respective roles. Here, we analysed the binding thermodynamics of Mn2+, Zn2+ and Mg2+ to Cut190 and their effects on the catalytic activity and thermal stability. The binding affinities of Mn2+ and Zn2+ were higher than that of Mg2+ and are all entropy driven with a binding stoichiometry of three, one and one for Zn2+, Mn2+ and Mg2+, respectively. The catalytic activity was measured in the presence of the respective metals, where the activity of 0.25 mM Mn2+ was comparable to that of 2.5 mM Ca2+. Our 3D Reference Interaction Site Model calculations suggested that all the ions exhibited a high occupancy rate for Site 2. Thus, Mn2+ and Mg2+ would most likely bind to Site 2 (contributes to stability) with high affinity, while to Sites 1 and 3 (contributes to activity) with low affinity. We elucidate the metal-dependent structural and functional properties of Cut190 and show the subtle balance on structure stability and flexibility is controlled by specific metal ions.