Structural Dynamics of the PET-Degrading Cutinase-like Enzyme from Saccharomonospora viridis AHK190 in Substrate-Bound States Elucidates the Ca2+-Driven Catalytic Cycle.

A cutinase-type polyesterase from Saccharomonospora viridis AHK190 (Cut190) has been shown to degrade the inner block of polyethylene terephthalate. A unique feature of Cut190 is that its function and stability are regulated by Ca2+ binding. Our previous crystal structure analysis of Cut190S226P showed that one Ca2+ binds to the enzyme, which induces large conformational changes in several loop regions to stabilize an open conformation [Miyakawa, T., et al. (2015) Appl. Microbiol. Biotechnol. 99, 4297]. In this study, to analyze the substrate recognition mechanism of Cut190, we determined the crystal structure of the inactive form of a Cut190 mutant, Cut190*S176A, in complex with calcium ions and/or substrates. We found that three calcium ions bind to Cut190*S176A, which is supported by analysis using native mass spectrometry experiments and 3D Reference Interaction Site Model calculations. The complex structures with the two substrates, monoethyl succinate and monoethyl adipate (engaged and open forms), presumably correspond to the pre- and post-reaction states, as the ester bond is close to the active site and pointing outward from the active site, respectively, for the two complexes. Ca2+ binding induces the pocket to open, enabling the substrate to access the pocket more easily. Molecular dynamics simulations suggest that a post-reaction state in the engaged form presumably exists between the experimentally observed forms, indicating that the substrate would be cleaved in the engaged form and then requires the enzyme to change to the open form to release the product, a process that Ca2+ can greatly accelerate.

Enzymatic hydrolysis of PET: functional roles of three Ca2+ ions bound to a cutinase-like enzyme, Cut190*, and its engineering for improved activity.

Cut190 from Saccharomonospora viridis AHK190 (Cut190) is the only cutinase that exhibits inactive (Ca2+-free) and active (Ca2+-bound) states, although other homologous cutinases always maintain the active states (Ca2+-free and bound). The X-ray crystallography of the S176A mutant of Cut190* (Cut190_S226P/R228S) showed that three Ca2+ ions were bound at sites 1-3 of the mutant. We analyzed the roles of three Ca2+ ions by mutation and concluded that they play different roles in Cut190* for activation (sites 1 and 3) and structural and thermal stabilization (sites 2 and 3). Based on these analyses, we elucidated the mechanism for the conformational change from the Ca2+-free inactive state to the Ca2+-bound active state, proposing the novel Ca2+ effect on structural dynamics of protein. The introduction of a disulfide bond at Asp250 and Glu296 in site 2 remarkably increased the melting temperatures of the mutant enzymes by more than 20-30 °C (while Ca2+-bound) and 4-14 °C (while Ca2+-free), indicating that a disulfide bond mimics the Ca2+ effect. Replacement of surface asparagine and glutamine with aspartic acid, glutamic acid, or histidine increased the melting temperatures. Engineered mutant enzymes were evaluated by an increase in melting temperatures and kinetic values, based on the hydrolysis of poly(butylene succinate-co-adipate) and microfiber polyethylene terephthalate (PET). A combined mutation, Q138A/D250C-E296C/Q123H/N202H, resulted in the highest thermostability, leading to the maximum degradation of PET film (more than 30%; approximately threefold at 70 °C, compared with that of Cut190* at 63 °C).