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Plastics pollution represents a global environmental crisis. In response, microbes are evolving the capacity to utilize synthetic polymers as carbon and energy sources. Recently, Ideonella sakaiensis was reported to secrete a two-enzyme system to deconstruct polyethylene terephthalate (PET) to its constituent monomers. Specifically, the I. sakaiensis PETase depolymerizes PET, liberating soluble products, including mono(2-hydroxyethyl) terephthalate (MHET), which is cleaved to terephthalic acid and ethylene glycol by MHETase. Here, we report a 1.6 Å resolution MHETase structure, illustrating that the MHETase core domain is similar to PETase, capped by a lid domain. Simulations of the catalytic itinerary predict that MHETase follows the canonical two-step serine hydrolase mechanism. Bioinformatics analysis suggests that MHETase evolved from ferulic acid esterases, and two homologous enzymes are shown to exhibit MHET turnover. Analysis of the two homologous enzymes and the MHETase S131G mutant demonstrates the importance of this residue for accommodation of MHET in the active site. We also demonstrate that the MHETase lid is crucial for hydrolysis of MHET and, furthermore, that MHETase does not turnover mono(2-hydroxyethyl)-furanoate or mono(2-hydroxyethyl)-isophthalate. A highly synergistic relationship between PETase and MHETase was observed for the conversion of amorphous PET film to monomers across all nonzero MHETase concentrations tested. Finally, we compare the performance of MHETase:PETase chimeric proteins of varying linker lengths, which all exhibit improved PET and MHET turnover relative to the free enzymes. Together, these results offer insights into the two-enzyme PET depolymerization system and will inform future efforts in the biological deconstruction and upcycling of mixed plastics.

Background:The recently discovered lytic polysaccharide monooxygenases (LPMOs) are important in enzymatic conversion of lignocellulosic biomass. Results: We describe structural and functional studies of NcLPMO9C, which cleaves both cellulose and certain hemicelluloses. Conclusion: NcLPMO9C has structural and functional features that correlate with the enzyme's catalytic capabilities. Significance: This study shows how LPMO active sites are tailored to varying functionalities and adds to a growing LPMO knowledge base.

Background: Lytic polysaccharide monooxygenases (LPMOs) represent a recently discovered enzymatic route to cleave carbohydrates. Results:We report the first basidiomycete LPMO structure and describe enzyme-cellulose interactions with simulation. Conclusion:We characterize the copper-containing active site and identify loops important for substrate recognition and binding. Significance: This structure is the first LPMO from a model basidiomycete fungus that contains many LPMO genes.

Plant cell-wall polysaccharides represent a vast source of food in nature. To depolymerize polysaccharides to soluble sugars, many organisms use multifunctional enzyme mixtures consisting of glycoside hydrolases, lytic polysaccharide mono-oxygenases, polysaccharide lyases, and carbohydrate esterases, as well as accessory, redox-active enzymes for lignin depolymerization. Many of these enzymes that degrade lignocellulose are multimodular with carbohydrate-binding modules (CBMs) and catalytic domains connected by flexible, glycosylated linkers. These linkers have long been thought to simply serve as a tether between structured domains or to act in an inchworm-like fashion during catalytic action. To examine linker function, we performed molecular dynamics (MD) simulations of the Trichoderma reesei Family 6 and Family 7 cellobiohydrolases (TrCel6A and TrCel7A, respectively) bound to cellulose. During these simulations, the glycosylated linkers bind directly to cellulose, suggesting a previously unknown role in enzyme action. The prediction from the MD simulations was examined experimentally by measuring the binding affinity of the Cel7A CBM and the natively glycosylated Cel7A CBM-linker. On crystalline cellulose, the glycosylated linker enhances the binding affinity over the CBM alone by an order of magnitude. The MD simulations before and after binding of the linker also suggest that the bound linker may affect enzyme action due to significant damping in the enzyme fluctuations. Together, these results suggest that glycosylated linkers in carbohydrate-active enzymes, which are intrinsically disordered proteins in solution, aid in dynamic binding during the enzymatic deconstruction of plant cell walls. biofuels | cellulase | post-translational modification | carbohydrate recognition

Background: Family 7 cellulases exhibit significant hydrolytic potential in cellulose degradation. Results: We report the Heterobasidion irregulare GH7 structure and compare it with other GH7 cellobiohydrolases with simulation. Conclusion: H. irregulare Cel7A exhibits intermediate dynamical and structural properties between Phanerochaete chrysosporium Cel7D and Hypocrea jecorina Cel7A. Significance: These results highlight regions of family 7 cellobiohydrolases important for carbohydrate processivity and association-dissociation rates on cellulose.

Background: Aromatic residues line glycoside hydrolase active sites mediating ligand binding. Results: Binding affinity is significantly altered upon tryptophan to alanine mutation, although relative to the location in the active site. Conclusion: Aromatic-carbohydrate interactions are employed in a variety of functionalities within the purview of ligand binding. Significance: Understanding the functional role of aromatic residues in the active site is necessary for the rational design of new carbohydrate-active enzymes.

Background: Nature employs processive and nonprocessive glycoside hydrolases to degrade polysaccharides. Results: We solved the Serratia marcescens nonprocessive chitinase (ChiC2) structure and used simulation to identify dynamic hallmarks of processivity in S. marcescens chitinases. Conclusion: Dynamic metrics complement structural insights in determining processivity. Significance: Identification of hallmarks of processivity is a key step toward development of a general, molecular-level theory of glycoside hydrolase processivity.