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A bacterium completely degrades poly(ethylene terephthalate) [Also see Report by Yoshida et al. ] An estimated 311 million tons of plastics are produced annually worldwide; 90% of these are derived from petrol. A considerable portion of these plastics is used for packaging (such as drinking bottles), but only ~14% is collected for recycling ( 1 ). Most plastics degrade extremely slowly, thus constituting a major environmental hazard ( 2 ), especially in the oceans, where microplastics are a matter of major concern ( 3 ). One potential solution for this problem is the synthesis of degradable plastics from renewable resources ( 4 ). This approach provides hope for the future but does not help to get rid of the plastics already in the environment. On page 1196 of this issue, Yoshida et al. ( 5 ) address this problem by reporting an organism that can fully degrade a widely used plastic.

Summary Polyethylene terephthalate (PET) is a mass‐produced synthetic polyester contributing remarkably to the accumulation of solid plastics waste and plastics pollution in the natural environments. Recently, bioremediation of plastics waste using engineered enzymes has emerged as an eco‐friendly alternative approach for the future plastic circular economy. Here we genetically engineered a thermophilic anaerobic bacterium, Clostridium thermocellum, to enable the secretory expression of a thermophilic cutinase (LCC), which was originally isolated from a plant compost metagenome and can degrade PET at up to 70°C. This engineered whole‐cell biocatalyst allowed a simultaneous high‐level expression of LCC and conspicuous degradation of commercial PET films at 60°C. After 14 days incubation of a batch culture, more than 60% of the initial mass of a PET film (approximately 50 mg) was converted into soluble monomer feedstocks, indicating a markedly higher degradation performance than previously reported whole‐cell‐based PET biodegradation systems using mesophilic bacteria or microalgae. Our findings provide clear evidence that, compared to mesophilic species, thermophilic microbes are a more promising synthetic microbial chassis for developing future biodegradation processes of PET waste. Promising bioremediation strategies for plastics waste are of great importance and requirements. In our study, we constructed a recombinant Clostridium thermocellum strain expressing a secretory cutinase (LCC) as a thermophilic whole‐cell biocatalyst to degrade PET under high‐temperature condition (60°C). To our knowledge, this biocatalysis system demonstrates the highest PET degradation efficiency compared to reported whole‐cell‐based systems and also enjoys a low‐cost advantage over the free enzyme‐based process.

Considerable research achievements were made to address the plastic crisis using biotechnology, but this is still limited to polyesters. This Comment aims to clarify important aspects related to myths and realities about plastic biodegradation and suggests distinct strategies for a bio-based circular plastic economy in the future.

Enzymatic depolymerization of polyethylene terephthalate (PET) towards monomer recycling offers a green route to a circular plastic economy, with scale-up currently underway. Yet, inconsistent assessment methods hinder clear comparisons between various PET hydrolases. This Perspective aims to identify critical gaps in this dynamic research field and outline key principles for selecting and tailoring novel enzymes, such as using uniform PET samples and standardizing reaction settings that mimic industrial conditions. Applying these guidelines will improve enzyme screening efficiency, increase data reproducibility, deepen the understanding of interfacial biocatalysis, and ultimately accelerate the development of more robust and cost-effective bio-based PET recycling methods. Enzymatic depolymerization of polyethylene terephthalate (PET) towards monomer recycling is a promising strategy for a bio-based circular plastic economy, but progress is limited by the lack of standard guidelines for assessing and comparing the depolymerization efficiency catalysed by various PET hydrolases. In this Perspective, the authors identify critical research gaps in sourcing novel PET hydrolases and specify crucial requirements for selecting and optimizing them for specific application scenarios.

The extreme durability of polyethylene terephthalate (PET) debris has rendered it a long-term environmental burden. At the same time, current recycling efforts still lack sustainability. Two recently discovered bacterial enzymes that specifically degrade PET represent a promising solution. First, Ideonella sakaiensis PETase, a structurally well-characterized consensus α/β-hydrolase fold enzyme, converts PET to mono-(2-hydroxyethyl) terephthalate (MHET). MHETase, the second key enzyme, hydrolyzes MHET to the PET educts terephthalate and ethylene glycol. Here, we report the crystal structures of active ligand-free MHETase and MHETase bound to a nonhydrolyzable MHET analog. MHETase, which is reminiscent of feruloyl esterases, possesses a classic α/β-hydrolase domain and a lid domain conferring substrate specificity. In the light of structure-based mapping of the active site, activity assays, mutagenesis studies and a first structure-guided alteration of substrate specificity towards bis-(2-hydroxyethyl) terephthalate (BHET) reported here, we anticipate MHETase to be a valuable resource to further advance enzymatic plastic degradation. Plastic polymer PET degrading enzymes are of great interest for achieving sustainable plastics recycling. Here, the authors present the crystal structures of the plastic degrading bacterial enzymes PETase, MHETase in its apo-form and MHETase bound to a non-hydrolyzable substrate analog.

Phytoplankton blooms provoke bacterioplankton blooms, from which bacterial biomass (necromass) is released via increased zooplankton grazing and viral lysis. While bacterial consumption of algal biomass during blooms is well-studied, little is known about the concurrent recycling of these substantial amounts of bacterial necromass. We demonstrate that bacterial biomass, such as bacterial alpha-glucan storage polysaccharides, generated from the consumption of algal organic matter, is reused and thus itself a major bacterial carbon source in vitro and during a diatom-dominated bloom. We highlight conserved enzymes and binding proteins of dominant bloom-responder clades that are presumably involved in the recycling of bacterial alpha-glucan by members of the bacterial community. We furthermore demonstrate that the corresponding protein machineries can be specifically induced by extracted alpha-glucan-rich bacterial polysaccharide extracts. This recycling of bacterial necromass likely constitutes a large-scale intra-population energy conservation mechanism that keeps substantial amounts of carbon in a dedicated part of the microbial loop. Phytoplankton blooms provoke bacterioplankton blooms, from which bacterial biomass (necromass) is released via zooplankton grazing and viral lysis. Here, Beidler et al. show that the bacterial biomass, including alpha-glucan polysaccharides generated from the consumption of algal organic matter, is reused by microbes in vitro and during a diatom-dominated bloom.

The use of transaminases to access pharmaceutically relevant chiral amines is an attractive alternative to transition-metal-catalysed asymmetric chemical synthesis. However, one major challenge is their limited substrate scope. Here we report the creation of highly active and stereoselective transaminases starting from fold class I. The transaminases were developed by extensive protein engineering followed by optimization of the identified motif. The resulting enzymes exhibited up to 8,900-fold higher activity than the starting scaffold and are highly stereoselective (up to >99.9% enantiomeric excess) in the asymmetric synthesis of a set of chiral amines bearing bulky substituents. These enzymes should therefore be suitable for use in the synthesis of a wide array of potential intermediates for pharmaceuticals. We also show that the motif can be engineered into other protein scaffolds with sequence identities as low as 70%, and as such should have a broad impact in the field of biocatalytic synthesis and enzyme engineering. A motif was identified in the scaffold of an ( S )-selective transaminase that enables the asymmetric synthesis of bulky chiral amines. This motif is transferable to other enzymes with as low as 70% sequence identity. The biocatalysts developed show high stereoselectivity and their synthetic potential was confirmed in preparative scale synthesis.

Marine macroalgae, particularly their complex polysaccharides, are an untapped renewable source of high-quality monosaccharides and related building blocks. To utilize this feedstock for industrial applications, the enzymatic depolymerization by marine microorganisms has been shown to be effective. A prime example is the common green alga Ulva , with its storage polysaccharide ulvan, which contains high quantities of L -rhamnose and D -glucuronic acid. As suitable high-throughput methods for analyzing the enzymatic degradation of complex polysaccharides are still lacking, a transcription factor–based biosensor is described here that utilizes the P rha BAD promoter native to E. coli , which is specific for L -rhamnose. This biosensor exhibited a linear response, enabling the quantification of L -rhamnose within a concentration range of 10–1000 µM. The introduction of a T7 stem-loop improved the performance, and various fluorescent reporter genes were studied. The optimized system was then used to evaluate various stages of the ulvan degradation cascade in terms of L -rhamnose release, confirming its applicability to complex sugar mixtures. A detectable fluorescence signal was only generated when all the necessary enzymes for breaking down the polymer into undecorated monosaccharides were present, highlighting the biosensor’s specificity. The application of this method to the degradation of Ulva sp. biomass samples of various origins was also successfully demonstrated. This establishes the biosensor as a promising method for further high-throughput investigations. Key points •  Development of an improved transcription factor-based biosensor for L-rhamnose. •  Biosensor application for the analysis of enzymatic polysaccharide degradation. •  Reliable quantification of L-rhamnose in complex carbohydrate mixtures. Graphical Abstract

Xylanases are important for the enzymatic breakdown of lignocellulose-based biomass to produce biofuels and other value-added products. We report functional and structural analyses of TsaGH11, an endo -1,4- β -xylanase from the hemicellulose-degrading bacterium, Thermoanaerobacterium saccharolyticum . TsaGH11 was shown to be a thermophilic enzyme that favors acidic conditions with maximum activity at pH 5.0 and 70 °C. It decomposes xylans from beechwood and oat spelts to xylose-containing oligosaccharides with specific activities of 5622.0 and 3959.3 U mg −1 , respectively. The kinetic parameters, K m and k cat towards beechwood xylan, are 12.9 mg mL −1 and 34,015.3 s −1 , respectively, resulting in k cat /K m value of 2658.7 mL mg −1  s −1 , higher by 10 2 –10 3 orders of magnitude compared to other reported GH11s investigated with the same substrate, demonstrating its superior catalytic performance. Crystal structures of TsaGH11 revealed a β-jelly roll fold, exhibiting open and close conformations of the substrate-binding site by distinct conformational flexibility to the thumb region of TsaGH11. In the room-temperature structure of TsaGH11 determined by serial synchrotron crystallography, the electron density map of the thumb domain of the TsaGH11 molecule, which does not affect crystal packing, is disordered, indicating that the thumb domain of TsaGH11 has high structural flexibility at room temperature, with the water molecules in the substrate-binding cleft being more disordered than those in the cryogenic structure. These results expand our knowledge of GH11 structural flexibility at room temperature and pave the way for its application in industrial biomass degradation.