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A Promiscuous Bacillus Esterase Reveals a Carboxylesterase Blueprint for PET Intermediate Degradation

This study characterizes BhEstB, a highly promiscuous *Bacillus halotolerans* esterase that, despite lacking typical PET-binding motifs and showing modest bulk polymer activity, exhibits exceptional efficiency in degrading PET intermediates (BHET and MHET), thereby establishing a new structural blueprint for enzymes transitioning from intermediate processing to true polymer degradation.

Original authors: Hadjira Bounabi, Bilal Yahiaoui, Meriem Gasmi, Meroua Safa Mechouche, Nico Bäse, Marno Gurschke, Rebecka Molitor, Pablo Pérez-García, Wolfgang R. Streit

Published 2026-07-27

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Hadjira Bounabi, Bilal Yahiaoui, Meriem Gasmi, Meroua Safa Mechouche, Nico Bäse, Marno Gurschke, Rebecka Molitor, Pablo Pérez-García, Wolfgang R. Streit

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Great Plastic Puzzle

Imagine the world's landfills are like a giant, tangled ball of yarn that never seems to unravel. One of the biggest knots in this ball is a plastic called PET (polyethylene terephthalate), the material used for water bottles and polyester clothes. For decades, this plastic has been a nightmare for nature because it's built to last, resisting the elements and the tiny microbes that usually eat away at organic waste. Scientists have been on a hunt for a "molecular scissors" — an enzyme — that can snip these plastic chains apart, turning them back into their original building blocks so they can be reused.

The challenge is that PET doesn't just fall apart in one go. It's like a long chain of heavy links. To break it down, you first need to cut the big chain into smaller, manageable pieces called intermediates. The most famous "scissors" found so far, like the Ideonella sakaiensis PETase, are great at cutting the big chain but often get stuck or slowed down by these smaller pieces. It's as if the scissors get jammed by the very debris they create. To solve this, researchers need a team of specialized workers: one to chop the big chain, and another super-efficient worker to quickly clear away the smaller pieces before they pile up and stop the whole process. This is where the search for a new, versatile enzyme comes in.

Meet BhEstB: The Plastic-Smashing "All-Rounder"

In this study, a team of researchers from the University of Hamburg and other institutions went on a treasure hunt through the genome of a salt-tolerant bacterium called Bacillus halotolerans (specifically a strain named HGR5). They were looking for a new kind of enzyme that could handle the tricky job of breaking down those intermediate plastic pieces. What they found was a surprise: a highly flexible enzyme they named BhEstB.

Think of BhEstB not as a specialized laser cutter, but as a versatile specialist in the bacterial world. While many known plastic-eating enzymes are picky eaters that only like the big, hard plastic chains, BhEstB is a "promiscuous" eater when it comes to specific aromatic esters. The researchers discovered that BhEstB is structurally different from the famous PETases we already know; it lacks a specific "grip" feature (a Trp-Met-Tyr/Phe motif) that usually helps enzymes latch onto the big plastic chains. Instead, it looks more like a standard carboxylesterase, a type of enzyme usually found helping bacteria digest fats and other natural oils. Importantly, it is not a generalist for all fats; the study confirmed it has no activity on long-chain pNP esters (C16–C18), proving it is highly specific to certain chemical structures rather than a broad-spectrum fat-eater.

The Big Findings:
The team put BhEstB to the test, and the results were impressive, especially for the "middleman" pieces of plastic.

The BHET Superstar: When faced with a molecule called BHET (bis(2-hydroxyethyl) terephthalate), which is a dimer or a two-link piece of the plastic chain, BhEstB went into overdrive. It could break down a massive 50 mM of BHET in just one hour at a comfortable 30°C. That's incredibly fast.
The MHET Machine: It was also a powerhouse against MHET (mono(2-hydroxyethyl) terephthalate), a single-link piece. It degraded 73% of 50 mM (roughly 36.5 mM) of MHET within 20 hours.
The Speedster: In terms of raw speed, BhEstB had a turnover rate (Kcat) of 15.5 +/- 1.25 s⁻¹ for BHET. This makes it one of the fastest BHET-hydrolyzing enzymes ever discovered.

The Trade-Off:
However, BhEstB isn't a perfect superhero for every job. When the researchers asked it to attack the big, solid plastic polymer (PET powder), it was only about 21% as effective as the gold-standard enzyme LCC (leaf-compost cutinase). It also struggled a bit with the solid plastic foil, showing almost no activity there. The paper suggests that BhEstB is not the enzyme you would use to dissolve a whole water bottle on its own. Instead, it shines as a specialist for clearing the "traffic jam" of intermediate pieces that other enzymes leave behind.

Why This Matters:
The paper suggests that BhEstB represents an evolutionary "blueprint." It shows how an enzyme might have started out as a generalist, good at breaking down small, soluble pieces (like BHET and MHET), and could potentially evolve into a true plastic-eater. Currently, it's best viewed as a crucial partner in a team. If you pair BhEstB with a strong "big chain cutter" (like an engineered LCC), BhEstB could swoop in and clear the intermediate debris, keeping the recycling process moving smoothly.

The researchers also used computer models to find the perfect conditions for BhEstB. They found that it works best at higher temperatures (around 65°C to 75°C) and slightly basic pH levels, though the enzyme itself starts to lose its shape and stop working if it gets too hot (its melting point is around 62.3°C). This suggests that while it's a great tool, it needs to be used carefully, perhaps in a controlled industrial setting where temperature can be managed.

In short, BhEstB isn't the magic bullet that will dissolve all our plastic waste overnight. But it is a vital piece of the puzzle, offering a new, highly efficient way to handle the specific intermediate steps that currently slow down plastic recycling. It proves that sometimes, the best way to solve a complex problem isn't with one giant hammer, but with a team of specialized tools working together.

Technical Summary: A Promiscuous Bacillus Esterase Reveals a Carboxylesterase Blueprint for PET Intermediate Degradation

Problem Statement
The enzymatic depolymerization of polyethylene terephthalate (PET) is a critical objective for circular plastic recycling. While engineered variants of Ideonella sakaiensis PETase (IsPETase) and leaf-compost cutinase (LCC) have demonstrated high efficiency in degrading bulk PET, their industrial application is often bottlenecked by the accumulation of intermediate breakdown products, specifically mono(2-hydroxyethyl) terephthalate (MHET) and bis(2-hydroxyethyl) terephthalate (BHET). MHET acts as a potent competitive inhibitor for many primary PET hydrolases. Although specialized MHETases exist (e.g., IsMHETase), they often lack activity toward the polymer or BHET, and many characterized MHETases exhibit low catalytic efficiency or poor thermostability. Furthermore, most known PET-active enzymes are highly specific to the polymer or its oligomers, lacking the "promiscuity" to efficiently process both the polymer and its monomeric/dimeric intermediates simultaneously. There is a need for biocatalysts that can bridge this gap, particularly those capable of high-throughput degradation of BHET and MHET at temperatures relevant to industrial upcycling.

Methodology
The study employed a multi-faceted approach combining bioinformatics, structural biology, and biochemical characterization:

Discovery and Screening: Researchers mined the genome of a halotolerant Bacillus halotolerans strain (HGR5), isolated from terrestrial hot springs in Algeria. Using Hidden Markov Model (HMM) searches against the Plastics-Active Enzymes Database (PAZy), they identified a novel esterase, BhEstB.
Structural Analysis: The 3D structure of BhEstB was modeled using AlphaFold3. Bioinformatic tools (CD-Search, SignalP, phylogenetic analysis via MEGA11) were used to compare BhEstB against known PETases, MHETases, and carboxylesterases (e.g., BsEstB, TfCa, PET46, IsMHETase). Molecular docking simulations (Attracting Cavities 2.0) were performed to assess substrate binding for MHET, BHET, and a PET trimer (3PET).
Biochemical Characterization: The recombinant BhEstB enzyme was expressed in E. coli, purified, and characterized for substrate specificity using p-nitrophenyl (pNP) esters of varying chain lengths. Thermal stability was assessed via NanoDSF.
Kinetic and Degradation Assays: Enzyme activity was quantified against BHET, MHET, and 3PET using UHPLC. Kinetic parameters ( $K_m$ , $k_{cat}$ ) were determined using both conventional and inverse Michaelis–Menten kinetics to account for substrate solubility issues. Degradation of semi-crystalline PET powder and amorphous PET foil was tested at various temperatures (30–70°C).
Optimization: The degradation of semi-crystalline PET powder by BhEstB was optimized using Central Composite Design (CCD) and Response Surface Methodology (RSM). A hybrid approach combining Artificial Neural Networks (ANN) and Genetic Algorithms (GA) was employed to refine the predictive model for optimal temperature, pH, and enzyme concentration.

Key Results

Structural Divergence: BhEstB is a carboxylesterase belonging to the $\alpha/\beta$ -hydrolase fold but is phylogenetically distinct from canonical PETases (which cluster in Pseudomonadota and Actinomycetota). It shares high sequence identity (83%) with Bacillus subtilis BsEstB. Unlike typical PETases, BhEstB lacks the characteristic Trp-Met-Tyr/Phe motif associated with PET binding but possesses a lid domain similar to TfCa.
Superior Intermediate Degradation: BhEstB exhibits exceptional activity toward PET intermediates. At 30°C, it degraded 50 mM BHET within 1 hour and 38 mM MHET within 24 hours. Its catalytic efficiency ( $k_{cat}$ ) for BHET was 15.5 s $^{-1}$ .
Comparative Performance: While BhEstB's activity on bulk PET powder was moderate (approximately 21% of wild-type LCC activity), it significantly outperformed the engineered variant LCCICCG and the archaeal feruloyl esterase PET46 in the turnover of BHET and MHET. It degraded 73% of MHET within 20 hours at 30°C, outperforming LCCICCG and PET46, though it was less efficient than the specialized IsMHETase.
Thermal Stability: BhEstB has a melting temperature ( $T_m$ ) of 62.3°C. It retains activity up to 50°C but loses function rapidly above this point. Its optimal activity on PET powder was observed at 60–70°C, near its thermal stability limit.
Kinetic Insights: Inverse Michaelis–Menten kinetics revealed a strong affinity for BHET ( $K_m$ of 0.1 mM in the insoluble substrate model), suggesting BhEstB follows an insoluble substrate model.
Optimization Modeling: While RSM indicated that higher enzyme concentrations and temperatures (>65°C) favored degradation, the ANN-GA hybrid model provided a more accurate prediction, estimating a maximum product release of 1,286 µM, closely matching experimental results (1,260 µM).

Significance and Claims
The authors posit that BhEstB represents a "blueprint" for an enzyme transitioning from monomeric intermediate processing toward true polymer activity. The study claims that BhEstB is one of the most efficient native BHET-hydrolyzing enzymes characterized to date. Its high promiscuity—possessing significant activity against BHET, MHET, and the polymer simultaneously—suggests it may serve as an evolutionary link or a versatile scaffold for developing multi-substrate biocatalysts.

The paper emphasizes that while BhEstB alone is not a replacement for high-performance PETases like LCCICCG for bulk polymer degradation, its unique profile makes it highly valuable for synergistic formulations. Specifically, it offers potential for "thermophilic plastic upcycling" when combined with other enzymes to overcome the kinetic bottlenecks caused by intermediate accumulation. The findings suggest that carboxylesterases from the Bacillus genus, which are widespread in nature and not necessarily evolved specifically for plastic, may hold untapped potential for the efficient degradation of PET intermediates, a critical step in closing the loop for PET recycling. The authors modestly conclude that BhEstB is a tool to improve industrial recycling efficiency when used in combination with other hydrolases or engineered strains, rather than a standalone solution for bulk PET depolymerization.

The Great Plastic Puzzle

Meet BhEstB: The Plastic-Smashing "All-Rounder"

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