AN ENGINEERED ENZYME FOR ENDLESS RECYCLING OF PET PLASTIC BOTTLES

A new enzyme with considerably improved catalytic performances and stability has been designed and developed with the aid of atomic resolution structures and molecular modelling and dynamics studies. The new enzyme can depolymerise plastics made of polyethylene terephthalate (PET) and facilitate its recycling into new bottles.

STRUCTURAL BIOLOGY

42 ESRF

Molecular basis of β-arrestin coupling to formoterol-bound β1-adrenoceptor, Y. Lee (a), T. Warne (a), R. Nehmé (a), S. Pandey (b), H. Dwivedi-Agnihotri (b), M. Chaturvedi (b), P.C. Edwards (a), J. García-Nafría (c,d), A.G. Leslie (a), A.K. Shukla (b) and C.G. Tate (a), Nature

583, 862-866 (2020); https://doi. org/10.1038/s41586-020-2419-1. (a) MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge (UK) (b) Department of Biological Sciences and Bioengineering, Indian Institute of Technology, Kanpur (India)

(c) Institute for Biocomputation and Physics of Complex Systems (BIFI), University of Zaragoza, Zaragoza (Spain) (d) Laboratorio de Microscopías Avanzadas, University of Zaragoza, Zaragoza (Spain)

PRINCIPAL PUBLICATION AND AUTHORS

can arise after activation of the target receptor. Each GPCR has the capacity to signal by at least two different pathways, which can be either a G protein pathway or a β-arrestin pathway. Signalling down these two pathways can give rise to different effects in the cell and ultimately different physiological responses. Pain relief by opiates is mediated by the G protein pathway and side effects are mediated by the arrestin pathway. Different drugs will activate the two different pathways to varying degrees, and this phenomenon is known as biased signalling. A biased agonist of the G protein pathway has reduced signalling down the arrestin pathway, while an arrestin-biased agonist has the opposite properties. However, the molecular basis for biased agonism is unknown. This work set out to study this using β1AR as a model GPCR.

To understand biased agonism at β1AR, two structures were determined: β1AR coupled to a G protein, and β1AR coupled to arrestin. It was also essential to use the same ligand in both structures so that any observed differences between the two structures were due to the coupled protein and not the ligand. Formoterol was chosen as the ligand because it is an arrestin-biased agonist of both β2AR and β1AR.

A nanobody (Nb80) was used as a G protein mimetic to determine the X-ray crystal structure of β1AR in a G protein-coupled conformation to an overall resolution of 2.9 Å at beamline ID30A-1. The β1AR used for the crystal structure was in detergent and formed well-diffracting crystals. However, this strategy could not be used for arrestin-coupled β1AR because arrestin coupling is inefficient to detergent-solubilised receptors and requires a lipid bilayer. Therefore, the arrestin-β1AR complex was made in lipid nanodiscs and the structure was determined by cryo-EM to an overall resolution of 3.3 Å. Careful comparison between the two structures showed that there were significant differences in the orthosteric binding pocket where formoterol binds and also on the intracellular face of the receptor where either arrestin or G protein couples (Figure 27).

These small differences could potentially be used to develop modified drugs that bind to the orthosteric binding pocket and signal through only the arrestin pathway. Another possibility is to target the region at the cytoplasmic face of the receptor, where the surfaces of the receptor when coupled to either a G protein or arrestin would be different.

Dating only from the 1950s, plastics have rapidly become the most produced synthetic materials in human history. While they have undoubtedly changed lives, the environment is the unfortunate witness that their end of life still remains a major challenge. In 2018, the global production of plastics reached 360 million tonnes, of which almost 200 accumulated in landfill and nature [1]. Polyethylene terephthalate (PET) is one of the most common thermoplastic polymers on the market, with 70 million tonnes per year produced for textile fibres and packaging. The main recycling process for PET is thermomechanical, and it only recycles part of

the waste (clear bottles). Moreover, this recycled PET shows decreased properties compared to virgin PET. PET hydrolases have been described as depolymerising this polymer into its monomers, ethylene glycol and terephthalic acid [2], and in 2016, a new PET hydrolase was discovered [3]. Nevertheless, these enzymes are still far from enabling economically viable enzymatic bio- recycling [4,5].

In this work, an enzyme capable of depolymerising PET in just a few hours was developed. A forgotten PETase, called Leaf Compost Cutinase (LCC) [6], whose efficiency