PETases are an

esterase An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis. A wide range of different esterases exist that differ in their substrate specificity, their protein structure ...

class of

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

s that

catalyze Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

of polyethylene terephthalate (PET) plastic to

monomer In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

ic mono-2-hydroxyethyl terephthalate (MHET). The idealized chemical reaction is (where n is the number of

s in the

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

chain): :(ethylene terephthalate)_n + H₂O → (ethylene terephthalate)_n-1 + MHET Trace amount of the PET breaks down to bis(2-hydroxyethyl) terephthalate (BHET). PETases can also break down PEF-plastic ( polyethylene-2,5-furandicarboxylate), which is a bioderived PET replacement, into the analogous . PETases can't catalyze the hydrolysis of

aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, ...

polyesters like

polybutylene succinate Polybutylene succinate (PBS) (sometimes written polytetramethylene succinate) is a thermoplastic polymer resin of the polyester family. PBS is a biodegradable aliphatic polyester with properties that are comparable to polypropylene. It may also ...

polylactic acid Polylactic acid, also known as poly(lactic acid) or polylactide (PLA), is a thermoplastic polyester with backbone formula or , formally obtained by condensation of lactic acid with loss of water (hence its name). It can also be prepared by ri ...

. Non-enzymatic natural degradation of PET will take hundreds of years, but PETases can degrade PET in matter of days.

History

The first PETase was discovered in 2016 from ''

Ideonella sakaiensis ''Ideonella sakaiensis'' is a bacterium from the genus'' Ideonella'' and family Comamonadaceae capable of breaking down and consuming the plastic polyethylene terephthalate (PET) using it as both a carbon and energy source. The bacterium was o ...

'' strain 201-F6

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

found from sludge samples collected close to a Japanese

PET bottle recycling Although PET is used in several applications, (principally textile fibres for apparel and upholstery, bottles and other rigid packaging, flexible packaging and electrical and electronic goods), as of 2022 only bottles are collected at a subst ...

site. Other types of PET degrading

hydrolase Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some common examples of hydrolase enzymes are es ...

s have been known before this discovery. These include hydrolases such as lipases, esterases, and cutinases. Discoveries of polyester degrading enzymes date at least as far back as 1975 (α-

chymotrypsin Chymotrypsin (, chymotrypsins A and B, alpha-chymar ophth, avazyme, chymar, chymotest, enzeon, quimar, quimotrase, alpha-chymar, alpha-chymotrypsin A, alpha-chymotrypsin) is a digestive enzyme component of pancreatic juice acting in the duod ...

) and 1977 (

lipase Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...

) for example. PET plastic was put into widespread use in the 1970s and it has been suggested that PETases in bacteria evolved only recently. PETase may have had past enzymatic activity associated with degradation of a waxy coating on plants.

Structure

As of April 2019, there were 17 known three-dimensional crystal structures of PETases
6QGC6ILX6ILW5YFE6EQD6EQE6EQF6EQG6EQH6ANE5XJH5YNS5XFY5XFZ5XG05XH2
an
5XH3
PETase exhibits shared qualities with both lipases and cutinases in that it possesses an α/β-hydrolase fold; although, the active-site cleft observed in PETase is more open than in cutinases. The ''Ideonella sakaiensis'' PETase is similar to dienelactone hydrolase, according to

Pfam Pfam is a database of protein families that includes their annotations and multiple sequence alignments generated using hidden Markov models. The most recent version, Pfam 35.0, was released in November 2021 and contains 19,632 families. Use ...

. According to ESTHER, it falls into the Polyesterase-lipase-cutinase family. There are approximately 69 PETase-like enzymes comprising a variety of diverse organisms, and there are two classifications of these enzymes including type I and type II. It is suggested that 57 enzymes fall into the type I category whereas the rest fall into the type II group, including the PETase enzyme found in the ''

.'' Within all 69 PETase-like enzymes, there exists the same three residues within the active site, suggesting that the catalytic mechanism is the same in all forms of PETase-like enzymes. PETase 5XH3 with HEMT-surface.png, Surface of the PETase double

mutant In biology, and especially in genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. It ...

( R103 G and S131 A) with HEMT (1-(2-hydroxyethyl) 4-methyl terephthalate) bound to its active site. HEMT is an analogue of MHET, and has an additional methanol

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

ified to it. PDBID: 5XH3. PETase active site.png,

Ribbon diagram Ribbon diagrams, also known as Richardson diagrams, are 3D schematic representations of protein structure and are one of the most common methods of protein depiction used today. The ribbon shows the overall path and organization of the protein b ...

of PETase with three residues Ser160, Asp206, and His237. The catalytic triad is represented by cyan-colored sticks. The active site is shown in orange to represent stimulation by a 2-HE(MHET)₄ molecule.

Mutations

In 2018 scientists from the University of Portsmouth with the collaboration of the National Renewable Energy Laboratory of the United States Department of Energy developed a mutant of this PETase that degrades PET faster than the one in its natural state. In this study it was also shown that PETases can degrade polyethylene 2,5-furandicarboxylate (PEF).

Biological pathway

In ''I. sakaiensis'', the resultant MHET is further broken down by the action of

MHETase The Enzyme MHETase is a hydrolase, which was discovered in 2016. It cleaves Mono-(2-hydroxyethyl)terephthalic acid, the PET degradation product by PETase, to ethylene glycol and terephthalic acid. This pair of enzymes, PETase and MHETase, enable ...

enzyme to

terephthalic acid Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced an ...

and

ethylene glycol Ethylene glycol ( IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an od ...

. * Laboratory experiments showed that chimeric proteins that artificially link a MHETase and a PETase outperform similar mixtures of free enzymes.

References

{{Esterases Hydrolases Recycling

History

Structure

Mutations

Biological pathway

See also

References