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All the bolded portions are the contributions added to the original wiki page for PETase.

PETases are an esterase class of enzymes that catalyze the hydrolysis of poly (ethylene terephthalate) (PET) plastic to monomeric mono-2-hydroxyethyl terephthalate (MHET). The idealized chemical reaction is (where n is the number of monomers in the polymer 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. PETases can't catalyze the hydrolysis of aliphatic polyesters like polybutylene succinate or polylactic acid.

History

The first PETase enzyme was discovered in 2016 from Ideonella sakaiensis strain 201-F6 bacteria found from sludge samples collected close to a Japanese PET bottle recycling site. Scientists suggested that the PETase enzyme may have had past enzymatic activity associated with degradation of a waxy coating on plants. Normally the natural degradation of PET without PETase will take hundred of years. PET (polyethylene terephthalate-plastics)is a very common source of many plastic items used in the daily life. PETase can degrade PET in a way that is not harmful to the environment. Other types of PET degrading hydolases 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) and 1977 (lipase) for example. PET plastic was put into widespread use in the 1970s and it has been suggested that PETases in bacteria evolved only recently.

Chemical Reaction - PETase Hydrolysis

Plastic breakdown by PETase

PETase hydrolyses PET (polyethylene terephthalate) into soluble building blocks due to reaction with water which is a bioconversion of plastics. PET is a polymer composed of ester bond-linked terephthalate (TPA) and ethylene glycol (EG). A high molecular weight and other properties make PET a great utilizing plastic. By the hydrolysis reaction PET hydrolyzing enzymes decompose PET into building blocks which is helpful for the environment. During the hydrolyzing PET, the enzyme produces mono-(2-hydroxyethyl) terephthalic acid (MHET), TPA ,and bis-2(hydroxyethyl) TPA (BHET). The novel bacteria called Ideonella sakaieensis is isolating and it utilize PET as an energy and carbon source. The deonella sakaieensis sticks to the surface of PET and keep a cutinase enzyme that allow PET to degrade. That reaction allows to degrade PET, and make it less harmful for the environment.

Structure

Ribbon diagram 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.

As of April 2018, there were 13 known three-dimensional crystal structures of PETases: 6EQD, 6EQE, 6EQF, 6EQG, 6EQH, 6ANE, 5XJH, 5YNS, 5XFY, 5XFZ, 5XG0, 5XH2 and 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. Scientists revolutionized the degradation rate of PET by PETase as a result of narrowing of the binding site through mutation of two active-site residues, although there are three comprising the active site. In the location of the active site, a catalytic triad is formed by the three residues Ser160, Asp206, and His237. The

MHET breakdown in I. sakaiensis

MHET is broken down in I. sakaiensis by the action of MHETase enzyme to terephthalic acid and ethylene glycol. The I. sakaiensis bacterium adhere to the PET surface and release a unique enzyme, similar to cutinase, with the ability to degrade PET. These are environmentally harmless as they are broken down further to produce carbon dioxide and water.

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).

Figure A. Classification of PETase-like enzymes. Figure B. A comparison of residues between two type II sub-classes and type I class of PETase-like enzymes.

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 Ideonella sakaiensis. 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.