Poly(ethylene terephthalate) (PET) is used extensively worldwide in plastic products, and its accumulation in the environment has become a global concern. Because the ability to enzymatically degrade PET has been thought to be limited to a few fungal species, biodegradation is not yet a viable remediation or recycling strategy. By screening natural microbial communities exposed to PET in the environment, we isolated a novel bacterium, Ideonella sakaiensis 201-F6, that is able to use PET as its major energy and carbon source. When grown on PET, this strain produces two enzymes capable of hydrolyzing PET and the reaction intermediate, mono(2-hydroxyethyl) terephthalic acid. Both enzymes are required to enzymatically convert PET efficiently into its two environmentally benign monomers, terephthalic acid and ethylene glycol.

Most petroleum-derived plastics, as exemplified by poly­(ethylene terephthalate) (PET), are chemically inactive and highly resistant to microbial attack. The accumulation of plastic waste results in environmental pollution and threatens ecosystems, referred to as the “microplastic issue”. Recently, PET hydrolytic enzymes (PHEs) have been identified and we reported PET degradation by a microbial consortium and its bacterial resident, Ideonella sakaiensis. Bioremediation may thus provide an alternative solution to recycling plastic waste. The mechanism of PET degradation into benign monomers by PET hydrolase and mono­(2-hydroxyethyl) terephthalic acid (MHET) hydrolase from I. sakaiensis has been elucidated; nevertheless, biodegradation may require additional development for commercialization owing to the low catalytic activity of these enzymes. Here, we introduce PET degrading microorganisms and the enzymes involved, along with the evolution of PHEs to address the issues that hamper microbial and enzymatic PET degradation. Potential applications of PET degradation are also discussed.

The molecular structure of the pepstatin-insensitive carboxyl peptidase from Scytalidium lignicolum, formerly known as scytalidopepsin B, was solved by multiple isomorphous replacement phasing methods and refined to an R factor of 0.230 (R free ‫؍‬ 0.246) at 2.1-Å resolution. In addition to the structure of the unbound peptidase, the structure of a product complex of cleaved angiotensin II bound in the active site of the enzyme was also determined. We propose the name scytalidocarboxyl peptidase B (SCP-B) for this enzyme. On the basis of conserved, catalytic residues identified at the active site, we suggest the name Eqolisin for the enzyme family. The previously uninvestigated SCP-B fold is that of a ␤-sandwich; each sheet has seven antiparallel strands. A tripeptide product, Ala-Ile-His, bound in the active site of SCP-B has allowed for identification of the catalytic residues and the residues in subsites S1, S2, and S3, which are important for substrate binding. The most likely hydrolytic mechanism involves nucleophilic attack of a general base (Glu-136)-activated water (OH ؊ ) on the si-face of the scissile peptide carbonylcarbon atom to form a tetrahedral intermediate. Electrophilic assistance and oxyanion stabilization is provided by the side-chain amide of Gln-53. Protonation of the leaving-group nitrogen is accomplished by the general acid function of the protonated carboxyl group of Glu-136. P epstatin-insensitive carboxyl peptidases (1-8) originally found by Murao and Oda can be classified into two groups, bacterial and fungal carboxyl peptidases. The 3D structures of the two members of the bacterial carboxyl peptidases have recently been determined (9, 10). From the tertiary folds, it was clear that these enzymes are members of the subtilisin family. Each enzyme has a glutamate residue acting as the general base instead of the histidine present in the subtilisins. The environments of the carboxylates contribute to the low pH optima of these enzymes.The fungal pepstatin-insensitive carboxyl peptidases (5) are represented by several enzymes, among them the enzyme that is the subject of this article. Formerly, this enzyme has been called Scytalidopepsin-B because of its low pH optimum and preponderance of acidic residues (8). It is isolated from Scytalidium lignicolum ATCC24568. We have chosen to rename this enzyme scytalidocarboxyl peptidase-B (SCP-B) to reflect its fungal source and active-site carboxyl groups. SCP-B is synthesized as a precursor consisting of two regions: an amino-terminal preprosegment of 54 amino acid residues and a mature enzyme consisting of 206 amino acid residues (11). The mature enzyme has no sequence similarity to the well known pepsin-like or retroviral aspartic peptidases, but it has significant similarity to the other fungal pepstatin-insensitive carboxyl peptidases (Fig. 1).SCP-B and the other carboxyl peptidases described here have been classified in family A4 of the aspartic endopeptidases in the MEROPS (http:͞͞merops.sanger.ac.uk). database. On the basis of our structur...