PETase S121E_D186H_R280A

Usage and Biology

The enzymes PETase and MHETase were first discovered in Ideonella sakaiensis in 2016 by a group of researchers in Japan. These enzymes were found to degrade polyethylene terephthalate (PET) into its monomers, terephthalic acid (TPA) and ethylene glycol (EG). PETase degrades PET into Mono-(2-hydroxyethyl)terephthalic acid (MHET), Bis(2-Hydroxyethyl) terephthalate (BHET) and TPA, the main product being MHET. MHET is further degraded by MHETase into TPA and EG. We are aiming to use mutants of these enzymes to degrade the microfibres that are coming off clothing during washing cycles. The enzymes would be secreted into a filter that captures the microfibres. This sequence is the Escherichia coli K12 (E. coli K12) codon optimized DNA of the R208A mutant of PETase, with an attached His tag. The His tag was attached in order to more easily identify the enzymes. These mutations have been reported in past papers to increase the thermostability of PETase.

The native predicted signal peptide (Met1-Ala33) was removed from the WT PETase sequence (Seo et al 2019) and replaced with a start codon (Met), however all mutations are numbered according to the full-length WT sequence. The amino acid sequence was submitted to Twist Bioscience who codon optimised the sequence for E. coli, ensuring that there were no forbidden restriction sites, BsaI or SapI, to allow for potential TypeIIS assembly. The resulting CDS was synthesised and cloned, by Twist, into pET28. This added a 63 AA His-tag and thrombin cleavage site to the N-terminal of the protein, a T7 promoter and T7 terminator.

Characterisation

In order to characterise our part and determine the rate of its activity and prove its functionality we have run a series of experiments. After transforming the Arctic Express, Rosetta Gami and BL21 DE3 strains of E. coli with our plasmid we induced the expression of the enzymes using IPTG. In order to confirm that the enzyme expression has been successful we ran a western blot which showed the presence of the enzyme in the soluble fractions of the sonicated cells. Afterwards the enzyme was purified and used in assays to show its functionality and determine the rate of its activity.

Western blot of the soluble fraction of Arctic Express strain showing expression of all mutants. The PageRuler Plus prestained protein ladder was used and labeled with the corresponding sizes. The negative control is labeled with 1. This part (PETase S121E_D186H_R280A) is labeled with 4. A clear band is visible with a size of about 30 kDa which is the size of PETase with the His tag attached to it.

Western blot of the soluble fraction of Rosetta Gami strain showing expression of all mutants. The PageRuler Plus prestained protein ladder was used and labeled with the corresponding sizes. The negative control is labeled with 1. This part (PETase S121E_D186H_R280A) is labeled with 4. A clear band is visible with a size of about 30 kDa which is the size of PETase with the His tag attached to it.

Expression in E.coli

Purification graphs

Nickel Affinity Chromatography

Nickel affinity trace showing the elution of the protein from the column with an increase in Imidazole concentration

Size Exclusion Chromatography (Superdex-200)

Further purification of the enzyme by size exclusion chromatography using a calibrated Superdex-200 column. The large peak at an elution volume of ~90 ml shows the monomeric form of the protein

The esterase activity assay shows the production of p-nitrophenol (A405nm) at different substrate concentrations

The specific activity of the enzyme at differing substrate concentrations

Thermal Stability Graphs

Thermal Stability

The thermal stability assay shows the production of p-nitrophenol (A405nm) after the pre-incubation of the enzyme at increasing temperatures before the esterase assay was carried out.