Enzyme Selection, Optimization, and Production toward Biodegradation of
Waste Poly(ethylene terephthalate) at Scale
Abstract
Poly(ethylene terephthalate) (PET) is one of the world’s most widely
used polyester plastics. Due to its chemical stability, PET is extremely
difficult to hydrolyze in a natural environment. Recent discoveries in
new polyester hydrolases and breakthroughs in enzyme engineering
strategies have inspired enormous research on biorecycling of PET. This
study summarizes our research efforts toward large-scale, efficient, and
economical biodegradation of waste PET, including PET hydrolase
selection and optimization, high-yield enzyme production, and
high-capacity enzymatic degradation of waste PET. First, genes encoding
PETase and MHETase from Ideonella sakaiensis and the ICCG variant
of leaf-branch compost cutinase (LCC) were codon-optimized and expressed
in Escherichia coli BL21(DE3) for high-yield production. To
further lower the enzyme production cost, a pelB leader sequence
was fused to LCC so that the enzyme can be secreted into the
medium to facilitate recovery. To help bind the enzyme on the
hydrophobic surface of PET, a substrate-binding module in a
polyhydroxyalkanoate depolymerase from Alcaligenes faecalis (PBM)
was fused to the C-terminus of LCC. The resulting four different
LCC variants (LCC, PelB-LCC, LCC-PBM, and PelB-LCC-PBM), together with
PETase and MHETase, were compared for PET degradation efficiency. A
fed-batch fermentation process was developed to produce the target
enzymes up to 1.2 g/L. Finally, the best enzyme, PelB-LCC, was selected
and used for the efficient degradation of 200 g/L recycled PET in a
well-controlled, stirred-tank reactor. The results will help develop an
economical and scalable biorecycling process toward a circular PET
economy.