Reaction pathways for the enzymatic degradation of poly(ethylene terephthalate): What characterizes an efficient PET-hydrolase?

Bioprocessing of polyester waste has emerged as a promising tool in the quest for a cyclic plastic economy. One key step is the enzymatic breakdown of the polymer, and this entails a complicated pathway with substrates, intermediates, and products of variable size and solubility. We have elucidated this pathway for poly(ethylene terephthalate) (PET) and four enzymes. Specifically, we combined different kinetic measurements and a novel stochastic model, and found that the ability to hydrolyze internal bonds in the polymer (endo-lytic activity) was a key parameter for overall enzyme performance. Endo-lytic activity promoted the release of soluble PET fragments with two or three aromatic rings, which, in turn, were broken down with remarkable efficiency (k_cat/K_M-values of about 10⁵  M^-1 s^-1 ) in the aqueous bulk. This meant that about 70% of the final, monoaromatic products was formed via soluble di- or tri-aromatic intermediates.