Enhanced enzymatic hydrolysis of pretreated polyester textile at high solids loading through fusion with a carbohydrate binding module

High solids loading is essential for economically and environmentally viable enzymatic recycling of poly(ethylene terephthalate) (PET), yet performance gains from enzyme-binding module fusions under industrially relevant conditions remain unclear. This study evaluated a rationally designed fusion between the thermophilic Saccharopolyspora flava cutinase and the type A carbohydrate‑binding module from Spirochaeta thermophila, selected for compatible temperature optima and substrate‑morphology preferences. The fusion enzyme retained the thermostability of its constituent domains and sustained activity at 50 °C over extended operation, while rapid loss of kinetic stability above this point accounted for poor performance at elevated temperatures. The fusion enzyme displayed increased enzyme binding capacity on both amorphous and semi‑crystalline PET, while affinity enhancement was substrate‑dependent and greatest for crystalline PET. However, enhanced adsorption on crystalline substrates did not translate into increased hydrolysis, indicating that catalytic turnover is limited by factors beyond binding, consistent with the Sabatier principle. In contrast, substantial improvements in PET depolymerisation were observed for amorphous substrates at industrially relevant (20 wt%) solids loadings under pH‑controlled reactor conditions. The largest enhancement occurred for a pre‑consumer PET textile following amorphisation and micronisation, far exceeding gains observed for amorphous films and powders. These results demonstrate that fusion enzymes have the potential to significantly enhance PET hydrolysis at high solids when substrate morphology permits productive enzyme-polymer interactions. Performance is therefore highly dependent on the specific pairing of enzyme and binding module, and on substrate properties, underscoring the need for evaluation under deployment‑relevant conditions.