An electronic device may have polymer structures such as polymer electronic device structures, gaskets for forming seals between display cover layers and housing structures, polymer structures that fill gaps between housing walls, adhesive, adhesive tapes, and other structures with polymer. To prevent degradation of the polymer from exposure to fatty acids and other potentially harmful materials, a protective enzyme may be incorporated into one or more of the polymer structures. The protective enzyme may be a lipoxygenase enzyme or other enzyme that degrades harmful substances such as fatty acids and thereby neutralizes the harmful substances and prevents damage to the polymer structures.