Kiss-Szeman_2025_Protein.Sci_34_e70320

Acylpeptide hydrolase (APEH) or acylaminoacyl-peptidase (AAP) is a serine hydrolase that regulates protein metabolism. It can also bind to and process unusual substrates, acting as a detoxifier. To better understand its promiscuous specificity, we determined the cryo-EM structures of mammalian APEH complexed with classical serine protease partners: a chloromethyl-ketone (CMK) inhibitor, an organophosphate (OP) pesticide (dichlorvos), and benzenesulfonyl-fluoride. Since CMK derivatives of N-acetylated peptides were suggested to induce apoptosis by inhibiting APEH, while OP complexes may serve as biomarkers of OP exposure and are linked to cognitive enhancement, these complexes carry physiological significance. We identified a unique strand-breaker Pro residue in the hydrolase domain, which relaxes the active site into a partially inactivated but more spacious conformation, transforming the classical serine protease apparatus into a versatile yet potent hydrolysis center with broad specificity, distinguishing the mammalian enzyme not only from other APEHs but also from serine alpha/beta hydrolases sharing essentially the same fold.