Catalytic Triad of Serine Proteases
It depends how you define an acid. For what it‘s worth, the chemical definition Google presents when I search is “a molecule or other species which can donate a proton or accept an electron pair in reactions”. On that basis the serine residue in the catalytic triad is acting as a weakly ionizing acid. 

Of course in aqueous solution a free serine hydroxyl group is not acidic, but one of the important features of enzyme catalysis is that amino acid residues are operating in a different environment that can enhance their activity. One factor is the proximity of other residues. Another is the effective dielectric constant can be different in the hydrophobic interior of a protein which can exclude water molecules.