Human carbonic anhydrase II (CA II) is a zinc metalloenzyme that catalyzes the reversible hydration/dehydration of CO2/HCO3 -. Although CA II has been extensively studied to investigate the proton-transfer process that occurs in the active site, its underlying mechanism is still not fully understood. In this presentation, we present the crystallographic structures of CA II using the High-pressure cryocrystallographic method. The structures reveal new intermediate solvent states of CA II that provide crystallographic snapshots during the restoration of the proton-transfer water network in the active site. In addition, the structures provide hints for the role of metal ions in CA II catalysis.