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Figure 1
The two copper ions (red–brown) bound to the surface of human α-carbonic anhydrase II. The copper to the right is situated at the classical active site where the zinc ion is normally bound by three histidine ligands (His94, His96 and His119). An NO2− ion is bound at this copper. The left copper is the second metal site where copper can bind in a square-planar nitro­genous setting like in a porphyrin. His64, which is found in two conformations, and a number of observed water molecules (blue dots), can facilitate the transport of electrons between the two metals.

IUCrJ
Volume 7| Part 2| March 2020| Pages 144-145
ISSN: 2052-2525
https://doi.org/10.1107/S2052252520002481