Hemerythrin crystal structure

From X-ray crystal structures the AT of the imidazole rings of the two histidine residues are coordinated to the Cu in plastocyanin (3, 4, 7), azurin (8), pseudoazurin (12), and CBP il3). However, in studies on Co(II)-substituted stellacyanin (71), it has been demonstrated that both histidines bind the metal via the N atom. Similar differences have been observed in the case of binuclear Fe proteins for example. Thus in ribonucleotide reductase the of histidine is coordinated, whereas in hemerythrin it is the N atom which is involved (85). In carbonic anhydrase the two coordinated imidazoles have and N atoms respectively bonded to the same Zn (85). The differences are most likely attributable to steric factors involving the polypeptide. 

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