Coordinated Nitrogen Atoms in Bimetallic Active Sites of Oxyhemocyanin and Oxyhemoerythrine

Oxyhemocyanin:
- Oxyhemocyanin is a respiratory protein found in the hemolymph of many arthropods and mollusks.
- It contains two copper atoms per subunit that form a binuclear center responsible for oxygen transport.
- Histidine amino acid residues are involved in coordinating the copper atoms in the bimetallic active site.
- In oxyhemocyanin, each copper atom is coordinated by three histidine residues, for a total of six coordinated nitrogen atoms.

Oxyhemoerythrine:
- Oxyhemoerythrine is a respiratory pigment found in some marine invertebrates, such as polychaetes and sipunculids.
- It contains iron atoms instead of copper, and each subunit has two iron atoms.
- The bimetallic center in oxyhemoerythrine is also coordinated by histidine residues.
- However, the coordination geometry and number of coordinated nitrogen atoms differ from oxyhemocyanin.
- In oxyhemoerythrine, each iron atom is coordinated by two histidine residues, for a total of four coordinated nitrogen atoms per iron atom.

Conclusion:
- The number of histidine amino acid nitrogen atoms co-coordinated to bimetallic active sites of oxyhemocyanin and oxyhemoerythrine respectively are six and four.
- These coordinated nitrogen atoms play a crucial role in stabilizing the bimetallic center and facilitating oxygen transport in these respiratory proteins.