How hemocyanin’s copper binds oxygen
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In many spiders, oxygen in the hemolymph is carried by hemocyanin, a copper-containing protein. When it binds oxygen, hemolymph can look blue; without oxygen, the protein is nearly colorless. In humans, oxygen is carried by iron-containing hemoglobin, which gives blood its red color.
At hemocyanin’s oxygen-binding site, two copper atoms interact with one oxygen molecule. Six histidine amino-acid residues coordinate those copper atoms. In a study of the spider *Cupiennius salei*, researchers determined its subunit sequences using RNA-copy DNA analysis, gel electrophoresis, and mass spectrometry.