Hemocyanin is an oxygen-transport metalloprotein present in some invertebrate animals. This protein contains :

Hemocyanin is a blue, extracellular oxygen-transport metalloprotein found in most molluscs and some arthropods [1]. Unlike hemoglobin, which uses iron in a heme group, hemocyanin utilizes a binuclear copper center to facilitate oxygen binding. Specifically, the active site of hemocyanin contains two copper atoms (ions) that are coordinated by six highly conserved histidine residues [2]. These two copper atoms work together to reversibly bind a single molecule of dioxygen (O2) [3]. In its deoxygenated state, the copper ions are in the Cu(I) oxidation state and the protein is colorless; upon binding oxygen, they are oxidized to Cu(II), giving the protein its characteristic blue color [2]. This type-3 copper protein structure is highly conserved across different invertebrate phyla despite differences in their overall quaternary structures [2].

Sources

1. [1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5899709/
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC12169132/
3. [3] https://www.sciencedirect.com/topics/immunology-and-microbiology/hemocyanin