Hemocyanin (Hc), a copper-containing extracellular multimeric protein, is the major protein component of hemolymph in different arachnid groups. Hc possesses 7 or 8 very well-characterized types of monomers with molecular weights ranging from 70 to 85 kDa, organized in hexamers or multiple of hexamers. The present chapter compiles the existing data with relation to the function of this protein in the arachnids. Hc has as main function the reversible transport of O2, but it shows many secondary though not less important functions. With reference to this, it has been described that Hc can transport hydrophobic molecules (lipid-derived hormones and lipids) to the different organs, having a key role in the lipid transport system. In arachnids, like in other arthropods and invertebrates, Hc has phenoloxidase function which is related to different metabolic processes such as melanin formation and defense against pathogens. In addition, Hc has additional defensive functions since it can serve as precursor for the production of antimicrobial peptides. In short, the evolution of this protein has led to the development of multiple functions essential for organisms possessing this protein.
Keywords: Antimicrobial peptides; Arachnids; Hemocyanin; Hexamers; Immune response; Lipid transport; Oxygen transport; Phenoloxidase.