Hemocyanin copper binding

Truchot, J.P. and F. Boitel. 1992. In vitro and in vivo effects of copper on hemocyanin-02 binding in the shore crab, Carcinus maenas. Comp. Biochem. Physiol. 103C 339-343. 

As is the case for other copper-binding proteins, the ligands are further oriented by hydrogen bonding to neighboring atoms. There is a local twofold axis relating the Cu(A) and Cu(B) sites of hemocyanin ... 

The active site of type 3 copper proteins consists of two copper atoms. Each of them is bound by three histidines provided by an antiparallel alpha-helix pair (Figure 1). Cu-A denotes the copper-binding site closer to the N-terminns, whereas Cu-B is the copper-binding site closer to the C-terminns. According to crystal structures of arthropod and mollnsc hemocyanins, the coppers bind a dioxygen molecnle in the same way as a peroxide in side-on... 

Copper-binding Regions of Hemocyanins and Tyrosinases. In the previous review by Lerch, the similar copper centers of hemocyanins and phenoloxidases were thoroughly discussed on the basis of sequence comparisons. Thus, the surroundings of Cu-A and Cu-B are different as already... 

Our biomimetic investigations have focused on the metalloproteins hemocyanin (He) (11-17) and tyrosinase (11,12,14,16,18,29), which contain two copper ions in their active center. The function of hemocyanin is to bind and transport dioxygen in the hemolymph of molluscs and arthropods. Studies employing EXAFS spectroscopy have shown that in the deoxy form, two (19-21) or three (13,21) imidazole units fiom protein histidine residues coordinate to each cuprous ion. Upon addition of O2 to give oxy-Hc, considerable changes take place in the coordination sphere giving rise to tetragonally coordinated Cu(II) ions... 

Evidence tom a variety of sources indicates that the active site of tyrosinase is very similar to that of hemocyanin, a dioxygen-binding protein found in molluscs and arthropods (15,16). This type of active site contains two copper ions, which are cuprous in the deoxy state, and which reversibly bind dioxygen, forming the oxy form of the enzyme or protein in which a peroxy ligand bridges between two cupric ions. 

The veiy different reactivities of hemocyanin and tyrosinase toward oxidizable substrates se n to be due to the presence of a substrate binding site in the latter. It appears, therefore, that the oxygenation of the phenol substrate occurs either by reaction of copper-bound pooxide or hydrc ieroxide with the ortho position of the copper-bound phenol (16) or that the 0-0 bond of the peroxide ligand is cleaved... 

The enzyme requires two copper ions per subunit for full expression of activity (18), but, unlike tyrosinase and hemocyanin, there is an absence of magnetic coupling between the two Cu(II) sites and both appear to be separate, isolated mononuclear copper sites (17). The process of dioxygen binding and activation appears to involve interaction of the doxygen molecule with only one copper ion, and it is also found that a proton is requir for the hydroxylation of substrate (19). 

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