Hemocyanin sources

Table 5.2 contains data about selected copper enzymes from the references noted. It should be understood that enzymes from different sources—that is, azurin from Alcaligenes denitrificans versus Pseudomonas aeruginosa, fungal versus tree laccase, or arthropodan versus molluscan hemocyanin—will differ from each other to various degrees. Azurins have similar tertiary structures—in contrast to arthropodan and molluscan hemocyanins, whose tertiary and quaternary structures show large deviations. Most copper enzymes contain one type of copper center, but laccase, ascorbate oxidase, and ceruloplasmin contain Type I, Type II, and Type III centers. For a more complete and specific listing of copper enzyme properties, see, for instance, the review article by Solomon et al.4... 

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. 

There are a number of excellent sources of information on copper proteins notable among them is the three-volume series Copper Proteins and Copper Enzymes (Lontie, 1984). A review of the state of structural knowledge in 1985 (Adman, 1985) included only the small blue copper proteins. A brief review of extended X-ray absorption fine structure (EXAFS) work on some of these proteins appeared in 1987 (Hasnain and Garner, 1987). A number of new structures have been solved by X-ray diffraction, and the structures of azurin and plastocyanin have been extended to higher resolution. The new structures include two additional type I proteins (pseudoazurin and cucumber basic blue protein), the type III copper protein hemocyanin, and the multi-copper blue oxidase ascorbate oxidase. Results are now available on a copper-containing nitrite reductase and galactose oxidase. 

The crystal structures of tyrosinase from Streptomyces castaneoghbisporus HUT 6202 and catechol oxidase from the sweet potato Ipomoea batata have been determined. They confirm that the coordination of the type-3 copper site in tyrosinase and catechol oxidase is very similar to that found in hemocyanin. This had been deduced before from the similarity of spectroscopic properties and a comparison of many tyrosinase and hemocyanin primary structures. On the basis of the biological source of the proteins seven different domain organizations could be identified. Plant catechol oxidases of different organisms have a sequence identity of about 40-60%. The sequence identity between catechol oxidases and mulluscan hemocyanins is about 35% over almost the whole length of the sequences. In contrast, the sequence identity between plant catechol oxidases and other type-3 copper proteins from any nonplant source is limited to the two copperbinding regions. 

Mussels and fish are capable of taking in great amounts of copper, and may be a valuable nutritional source of that metal for humans (Han and Hung 1990). Copper is part of hemocuprein (hemocyanin), a constitutive prosthetic element in the respiratory system of squid, crabs, and some fishes, and is analogous to iron in hemoglobin (Frausto de Silva and Williams 1991). Fish absorb copper directly from water and accumulate it in greater amount than mercury, lead, and cadmium (Mount et al. 1994). Also, terrestrial Crustacea accumulate copper more then the littoral. 

Hemocyanin from molluscan and crustacean sources is able to decompose hydrogen peroxide catalytically (Ghiretti, 1956 Felsenfeld and Printz, 1959). This catalase-like action is due to the presence of copper in the hemocyanin molecule. However, equivalent amounts of copper alone have no effect on the decomposition of HgOj. Among the copper complexes of the amino acid tested, only Cu-arginine is active. 

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