Hemocyanin arthropod

Contrary to the results for molluscan hemocyanin, arthropod hemocyanin does not undergo rapid aging upon fluoride treatment as evidenced by the lack of change in the optical spectrum84. However, an interaction of F- and arthropod oxyhemocyanin was observed by NMR studies82 (vide ante) and the maintenance of the 340 nm absorption is due to the retention of 02 by the oxyhemocyanin in spite of the presence ofF". 

Oxygen transport (binuclear) Hemocyanin Mollusks and arthropods Dioxygen transport... 

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. 

Hemocyanin O2 transport Arthropods, mollusks Preaux and Gielens (1984)... 

The arthropod hemocyanins have yielded the most to diffraction studies. Studies of Limulus hemocyanin by Magnus and Love (1983, cited by Preaux and Gielens, 1984) showed a kidney bean-shaped subunit which is consistent with both electron micrographs and the structure of hemocyanin from Panuliris interruptus (spiny lobster). 

An attempt is being made to find crystals of an arthropod hemocyanin which diffract to high resolution and have identical subunits in the asymmetric unit. To that end Buisson et al. (1989) crystallized subunit Aa6 of Androctonus australie hemocyanin in two forms one which diffracts only to 8 A and one which diffracts to 3 A. 

By far the most definitive study on an arthropod hemocyanin has been that by Volbeda and Hoi (1989b), a crystallographic tour de force. Crystals of subunit b can be formed from solutions of native hemocyanin which contain three types of subunits (a, b, and c). Two subunits (a and b) are nearly identical (3% difference in sequence), whereas subunit c differs more. Subunits a and b are glycosylated at a single residue (Asn-167). While the Panuliris form has been shown to be deoxy (Volbeda et al., 1989), unpublished observations indicate that the horseshoe crab structure Limulus) is in the oxygenated state (K. Magnus, personal communication, 1988, cited by Volbeda and Hoi, 1988). 

Immunocyanin is a stable modification of keyhole limpet hemocyanin, a non-heme, oxygencarrying copper protein found in arthropods and mollusca. It is an aspecific activator of both cellular and humoral reactions. Immunocyanin is used for the local treatment of bladder cancer. Its systemic adverse effects are usually limited to some mild fever. 

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

Like hemerythrin, hemocyanin is an oxygen transport non-heme-containing protein found in some arthropods and molluscs (104,105). In the 02-bound form, hemocyanin contains an antiferromagnetically coupled binuclear copper(II) system (106) ligated by histidine residues, with a sideways / 2-v2 V2 peroxo group bound to both Cu11 centers (104), which superseded the previous model (107). 

The interest in catechol oxidase, as well as in other copper proteins with the type 3 active site, is to a large extent due to their ability to process dioxygen from air at ambient conditions. While hemocyanin is an oxygen carrier in the hemolymph of some arthropods and mollusks, catechol oxidase and tyrosinase utilize it to perform the selective oxidation of organic substrates, for example, phenols and catechols. Therefore, establishment of structure-activity relationships for these enzymes and a complete elucidation of the mechanisms of enzymatic conversions through the development of synthetic models are expected to contribute greatly to the design of oxidation catalysts for potential industrial applications. 

Burmester, T. (2001). Molecular evolution of the arthropod hemocyanin superfamily. Molecular Biology <6 Evolution 18 184-195. 

Burmester, T. and K. Scheller (1996). Common origin of arthropod tyrosinase, arthropod hemocyanin, insect hexamerin, and dipeteran arylphorin receptor. J. Mol. Evol. 42 713-728. 

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... 

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