Hemocyanin dioxygen binding

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 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). 

Takano, Y., Yamaguchi, K., Hybrid density functional study of ligand coordination effects on the magnetic couplings and the Dioxygen binding of the models of hemocyanin. Int. J. Quantum Chem. 2007, 107, 3103-3119. 

Figure 1 Schematic representation of the dioxygen binding dinuclear copper enzymes Hemocyanin (He), Tyrosinase (Tyr), and Catechol oxidase (Co)...

Fig. 19.12 The copper-dioxygen binding site in hemocyanin from PunitNms iniermptut. The copper, uoms are indicated by open circles. Ihe histiduius by peniagons. imj the protein chain by ribbons. [From VoJbeda, A. Hoi, W. G. J. J Mol. Biol. 1989.20V, 299-279. Reproduced v/ith permission.]...

Figure 3 Hierarchies in the structures of hemocyanins from arthropods and molluscs. A kidney shaped arthropod subunit (Mr 72 kDa) binds one molecule of dioxygen. Depending on the species, the subunits associate to 1 x 6, 2 x 6, 4 x 6, 6 x 6 and 8 X 6-meric hemocyanins found freely dissolved in the hemolymph. A molluscan hemocyanin subunit (Mr 400 kDa) folds into eight functional units each carrying one active site (two cannot be seen being hidden in the interior of the cylinder). Ten of these subunits form cylinders. Depending on the species, decamers or didecamers are found. Thus, hemocyanins can bind up to 160 dioxygen molecules with a cooperativity h of over 11...

Details about the spectroscopic characterization of the (p-rj rj -peroxo) dicopper(II) complexes are presented later. It has been noted that the dioxygen binding of tyrosinase is reversible as in the case of oxy-hemocyanin (Scheme i) 32,3s,36 fonn of the enzyme is converted into the... 

A apart. Dioxygen binding results in the two-electron reduced peroxide in a side-on bound dicopper(II) structure (Fig. 1). Crystallographic information is (also) established for the reduced, as well as the oxidized met forms (3-5). Based on their spectroscopic similarities, a similar active-site structure is proposed for the monooxygenase tyrosinase (Tyr) X-ray structures are available for catechol oxidase (CO) depicting a similar active-site structure to He and Tyr (6). However, unlike the reversible binding reactivity of hemocyanin, tyrosinase catalyzes the aromatic hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (l-DOPA), and can further perform catechol oxidase activity which oxidizes the catechol to the quinone (Fig. 2) (7). 

Hemocyanins are large, multisubunit dioxygen transporting proteins found in the hemolymph of many invertebrate species of the phyla of molluscs and arthropods 14). The subunits of molluscan hemocyanins contain functional units with a molecular weight of about 50,000 Da, each of which contains a dioxygen binding dicopper center. Arthropodal hemocyanins occur as hexamers, or multihexamers of subunits with a molecular weight of about 75,000 Da. As shown by the comparison of various X-ray crystal structures of the proteins from Panulirus interruptus (IS), Limulus polyphemus... 

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