Oxyhemocyanins

The latter binding mode has been observed for oxyhemocyanin (77) and in a dicopper(II) peroxide complex (78). Figure 2 illustrates structures for the peroxo intermediate which are most compatible with its spectroscopic parameters, as well as another, less likely, possibility (51). 

Our earlier research on the coupled binuclear copper proteins generated a series of protein derivatives in which the active site was systematically varied and subjected to a variety of spectroscopic probes. These studies developed a Spectroscopically Effective Model for the oxyhemocyanin active slte.(l) The coupled binuclear copper active site in tyrosinase was farther shown to be extremely similar to that of the hemocyanlns with differences in reactivity correlating to active site accessibility, and to the monophenol coordinating directly to the copper(II) of the oxytyroslnase site.(2) These studies have been presented in a number of reviews.(3) In the first part of this chapter, we summarize some of our more recent results related to the unique spectral features of oxyhemocyanin, and use... 

Figure 1. Comparison of absorption (top) and EPR (bottom) spectra of normal tetragonal copper(II) and oxyhemocyanin.

Figure 2. Absorption and circular dlchrolsm spectra of oxyhemocyanin compared with absorption spectrum of methemocyanln.

Oxygenic photosynthetic organisms, [2Fe-2S] ferredoxins, 38 224-233 Oxygenyl ion, preparation of, 9 229 Oxyhalides, of berkelium, 28 49, 51-53 Oxyhalogeno cations, 9 276-279 Oxyhemerythrin, 40 373-374, 45 84 XAS, 36 325 Oxyhemocyanin, 40 363 m-peroxo dinuclear copper complexes as models for, 39 41-52 physicochemical properties, 39 47-48 Oxyhemocyanins, XAS, 36 326-327 Oxyhemoglobin, 21 135 Oxyiodonium cations, 9 277 Oxymanganese phthalocyanine, strucmre of, 7 31-35... 

The structure of oxyhemocyanin has recently been determined.50 It presents yet a third mode of binding between oxygen-carrying meuil atoms and the dioxygen moiety cole. The latter oxidizes each copperfl) to copper(ll) and is in turn reduced to the... 

The 46-kDa monomeric tyrosinase of Neurospom contains a pair of spin-coupled Cu(II) ions.568 569 The structure of this copper pair (type 3 copper) has many properties in common with the copper pair in hemo-cyanin.569a For example, in the absence of other substrates, tyrosinase binds 02 to form "oxytyrosinase," a compound with properties resembling those of oxyhemocyanin and containing a bound peroxide dianion.569... 

Figure 16 (a) The molecular geometry of n5lCu Ci (D"pen)12Cl)-nH20 391 (b) notation for the decay to oxyhemocyanin dinuclear copper species and (c) the structural relationships between (i) HCu Pc (ii) Cu Pc (iii)... 

Figure 1 Limulus II deoxyhemocyanin and oxyhemocyanin Cu202 structures.

The properties of Kitajima s p-p2 Ti2-pcroxo dicopper(II) complex lead to the conclusion that this is the likely structure in oxyhemocyanin and oxytyrosi-nase this is perhaps the most important contribution from this type of model chemistry. A distorted or closely related peroxo-dicopper(II) species appears to be involved in aromatic hydroxylation proceeding in a well-characterized tyrosinase model system. 

A new oxyhemocyanin molluskan (octopus) protein x-ray structure has been reported [188a]. The active-site is very similar to that observed for the horseshoe crab structure one of the six histidine ligands is involved in a thioether bridge, i.e., to a nearby cysteine. 

The structure of the proposed intermediate (Figure 3) consists of substrate molecules, 02, and two Cu atoms as an active center and can be characterized as follows (1) The value of kD is maximized at a 2 1 [imidazole residue] to [Cu] ratio. (2) Copper(II) is not reduced by an excess of DOPA in the absence of 02. (3) Copper(II) is present in the steady state of the reaction. (4) The value of kD increases with the addition of H202 and new absorption appears at 450 nm, similar to the behavior of oxyhemocyanin. 

A. ix-Peroxo Dinuclear Copper Complexes as Models for Oxyhemocyanin and... 

However, some other structures also have been proposed in opposition to 1. Lontie and Vanquickenborne, for example, suggested the structure represented by 2 for the active site of Helix pomatia oxyhemocyanin (3). 

Recently, we also found that the blue solution of oxyhemocyanin turns red-purple upon addition of a large quantity of ethyleneglycol. Characterization of the purple hemocyanin should contribute to the understanding of the structural aspects of native hemocyanin. 

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