Hemocyanin metal ligands

Hemocyanin Tyrosinase Ribonucleotide reductase ° Ferridoxin Andrenodoxin Rubredoxin Blue copper Transferrins and other Fe(III)-tyrosinate proteins " transfer transition dination geometry of metal ligands... 

In a study of octopus hemocyanin (Salvato et al., 1989), a green half-methemocyanin could be formed in the presence of slight molar excess of sodium nitrite and ascorbate. This form was believed to consist of cuprous and cupric metals, but the amoung of NO directly bound was less than 0.1 mol. NOi was believed to be the oxidizing species its relationship to the endogenous ligand was not addressed. Apparently, hemocyanins are most likely to be found in deoxy, or reduced, forms, and it is difficult to oxidize them. 

The binding of CO to hemocyanin is of interest. The two-coordinate copper centres in deoxyhemocyanin might each be expected to bind CO. However, some two-coordinate complexes of Cu1 with heterocyclic donor ligands show lack of reactivity towards CO in the absence of additional ligands, probably because the metal does not receive enough electron density to back bond to CO.1295... 

A trans-n- 1,2-Peroxo Dicopper(II) Complex. Our own efforts have resulted in the structural and spectroscopic characterization of five types of copper-dioxygen complexes (6), distinguished on the basis of the ligands used for their synthesis and on their distinctive structures or physical properties. Thus, the manner in which hemocyanin binds 02 is not the only one possible, and it is of considerable interest to deduce the structures, along with associated spectroscopy and reactivity of a variety of types. Dioxygen can bind to dinuclear transition metals in a variety of structural modes, shown in Figure 2. As mentioned, mode C is present in oxy-Hc and Kitajima s model complex (Scheme 1), whereas we have structural and spectroscopic evidence for types A (30-32), B (33-35), and F (36-38) for peroxo 022- binding, and mode D (39, 40) in the case of hydroperoxo (OOH ) complexes. 

There are three reasonable combinations of metal oxidation states for oxidized Type 3 copper that are consistent with spectral and redox data (1) Cu(I) Cu(I) with some other group, e.g., disulfide, functioning as a two-electron acceptor (2) Cu(I)-Cu(III) where Cu(III) is low spin and (3) an antiferromagnetically coupled Cu(II)-Cu(II) dimer. Magnetic susceptibility studies on Rhus vernicifera laccase have established that the two Type 3 copper atoms in this enzyme are present as an antiferromagnetically coupled Cu(II) dimer (4). The Type 3 copper atoms of hemocyanin and tyrosinase appear to be similarly coupled and separated by 3-5 A (5,6,7). Further structural information on the Type 3 copper chromophore is scanty neither the identity of the ligands nor the geometry of the site has been ascertained. There is likewise a paucity of literature on binuclear copper complexes that exhibit structural features expected for Type 3 copper. 

Binuclear metal complexes are interesting from several points of view, in particular if it is possible to keep the two metal ions not to far from each other [1]. Under such conditions metal-metal interactions are expected and they should reflect themselves in their magnetic and electrochemical properties. In addition two metal ions kept at a fixed distance often allow the specifical binding of a bridging ligand and thus recognition of a molecule and sometimes also its activation. Thus they can be used as models for the study of the structure and reactivity of metalloproteins such as laccase, hemocyanin, hemerythrin and urease. 

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