Hemocyanin bridging ligands

Figure 101 The absorption spectra of half-met-hemocyanin protein plus added bridging ligands 15 K IT —...

Reduced hemocyanin (i.e., deoxy-Hc) is colorless, indicative of a 3d10 copper(I) formulation. Crystal structures of two deoxy forms are now available, and an investigation of the horseshoe crab Limulus II protein indicates that the two Cu(I) ions are 4.6 A apart, each found in a trigonal-planar coordination environment with Cu—NHis 2.0 A (17). There is no bridging ligand in this form, and cooperative effects of 02 binding are probably initiated and transmitted as a result of movement... 

The role of a bridging ligand in determining the magnetic properties of oxidized hemocyanin has been studied using azide substitution in the active site154 155). 

Phenoxy bridging ligands, (69), have been synthesized and used to form binuclear copper complexes as models for certain copper-containing enzymes (e.g., hemocyanin and tryrosinase).202,205... 

Hemocyanine and tyrosinase are of this type, Hemocyanine is the oxygen carrier in molluscs and arthropods. Two Cu ions are involved in a subunit and bind one O2. A deoxy structure of a lobster hemocyanine from Panulitrus interruptus has been reported [43] for which both the Cu are monovalent. Three His coordinate to each Cu, and two Cu and four His are located in the same plane (Fig. 2-23). The two copper ions are not equivalent and no bridging ligand exist between them. Tyrosinase oxidizes phenols to orthoquinones. O2 is coordinated between two Cu ions as a p-peroxo structure. 

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. 

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. 

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