Hemocyanin oxidation states

The general feature of the active site of the purple hemocyanin can be elucidated as follows. The purple hemocyanin is an equilibrium mixture of approximately 60% of a slightly deformed species (abbreviated as A) and approximately 40% of a rather seriously deformed species (abbreviated as B). The species A exhibits the Raman peak for the O—O stretching vibration at around 750 cm-1, being ESR-inactive and responsible for the purple color. Though the species turns red upon the addition of NCS-, the oxidation state of the two coppers at the active site probably is kept as Cu(II) in an ESR-inactive structure. To satisfy this requirement, imidazole cannot be the bridg-... 

Active sites containing two copper ions that are antiferromagneti-cally coupled in the oxidized state are often referred to as type 3 copper sites (129). It has recently become evident that these centers cannot all be considered alike and that in the blue copper oxidases the type 3 sites are in fact part of a tricopper cluster these will be considered in Section VA. The proteins containing dinuclear type 3 copper sites comprise hemocyanin and a number of oxygenase enzymes, of which the best known are tyrosinase and dopamine j8-hydroxylase. 

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. 

Copper is an essential element, being active in many enzymes and hemocyanin. Copper is an essential nutrient element to animals and plants. However, high Cu accumulation in animals and plants can be toxic. Copper is found in three oxidation states including cupric (+2), cuprous (+1), and elemental Cu (0). Cu+ and Cu + are the most important forms and are involved in oxidation-reduction reactions in soils and sediments (Figure 12.7). Cu+ and Cu + can exist in aqueous systems, although the latter is much more dominant. Copper is widely distributed in nature in its elemental state and in the form of sulfide, arsenite, chloride, and carbonates. The earth s crust on an average contains approximately 50 ppm copper. Soil and sediment contain approximately... 

Manganese compounds are used as oxidant and catalyst in the various electron-transfer reactions in vivo involving the Mn(II), Mn(III), or Mn(IV) oxidation states. In particular, manganese complexes play an important role as oxygen carrier, similar to iron and copper porphyrins such as hemoglobin and hemocyanin. Manganese compounds have a pecuhar nature for the reaction with molecular oxygen. 

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... 

The substrate-binding site in nitrous oxide reductase is a dinuclear copper unit that, in some states, also exists as a mixed-valence species (160y, this center does not show a seven-line EPR spectrum and is similar to the (Class II) mixed-valence derivatives of hemocyanin (156). 

---