Dinuclear copper proteins electronic structure

Type 1 copper proteins are the class of proteins for which cupredoxins were originally named. Type 1 copper proteins include both proteins with known electron transfer function (e.g., plastocyanin and rusticyanin), and proteins whose biological functions have not been determined conclusively (e.g., stellacyanin and plantacyanin). Although these proteins with unknown function cannot be called cupredoxins by the strict functional definition, they have been classified as cupredoxins because they share the same overall structural fold and metal-binding sites as cupredoxins. In addition, many multidomain proteins, such as laccase, ascorbate oxidase, and ceruloplasmin, contain multiple metal centers, one of which is a type 1 copper. Those cupredoxin centers are also included here. Finally, both the Cua center in cytochrome c oxidase (CcO) and nitrous oxide reductase (N2OR), and the red copper center in nitrocyanin will be discussed in this chapter because their metal centers are structurally related to the type 1 copper center and the protein domain that contains both centers share the same overall structural motif as those of cupredoxins. The Cua center also functions as an electron transfer agent. Like ferredoxins, which contain either dinuclear or tetranuclear iron-sulfur centers, cupredoxins may include either the mononuclear or the dinuclear copper center in their metal-binding sites. 

The above results and analysis on the low A in mononuclear and dinuclear cupredoxins have also been supported by numerous structural analyses of both oxidized and reduced cupredoxins. Both X-ray crystallography and XAS " of the metal-binding sites in the cupredoxins clearly show minimal difference between the oxidized and reduced proteins. Finally, in addition to the unique geometry and valence delocalization discussed above, which affect inner-sphere reorganization energy, other factors that may influence outer-sphere reorganization energy may also play an important role in cupredoxins. The factors include exclusion of water or solvent from the copper center in the folded proteins. This factor rules out electron transfer as a putative function for the red copper protein nitrosocyanin because its copper center is solvent accessible. 

Recently several laccases have been crystallized and their three-dimensional crystal structures are now available 13,33-35). Laccase belongs to the class of multicopper-oxidases and contains four copper centers per protein molecule type 1 (Tl) or blue copper, type 2 (T2) or normal copper, and type 3 (T3) or coupled dinuclear copper centers 36). It catalyzes the oxidation of electron rich aromatic substrates, usually phenols or aromatic amines, via four single electron oxidation steps concomitant with the four electron reduction of O2 to H2O 15,37). Electron transfer takes place at the Tl site, which is followed by electron transfer to the T2/T3 assembly. This is where the reduction of dioxygen takes place. 

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