Blue copper proteins halocyanin

The first class is cupredoxins—single-domain blue copper proteins composed of only one BCB domain. These proteins include plastocyanin, azurin, pseudoazurin, amicyanin, auracyanins, rusticyanin, halocyanin, and sulfocyanin (see Section IV). Plantacyanin of the phytocyanin family (Section V), subunit II of the cytochrome c oxidase, and the recently characterized nitrosocyanin also fall into this class. The last two are single BCB domain polypeptides closely related structurally to cupredoxins, but harboring, respectively, a binuclear copper site known as CuA and a novel type of copper-binding site called red (see Sections IX and X). 

Halocyanin and sulfocyanin are archaebacterial cupre-doxins that are attached to peripheral membranes through a lipid anchor at their N-terminus. Halocyanin was the first cupredoxin purified from an archaeon, haloalkaliphilic Natronobacterium pharaonis These cells live in high pH (around 10-11) and in extreme salinity (30%) environments. The presence of the blue copper protein, sulfocyanin in Sul-folobus acidocaldarius was first predicted from its gene sequence. It has been subsequently purified as a recombinant protein and shown to bind a single copper ion with spectroscopic properties typical for a blue copper site. ... 

Fig. 37. Phylogenetic family tree of the four small blue protein families. The roots as well as the branching points to auracyanin, rusticyanin, halocyanin, and umecyanin are currently not known. Plastocyanin occurs in bacteria as well as in eukaryotes. Ascorbate oxidase was used as reference protein in creating the diagram. The plant allergen Ra3 is a non-copper protein. Halocyanin is currently the only known archaeate copper protein. Adapted from Ryden and Hunt 1993 [71]...

Small blue proteins are involved in various biochemical processes. Where their physiological function is known, it is that of single-electron transport proteins. The range of their redox potentials reaches from +183 mV (Halocyanin [18], + 184 mV Stellacyanin [68] to 680 mV (Rusticyanin [68, 69]) as compared to Cu2+/Cu+, E° = +153 mV. Very few redox proteins function in this range. This feature, and their characteristic blue color are the product of the type 1 copper center, the only redox-active group in these proteins. During electron... 

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