Halocyanin

Haloacid dehalogenase(s) 590 mechanism of 590 Haloalkane dehalogenase(s) 591 active site structure 591 Halocyanin 883 Haloperoxidases 855, 889 Hammerhead ribozyme 649, 651s mechanism of action 651 Hammett equation 308... 

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). 

AE005106 halocyanin-like, HCPB hcpB/ VNG2196G... 

Fig. 4. Multiple sequence alignment of the precursors of seven cupredoxin sequences (hcpB through hcpG) and two BCB domains of the hcpA gene product (NDl and ND2) identified in the genome of an archaeon, Halohacterium sp. NRC-1, with that of halocyanin (Nhal) from Natronobacterium pharaonis and plastocyanin from a cyanobacterium, Synechocystis sp. PCC 6803.

The sequences of auracyanin B, halocyanin, and sulfocyanin deduced from their gene sequences reveal an unusually long N-terminal extension featuring a hydrophobic domain similar to signal peptides found in other... 

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. ... 

The sequences of halocyanin and sulfocyanin, deduced from their gene sequences, have unusually long N-terminal extensions. These extensions feature a hydrophobic signal peptide which is followed by a segment that in the case of halocyanin has an Asn-Gly doublet occurring consecutively seven times. In sulfocyanin, this segment is rich in Ser residues. It is believed that these extensions are covalently modified by a lipid moiety which anchors the proteins to the cell membrane. 

Like halocyanin and sulfocyanin, auracyanin B has an N-terminal extension that is thought to anchor the protein to the cell membrane. In contrast to those proteins, however, this extension is rich in Pro, Ala, and Thr residues. In auracyanin A, they are Gly residues. ... 

In aqueous solution, the redox potential of the redox couple Cu2+/Cu+ is E° = +153 mV [6]. In contrast to this, the range of redox potentials found in copper proteins and enzymes extend from +183 mV in halocyanin [18] to + 785 mV in... 

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... 

Most currently known small blue proteins are found either in bacteria or in eukarya. However, a small blue protein, halocyanin [74], was recently discovered in Natronobacterium pharaonis [75], The fact that this bacterium belongs to the kingdom of the archaea will require new deliberations as to when and where the precursor of the small blue proteins originated. 

The sequence homologies of plastocyanin, pseudoazurin, amicyanin, and halocyanin are reflected in the similarities of their tertiary structures and in the crystal structures of plastocyanin [22], pseudoazurin [76, 77] and amicyanin [20,78], The peptide chains of plastocyanin and pseudoazurin consist of eight, and that of amicyanin nine, /1-sheets which are connected by loops. The protein consists of two sandwiched planes, formed by strands 2,8,7, and 4 on the one side and strands 2,1,3, and 6 on the other. Strand 2 belongs to both planes, and strands 0 and 5 are not part of the sandwich [20,71,78]. Strand 0 is the N-ter-minal strand in amicyanin which is formally an elongation of the peptide chain as compared to plastocyanin and pseudoazurin (Fig. 13). 

The post-translational processing of halocyanin includes a lipid-modification of a cysteine residue which possibly anchors the protein to a membrane. A spacer of seven Asn-Glu-repeats neighbors the modified cysteine, increasing the distance between protein and membrane and thus improves the interactions between halocyanin and electron-donors / electron-acceptors [74]. 

In all three proteins, the type 1 copper is coordinated in a distorted tetrahedron in which the Cu2+ ion is situated 0.35 (Pcy), 0.43 (Paz), and 0.40 A (Acy) above the plane formed by both histidines and cysteine, towards the methionine residue [20]. In azurin, the distance between the methionine ligand and copper ion is markedly larger than for the members of the plastocyanin family, resulting in a more trigonal pyramidal conformation [20,21], a ligand stereochemistry which also occurs in halocyanin [18,94], Other factors influence the redox potentials as well, e.g., the hydrophobicity of the region surrounding the copper center. However, so little is known about the collaboration between the various factors that it is currently not possible to predict accurately the probable redox potentials of type 1 copper proteins. 

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]...

Halocyanins have been isolated from a haloalkaliphillic archaebacterium." They may be involved in electron transfer in respiration chains that include cytochrome be and a terminal oxidase cytochrome Uclacyanins have been isolated from Arabidopsis plant." They have... 

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