Cucumber basic blue protein ligands

The crystal structure of cucumber basic blue protein has now been refined to 3.0 A resolution (Adman, 1985). The protein consists of eight strands, only five of which form a P-sandwich and the protein has less P-sheet character than plastocyanin or azurin. The ligands to copper are provided by the side chains of His-39, Cys-79, His-84 and Met-89. The copper site has the N2SS coordination seen in plastocyanin. The imidazole rings of the His-39 and His-94 residues are exposed to the solvent providing a likely entry site for electon transfer to the copper centre. 

Solvent is usually excluded from the blue copper site, which is buried 6 A inside the protein, having only the His ligand from the copperbinding loop exposed to the surface. The phytocyanins, stellacyanin and plantacyanin (cucumber basic protein), are exceptions, in which both His ligands are solvent exposed and the copper ion is only 3 A beneath the protein surface. This situation makes the copper center in this family of blue copper proteins more accessible to low-molecular-weight solutes (see Section V). 

Redox potentials for the different copper centers in the blue oxidases have been determined for all members of the group but in each case only for a limited number of species. The available data are summarized in Table VI 120, 121). The redox potentials for the type-1 copper of tree laccase and ascorbate oxidase are in the range of 330-400 mV and comparable to the values determined for the small blue copper proteins plastocyanin, azurin, and cucumber basic protein (for redox potentials of small blue copper proteins, see the review of Sykes 122)). The high potential for the fungal Polyporus laccase is probably due to a leucine or phenylalanine residue at the fourth coordination position, which has been observed in the amino-acid sequences of fungal laccases from other species (see Table IV and Section V.B). Two different redox potentials for the type-1 copper were observed for human ceruloplasmin 105). The 490-mV potential can be assigned to the two type-1 copper sites with methionine ligand and the 580-mV potential to the type-1 center with the isosteric leucine at this position (see Section V.B). The... 

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