Transfers electrons from cytochrome c551 to cytochrome oxidase. Exhibits a broad peak at 623 nm. The above maxima are for the oxidized form. E(0) is +306 mV for copper at pH 7.0, +280 mV for copper at pH 8.0, +291 mV for ADH IIB-azurin complex, +286 mV at pH 8.0 and 0.3 M KCl and +298 mV for ADH IIB-azurin complex. Monomer with an intramolecular disulfide bond but can also exist a dimer with an intermolecular disulfide bond. Mass determined without bound copper ion.