Halocyanin, an archaebacterial blue copper protein (type I) from Natronobacterium pharaonis

A small blue copper protein with a molecular mass of about 15.5 kDa has been isolated from the haloalkaliphilic Natronobacterium pharaonis. This protein, which was named halocyanin, contains one Cu2+. The circular dichroism spectrum in the region of the absorption of the peptide bond reveals mainly beta-structural elements. The absorption spectrum in the visible range has three maxima with the main band at 600 nm. The circular dichroism spectrum in this region is characterized by four extrema with maxima at 413 and 590 nm and minima at 477 nm as well as 721 nm. The electron paramagnetic resonance spectrum displays a hyperfine structure which shows a close resemblance to those of plastocyanin. The cumulative spectral data agree well with those of other small blue copper proteins with axial symmetry of the Cu ligands. In analogy, one can deduce that halocyanin may adopt a type I copper binding site with two His, one Met, and one Cys as probable ligands. This conclusion is confirmed by the C-terminal sequence which contains, homologous to other known sequences of type I copper proteins, three of the four copper ligands. Halocyanin can only be removed from the membrane by mild treatment with detergents. This observation indicates that halocyanin is a peripheral membrane protein which serves as a mobile electron carrier. The presence of type I blue copper proteins is archaebacteria might lead to further insights into their phylogenetic origin.