Metal-binding properties of an engineered purple Cu(A) center in azurin

A Cu(A) center engineered into Pseudomonas aeruginosa azurin was studied by metal substitution. Metal-binding properties were determined by electronic absorption (UV-vis) and electrospray ionization mass spectrometry (ESI-MS). The metal-binding site readily binds thiophilic metal ions, such as Hg(II), Ag(I), Cu(I), Cd(II), and Au(I). Harder metal ions, like Co(II), bind to apo-Cu(A)-azurin only under basic conditions (pH 9.1-9.2). The results obtained from these studies indicate that two factors influence metal binding in Cu(A) azurin: (1) the site favors metal combinations which produce an overall +3 charge, and (2) the site binds soft, thiophilic metal ions. The results demonstrate the remarkable ability of the Cu(A) center to maintain valence delocalization of its native metal ions and to ensure redox accessibility of only one of the two redox couples (i.e., [Cu(1.5)···Cu(1.5)]⇆ [Cu (I)···Cu(I)]) under physiological conditions. These findings may lead to the preparation of new metal ion derivatives and can serve as a basis for understanding this efficient electron transfer center.