The axial ligand and extent of protein folding determine whether Zn or Cu binds to amicyanin

M98Q amicyanin is isolated with zinc bound to its type 1 copper-binding site. The influence of the axial ligand of the type 1 copper site on metal specificity is strongest prior to the completion of protein folding and adoption of the final type 1 site geometry. The preference for zinc over copper correlated with the selectivity of apoamicyanin in vitro in the partially folded, rather than the completely folded state. These results suggest that metal incorporation in vivo occurs during protein folding in the periplasm and not to a preformed type 1 site.