The interaction of exogenous Cu(II) with stable T-state Ni(II)- and Cu(II)-reconstituted hemoglobins has been studied. The relative binding affinities for the two human hemoglobin Cu(II) binding sites are found to be reversed in these hemoglobins relative to native iron(II) hemoglobin A. Nickel hemoglobin, modified by N-ethylmaleimide (NEM), iodoacetamide, and carboxypeptidase A, is used to establish that the observed differences can be attributed to the protein quaternary conformation and not to the metal substitution. Magnetic interactions between the Cu(II) responsible for oxidation and the metal-heme center suggest that the Cu(II) is closer to the heme in T-state hemoglobin than R-state hemoglobin. This finding suggests a pathway for T-state heme oxidation which does not require the β-93 sulfhydryl group, consistent with rapid Cu(II) oxidation for NEM-reacted deoxyhemoglobin. © 1989, American Chemical Society. All rights reserved.