Heterometallic hybrids of homometallic human hemoglobins

Hybridization experiments between normal Hb tetramers (Fe²⁺Hb) and those with four metal-substituted hemes (i.e., replacement of Fe²⁺ by Co²⁺, Mg²⁺, Mn²⁺, Mn³⁺, Ni²⁺, or Zn²⁺) have revealed unexpected behavior. These homometallic Hbs have previously served as models that mimic the deoxy or oxy properties of normal Fe²⁺Hb. In this study, hybrids were composed of one α¹β¹ dimer that is metal-substituted at both hemes, in association with a second dimer α²β² that has normal Fe²⁺ hemes. Both metal- substituted subunits are unligated, whereas the two Fe²⁺ subunits either are both unligated or both ligated with O₂, CO, or CN. It was found that four of the metal-substituted Hbs (Mg²⁺Hb, Mn²⁺Hb, Ni²⁺Hb, and Zn²⁺Hb) did not form detectable amounts of heterometallic hybrids with normal Fe²⁺Hb even though (i) their homometallic parents formed tight tetrameric complexes with stabilities similar to that of Fe²⁺Hb and (ii) hybrids with metal substitution at both α sites or both β sites are known to form readily. This striking positional effect was independent of whether the normal Fe²⁺ hemes were ligated and of which ligand was used. These findings indicate that surprisingly large changes in tetramer behavior can arise from small and subtle perturbations at the heme sites. Possible origins of these effects are considered.