HETEROMETALLIC HYBRIDS OF HOMOMETALLIC HUMAN HEMOGLOBINS

Hybridization experiments between normal Hb tetramers (Fe(2+)Hb) and t hose with four metal-substituted hemes (i.e., replacement of Fe2+ by C o2+, Mg2+, Mn2+ Mn3+, Ni2+, or Zn2+) have revealed unexpected behavior . These homometallic Hbs have previously served as models that mimic t he deoxy or oxy properties of normal Fe(2+)Hb. In this study, hybrids were composed of one alpha(1) beta(1) dimer that is metal-substituted at both hemes, in association with a second dimer alpha(2) beta(2) tha t has normal Fe2+ hemes. Both metal-substituted subunits are unligated , whereas the two Fe2+ subunits either are both unligated or both liga ted with Oz, CO, or CN. It was found that four of the metal-substitute d Hbs (Mg(2+)Hb, Mn(2+)Hb, Ni(2+)Hb, and Zn(2+)Hb) did not form detect able amounts of heterometallic hybrids with normal Fe(2+)Hb even thoug h (i) their homometallic parents formed tight tetrameric complexes wit h stabilities similar to that of Fe(2+)Hb and (ii) hybrids with metal substitution at both alpha sites or both beta sites are known to form readily. This striking positional effect was independent of whether th e normal Fe2+ hemes were ligated and of which ligand was used. These f indings indicate that surprisingly large changes in tetramer behavior can arise from small and subtle perturbations at the heme sites. Possi ble origins of these effects are considered.