Expression and properties of recombinant HbA(2) (alpha(2)delta(2)) and hybrids containing delta-beta sequences

Hemoglobin A(2) (alpha (2)delta (2)), which is present at low concentration (1-2%) in the circulating red cells of normal individuals, has two importa nt features that merit its study, i.e., it inhibits polymerization of sickl e HbS and its elevated concentration in some thalassemias is a useful clini cal diagnostic. However, reports on its functional properties regarding O-2 binding are conflicting. We have attempted to resolve these discrepancies by expressing, for the first time, recombinant hemoglobin A(2) and systemat ically studying its functional properties. The construct expressing HbA(2) contains only alpha and delta genes so that the extensive purification requ ired to isolate natural HbA(2) is circumvented. Although natural hemoglobin A(2) is expressed at low levels in vivo, the amount of recombinant alpha ( 2)delta (2) expressed in yeast is similar to that found for adult hemoglobi n A and for fetal hemoglobin F when the alpha + beta or the alpha + gamma g enes, respectively, are present on the construct. Recombinant HbA(2) is sta ble, i.e., not easily oxidized, and it is a cooperative functional hemoglob in with tetramer-dimer dissociation properties like those of adult HbA. How ever, its intrinsic oxygen affinity and response to the allosteric regulato rs chloride and 2,3-diphosphoglycerate are lower than the corresponding pro perties for adult hemoglobin. Molecular modeling studies which attempt to u nderstand these properties of HbA(2) are described.