Component D of chicken hemoglobin and the hemoglobin of the embryonic Tammar wallaby (Macropus eugenii) self‐associate upon deoxygenation: Effect on oxygen binding

Extensive measurements of oxygen binding by some vertebrate hemoglobins (Hbs) have suggested an unusually high degree of cooperativity with reported Hill coefficients, nH, greater than 4.0. We have reexamined this possibility of “super-cooperativity” with chicken Hb components A (αA2β2) and D (αD2β2). Prior studies have shown that component D but not A self-associates to dimers of tetramers upon deoxygenation. This self-association is reflected in the oxygen equilibrium of Hb D which shows a maximal nH, greater than 4.0 at ∼4 mM heme concentration. In contrast, component A has maximal nH value below 3. The value of the maximal nH for Hb D increases linearly with the fraction of octamer present in the deoxy Hb. We anticipate that deoxygenation-dependent self-association will be shown to be a general property of Hb D from birds and reptiles. Neither oxygen equilibria nor sedimentation measurements show any evidence that components A and D interact to form a complex when deoxygenated. We have also reexamined the oxygen equilibria of Hbs of an embryonic marsupial, the wallaby. The equilibria in red cells have been reported to have Hill coefficients as high as 5–6. Although our oxygen equilibrium measurements of solutions of unfractionated wallaby Hb at a concentration of ∼1 mM show no nH values greater than ∼3.0, sedimentation velocity measurements provide clear evidence for deoxygenation-dependent self-association. Proteins 2008. © 2007 Wiley-Liss, Inc.