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Component D of chicken hemoglobin and the hemoglobin of the embryonic Tammar wallaby ( Macropus eugenii ) self‐associate upon deoxygenation: Effect on oxygen binding
Author(s) -
Rana Mitra S.,
Knapp James E.,
Holland Robert A. B.,
Riggs Austen F.
Publication year - 2007
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.21793
Subject(s) - cooperativity , deoxygenation , hemoglobin , oxygen , bohr effect , cooperative binding , biology , biophysics , tammar wallaby , oxygen transport , chemistry , biochemistry , binding site , marsupial , zoology , organic chemistry , oxygen–haemoglobin dissociation curve , catalysis
Extensive measurements of oxygen binding by some vertebrate hemoglobins (Hbs) have suggested an unusually high degree of cooperativity with reported Hill coefficients, n H , greater than 4.0. We have reexamined this possibility of “super‐cooperativity” with chicken Hb components A (α A 2 β 2 ) and D (α D 2 β 2 ). 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 n H , greater than 4.0 at ∼4 m M heme concentration. In contrast, component A has maximal n H value below 3. The value of the maximal n H 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 m M show no n H values greater than ∼3.0, sedimentation velocity measurements provide clear evidence for deoxygenation‐dependent self‐association. Proteins 2008. © 2007 Wiley‐Liss, Inc.