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The importance of intraerythrocytic organic phosphates in the allosteric control of oxygen binding to vertebrate hemoglobin (Hb) is well recognized and is correlated with conformational changes of the tetramer. ATP is a major allosteric effector of snake Hb, since the absence of this nucleotide abolishes the Hb cooperativity. This effect may be related to the molecular weight of about 32,000 for this Hb, which is compatible with the dimeric form. ATP induces a pH-dependent tetramerization of deoxyHb that leads to the recovery of cooperativity. This phenomenon may be partially explained by two amino acid replacements in the β chains (CD2 Glu-43 → Thr and G3 Glu-101 → Val), which result in the loss of two negative charges at the α₁β₂ interface and favors the dissociation into dimers. The ATP-dependent dimer ↔ tetramer may be physiologically important among ancient animal groups that have similar mutations …