Allosteric Transition in Hemoglobin (α^A₂β¹₂) from the Rhynchocephalian Reptile Relict Sphenodon Punctatus

Abstract

The major hemoglobin component Hb A of the tuatara, Sphenodon punctatus. a relict of the rhynchocephalian reptiles that lived 200 million years ago, was investigated in the light of the apparent contradiction inherent in an effect of organic phosphate cofactors on the oxygen affinity of hemoglobins exhibiting hyperbolic oxygen equilibrium curves. The heterotropic allosteric effect of ATP, the major erythrocytic cofactor in the tuatara, is shown to be correlated with distinct homotropic interactions (Hill's cooperativity coefficient at half-saturation, n₅₀, attaining 1.3–1.5 above pH 7.9, and with free energies of heme-heme interaction (4.7 kJ.mole⁻¹ at pH 7.2) which resemble those in other vertebrate hemoglobins that exhibit higher n₅₀ values. Curiously, chloride ions increase hemoglobin-oxygen affinity below pH ± 7.2. The possible mechanisms underlying the effects of ATP and chloride are discussed.