A polymerising Root-effect fish hemoglobin with high subunit heterogeneity. Correlation with primary structure

The blood of the teleost Chelodonichthys kumu, living in the temperate waters of New Zealand, contains a single hemoglobin. The complete amino acid sequence of the ? and ? chain has been established. The presence of a reactive Cys in the external position ?CD8(49) causes polymerisation through intermolecular disulfide bridges between ? chains, with no alteration of functional features. C. kumu Root-effect hemoglobin displays very low or no subunit co-operativity in the physiological pH range. Kinetic experiments on the oxygen dissociation and binding of carbon monoxide show a marked, pH-dependent functional heterogeneity of the two chains, which contributes to the observed reduction of co-operativity. In contrast, kinetic heterogeneity was not observed in the process of CO dissociation, indicating that functional differences between the subunits are detectable only for the dynamic ligand association pathway. The allosteric effector, ATP, seems to increase the pK(a) of the proton-linked effect on the slow-reacting subunit, affecting the quaternary equilibrium through stabilisation of the T state at lower pH, rather than enhancing the functional heterogeneity itself. In position E11 of both chains, Val (usually present at the distal side of the heme), is substituted by Ile. Although this residue has been shown not to significantly alter ligand binding to the ? chain, to some extent it can perturb the access of oxygen to the ? chain. Thus, this substitution may be the main reason for subunit functional heterogeneity.