Hb Bruxelles, deletion of Phe(beta 42), shows a low oxygen affinity and low cooperativity of ligand binding

Functional studies of partially purified hemoglobin (Hb) Bruxelles, phe(beta 42) (CD1) --> 0 indicate a major shift in the allosteric equilibrium toward the deoxy (T state) conformation. While Hb A shows a roughly symmetrical oxygenation curve with maximum cooperativity near half-saturation, Hb Bruxelles shows mainly properties of the low affinity (T state) form, The oxygen equilibrium curves for purified (> 80%) Hb Bruxelles show little cooperativity and a P-50 (without 2,3-diphosphoglycerate) about twice that of Hb A. The low cooperativity for Hb Bruxelles is partially compensated by an increase in oxygen affinity of the deoxy conformation and a lower 2,3-diphosphoglycerate effect. The beta chains of normal Hb have consecutive phenylalanine residues at positions 41 and 42. DNA sequencing studies of Db Bruxelles showed a deletion of the codon TTT, which corresponds to residue Phe(42). The CO rebinding kinetics after flash photolysis show mainly the slow phase, characteristic of CO binding to the deoxy conformation, In phosphate buffer at pH 7, the slow phase dominates even at low photolysis levels, where the main reaction is ligand binding to the triply liganded form, This indicates a switchover point, from the deoxy to oxy conformation, occurring beyond three ligands for Hb Bruxelles, There are few natural mutants that show a change in the oxygen affinity and cooperativity as large as that observed for Hb Bruxelles.