Hb Bruxelles: alpha 2A beta (2)41 or 42(C7 or CD1)Phe deleted

Hemoglobin variants: biochemical properties and clinical correlates

Diseases affecting hemoglobin synthesis and function are extremely common worldwide. More than 1000 naturally occurring human hemoglobin variants with single amino acid substitutions throughout the molecule have been discovered, mainly through their clinical and/or laboratory manifestations. These variants alter hemoglobin structure and biochemical properties with physiological effects ranging from insignificant to severe. Studies of these mutations in patients and in the laboratory have produced a wealth of information on hemoglobin biochemistry and biology with significant implications for hematology practice. More generally, landmark studies of hemoglobin performed over the past 60 years have established important paradigms for the disciplines of structural biology, genetics, biochemistry, and medicine. Here we review the major classes of hemoglobin variants, emphasizing general concepts and illustrative examples.

Hb Bruxelles, deletion of Phebeta42, shows a low oxygen affinity and low cooperativity of ligand binding

Functional studies of partially purified hemoglobin (Hb) Bruxelles, Phebeta42 (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 P50 (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 Hb Bruxelles showed a deletion of the codon TTT, which corresponds to residue Phe42. 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.