International Hemoglobin Information Center. Variants of the alpha chain

Advanced analytical methods for hemoglobin variants

Hemoglobin variants are the protein mutations most often encountered in the clinical scene. They have been useful for developing methods to analyze mutant proteins because of their size and ease of collection in large amounts. Improvements in analytical methods have been directed toward higher resolution in electrophoresis and shorter elution times in chromatography. More importantly, in the last 20 years, hemoglobin variants have been used in the development of mass spectrometric strategies for analyzing protein mutations. This approach consists of a series of steps: measurement of the molecular mass of globins to detect or confirm the presence of mutations, peptide mass mapping or peptide mass fingerprinting of an enzymatic digest to identify mutated peptides, and tandem mass spectrometry to determine or confirm the site and type of mutation. The mass spectrometric strategy has enabled rapid analysis and demonstrated a superb ability to detect a number of hemoglobin variants, particularly those without a change in electrophoretic or chromatographic properties. Even with the recent advances in DNA analysis, protein analysis is still essential, because post-translational modifications following amino acid substitutions can occur including N-terminal acetylation, deamidation and oxidation-mediated processes.

Hb Godavari [alpha 95(G2)Pro-->Thr]: a neutral amino acid substitution in the alpha 1 beta 2 interface that modifies the electrophoretic mobility of hemoglobin

Hb Godavari [alpha 95(G2)Pro-->Thr] was characterized independently in two families of different ethnic origin. The first case, found in the Netherlands, involved an Indian patient. The second one was identified a few months later in an African family from Mali, living in France. Hb Godavari is the fourth example of a substitution involving neutral residues at position alpha 95(G2). In all these variants the electrophoretic pattern suggested that the structural modification unmasks a charged residue buried in the alpha 1 beta 2 contact area. The oxygen affinity of this abnormal hemoglobin was approximately 10% higher than that of Hb A; in the absence of 2,3-diphosphoglycerate, its cooperativity was moderately decreased, suggesting a slightly unstable T state.

Influence of the A helix structure on the polymerization of hemoglobin S

Hb S variants containing Lys-beta132 --> Ala or Asn substitutions were engineered to evaluate the consequences of the A helix destabilization in the polymerization process. Previous studies suggested that the loss of the Glu-beta7-Lys-beta132 salt bridge in the recombinant Hb betaE6V/E7A could be responsible for the destabilization of the A helix. The recombinant Hb (rHb) S/beta132 variants polymerized with an increased delay time as well as decreased maximum absorbance and Hb solubility values similar to that of Hb S. These data indicate that the strength of the donor-acceptor site interaction may be reduced due to an altered conformation of the A helix. The question arises whether this alteration leads to a true inhibition of the polymerization process or to qualitatively different polymers. The oxygen affinity of the beta132 mutated rHbs was similar to that of Hb A and S, whereas the cooperativity and effects of organic phosphates were reduced. This could be attributed to modifications in the central cavity due to loss of the positively charged lysine. Since Lys-beta132 is involved in the stabilization of the alpha1-beta1 interface, the loss of the beta132(H10)-beta128(H6) salt bridge may be responsible for the marked thermal instability of the beta132 mutated rHbs.

Recombinant [Phe(beta)63]hemoglobin shows rapid oxidation of the beta chains and low-affinity, non-cooperative oxygen binding to the alpha subunits

We have engineered alpha2beta2 [Phe63]hemoglobin by changing the highly conserved distal histidine of the beta chains to a phenylalanine. The mutant tetramer binds four high-affinity ligands, such as CO or NO, to the ferrous form, or CN to the oxidized iron; however, it binds only two low-affinity ligands, oxygen and azide. The absorption spectrum of the ferrous deoxy or ferric forms are not normal, displaying an enhanced absorption of the visible band near 560 nm. Half of the autooxidation process, attributed to the mutated beta subunits, is over 1000-fold faster than for Hb A. The mutant Hb exhibits non-cooperative binding of two oxygens with an affinity about fivefold lower than those of HbA valency hybrids (alpha met beta)2. Functional properties of this mutant Hb resemble those of Hb Saskatoon ([Tyr63]Hb) [Suzuki, T., Hayashi, A., Shimizu, A. & Yamamura, Y. (1966) Biochim. Biophys. Acta 127, 280-282]. Flash-photolysis experiments also indicate non-cooperative behaviour: the CO-recombination kinetics were independent of the fraction dissociated. Furthermore, the amplitude of the CO bimolecular phase was the same for the (alpha(CO)metbeta)2 valency hybrid or the (alphaCO betaCO)2 form, suggesting mainly geminate CO-recombination kinetics to the beta chains. EPR and Resonance Raman spectra did not show evidence for a hemichrome, normally considered as a six-coordinated iron with low-spin character. The EPR and resonance Raman spectra for the mutated beta subunits demonstrate the presence of a high-spin compound in the ferric and deoxy ferrous forms. In particular, the ferrous mutated beta subunits are penta-coordinated. The abnormal absorption spectra are possibly due to an interaction between the porphyrin and the phenyl ring in the distal position rather than to direct binding to the iron.