Hemoglobin J Altgeld Gardens. A hemoglobin variant with a substitution of the proximal histidine of the beta-chain

Roles of Fe-Histidine bonds in stability of hemoglobin: Recognition of protein flexibility by Q Sepharose

Using various mutants, we investigated to date the roles of the Fe-histidine (F8) bonds in cooperative O₂ binding of human hemoglobin (Hb) and differences in roles between α- and β-subunits in the α₂β₂ tetramer. An Hb variant with a mutation in the heme cavity exhibited an unexpected feature. When the β mutant rHb (βH92G), in which the proximal histidine (His F8) of the β-subunit is replaced by glycine (Gly), was subjected to ion-exchange chromatography (Q Sepharose column) and eluted with an NaCl concentration gradient in the presence of imidazole, yielded two large peaks, whereas the corresponding α-mutant, rHb (αH87G), gave a single peak similar to Hb A. The β-mutant rHb proteins under each peak had identical isoelectric points according to isoelectric focusing electrophoresis. Proteins under each peak were further characterized by Sephadex G-75 gel filtration, far-UV CD, ¹H NMR, and resonance Raman spectroscopy. We found that rHb (βH92G) exists as a mixture of αβ-dimers and α₂β₂ tetramers, and that hemes are released from β-subunits in a fraction of the dimers. An approximate amount of released hemes were estimated to be as large as 30% with Raman relative intensities. It is stressed that Q Sepharose columns can distinguish differences in structural flexibility of proteins having identical isoelectric points by altering the exit rates from the porous beads. Thus, the role of Fe-His (F8) bonds in stabilizing the Hb tetramer first described by Barrick et al. was confirmed in this study. In addition, it was found in this study that a specific Fe-His bond in the β-subunit minimizes globin structural flexibility.

Hb Auckland [alpha 87(F8) His-->Asn]: a new mutation of the proximal histidine identified by electrospray mass spectrometry

Hb Auckland is a newly described unstable hemoglobin with a mutation of alpha 97(F8)His-->Asn. This substitution, involving the proximal histidine, does not lead to methemoglobinemia, but to instability and accelerated heme loss. The clinical picture is of a mild compensated hemolytic anemia. The presence of an abnormal hemoglobin was first demonstrated by the isopropanol stability test and confirmed by electrospray ionization mass spectrometry of total lysate. This showed that 14% of the alpha chains had a mass of 15,103.4 Da, i.e. 23 Da less than normal. Examination of tryptic digests showed an identical decrease in mass for peptide alpha T-9 (from 2,997.4 to 2,974.5 Da). Subdigestion with endoproteinase Asp-N located the 23 Da loss to residues alpha 85-90, and further digestion with thermolysin identified the mutation as His-->Asn at position 87 of the alpha chain. This was confirmed by sequence analysis of the peptide alpha 85-90.

Hemoglobin Redondo [beta 92(F8) His----Asn]: an unstable hemoglobin variant associated with heme loss which occurs in two forms

Hb Redondo [beta 92(F8) His----Asn] illustrates how post-translational structural modifications may modify the phenotypic expression of an unstable hemoglobin. This variant was found in a Portuguese patient suffering from a chronic hemolytic anemia. The electrophoretic pattern demonstrated that it occurred in two forms, both being semi-hemoglobins: the fastest one migrating like HbS and the other like HbA2; after a few days of storage at 4 degrees C the intensity of the slowest Hb fraction decreased while that of the other increased proportionally. In both cases, the RP-HPLC analysis of the tryptic digest of the aminoethylated beta chains demonstrated the presence of an abnormal beta T10 peptide carrying a His----Asx substitution. Microsequence study of these two peptides demonstrated that the slowest abnormal Hb fraction had a beta 92 His----Asn substitution and the fastest a His----Asp at the same site. All these results suggest that the beta 92 His----Asn variant loses readily its heme group and that a deamidation occurs rapidly in vitro, yielding a beta 92 Asp semi-hemoglobin. The oxygen affinity of the patient's red blood cells was increased, leading to a stimulation of erythropoiesis and to a macrocytic hemolytic disease.

Hb Isehara (or Hb Redondo) [beta 92 (F8) His----Asn]: an unstable variant with a proximal histidine substitution at the heme contact

A 50-year-old Japanese female patient was found to have hemolytic anemia. Isoelectrofocusing of her hemolysate revealed two abnormal hemoglobin bands, one of which was very close to the Hb A2 band, and the other between the Hb A2 and Hb F bands. CM-cellulose column chromatography of the globin prepared from the abnormal hemoglobin showed that the abnormal chain eluted faster than the normal beta and delta chains; the beta X chain, however, did not separate from the normal beta chain in urea cellulose acetate electrophoresis. An instability test of the patient's hemolysate revealed the presence of an unstable component. Structural analysis of the abnormal beta chain indicated that the histidine residue at beta 92(F8) was replaced by an asparagine or aspartic acid residue. DNA amplified by polymerase chain reaction was sequenced by the dideoxy method. The nucleotide sequence of the beta 92 codon was AAC instead of CAC, suggesting that the amino acid substitution corresponded to His----Asn, which is the same as is found in Hb Redondo or beta 92(F8)His----Asn----Asp.

beta-Thalassemia present in cis to a new beta-chain structural variant, Hb Vicksburg [beta 75 (E19)Leu leads to 0]

Hemoglobin Vicksburg was discovered in a 6-year-old Black boy who had been anemic since infancy. Examination of his hemolysate revealed 87.5% Hb F, 2.4% Hb A2, and 7.6% Hb Vicksburg, which had the electrophoretic and chromatographic properties of Hb A. Structural analysis of Hb Vicksburg demonstrated a deletion of leucine at beta 75(E19), a new variant. Hb Vicksburg was neither unstable nor subject to posttranslational degradation. The alpha/non-alpha biosynthetic ratio was 2.6. Because the proband appeared to be a mixed heterozygote for Hb Vicksburg and beta 0-thalassemia, Hb Vicksburg should have comprised the major portion of the hemolysate. Thus, Hb Vicksburg was synthesized at a rate considerably lower than would be expected on the basis of gene dosage. There was no reason to suspect abnormal translation of beta Vicksburg mRNA; in individuals with Hb St. Antoine (beta 74 and beta 75 deleted), the abnormal hemoglobin comprised 25% of the hemolysate in the simple heterozygote yet was unstable. Deletion of beta 75, therefore, would not in itself appear to lead to diminished synthesis. There was a profound deficit of beta Vicksburg mRNA when measured by liquid hybridization analysis with beta cDNA. The most plausible explanation for the low output of Hb Vicksburg is that a mutation for beta +-thalassemia is present in cis to the structural mutation.