Hemoglobin Hirose, a human hemoglobin variant with a substitution at the alpha1beta2 interface. Subunit dissociation and the equilibria and kinetics of ligand binding

Allosteric free energy changes at the alpha 1 beta 2 interface of human hemoglobin probed by proton exchange of Trp beta 37

The energetic changes that occur on ligand binding in human hemoglobin have been investigated by measurements of the exchange rates of the indole proton of Trpbeta37(C3). The Trpbeta37 residues are located in helices C of the beta-subunits and are involved in contacts with the segments FG of the alpha-subunits at the interdimeric alpha1beta2 and alpha2beta1 interfaces of the hemoglobin tetramer. In the quaternary structure change that accompanies ligand binding to hemoglobin, these contacts undergo minimal changes in relative orientation and in packing, thereby acting as hinges, or flexible joints. The exchange rates of the indole proton of Trpbeta37(C3) were measured by nuclear magnetic resonance spectroscopy, in both deoxygenated and ligated hemoglobin. The results indicate that, at 15 degrees C, the exchange rate is increased from 9.0. 10(-6) to 3.3. 10(-4) s(-1) upon ligand binding to hemoglobin. This change suggests that the structural units at the hinge regions of the alpha1beta2/alpha2beta1 interfaces containing Trpbeta37(C3) are specifically stabilized in unligated hemoglobin, and experience a change in structural free energy of approximately 4 kcal/(mol tetramer) upon ligand binding. Therefore, the hinge regions of the alpha1beta2/alpha2beta1 interfaces could play a role in the transmission of free energy through the hemoglobin molecule during its allosteric transition.

Changes of tyrosine and tryptophan residues in human hemoglobin by oxygen binding: near- and far-UV circular dichroism of isolated chains and recombined hemoglobin

In order to assign the circular dichroism (CD) spectral change in the region between 280 and 300 nm of human adult hemoglobin (Hb A) upon the quaternary structure transition induced by oxygen binding, the near- and far-UV CD spectra of the isolated chains and the recombined hemoglobin were examined. Deoxygenation made the negative CD band at 290 nm of oxy-alpha chain deeper. On the other hand, positive CD bands of oxy-beta chain at the 280 to approximately 300 nm became negative upon deoxygenation. These changes were interpreted as being due to environmental alterations of tyrosine (Tyr) and/or tryptophan (Trp) perturbed by tertiary structural changes from the oxy to deoxy form in isolated chains, referring to the CD spectra of model compounds. From the difference between CD bands of the arithmetic mean of deoxy isolated chains and the CD band of deoxyHb tetramer, the contribution of tertiary structural change to the negative CD band of deoxyHb A at 287 nm was estimated to be 50%. This finding has revealed that the net contribution of quaternary structure transition to the negative band is 50%. In far-UV CD spectra, the environmental changes of aromatic residues upon the quaternary structure transition were also detected as a negative band at 225 nm.

Contribution of alpha140Tyr and beta37Trp to the near-UV CD spectra on quaternary structure transition of human hemoglobin A

The CD band of human adult hemoglobin (Hb A) at 280 approximately 290 nm shows a pronounced change from a small positive band to a definite negative band on the oxy (R) to deoxy (T) structural transition. This change has been suggested to be due to environmental alteration of Tyrs (alpha42, alpha140, and beta145) or beta37 Trp residues located at the alpha1beta2 subunit interface by deoxygenation. In order to evaluate contributions of alpha140Tyr and beta37Trp to this change of CD band, we compared the CD spectra of two mutant Hbs, Hb Rouen (alpha140Tyr-->His) and Hb Hirose (beta37Trp-->Ser) with those of Hb A. Both mutant Hbs gave a distinct, but smaller negative CD band at 287nm in the deoxy form than that of deoxyHb A. Contributions of alpha140Tyr and beta37Trp to the negative band at the 280 approximately 290 nm region were estimated from difference spectra to be 30% and 26%, respectively. These results indicate that the other aromatic amino acid residues, alpha42Tyr and beta145Tyr, at the alpha1beta2 interface, are also responsible for the change of the CD band upon the R-->T transition of Hb A.

Substitution of the heme binding module in hemoglobin alpha- and beta-subunits. Implication for different regulation mechanisms of the heme proximal structure between hemoglobin and myoglobin

In our previous work, we demonstrated that the replacement of the "heme binding module," a segment from F1 to G5 site, in myoglobin with that of hemoglobin alpha-subunit converted the heme proximal structure of myoglobin into the alpha-subunit type (Inaba, K., Ishimori, K. and Morishima, I. (1998) J. Mol. Biol. 283, 311-327). To further examine the structural regulation by the heme binding module in hemoglobin, we synthesized the betaalpha(HBM)-subunit, in which the heme binding module (HBM) of hemoglobin beta-subunit was replaced by that of hemoglobin alpha-subunit. Based on the gel chromatography, the betaalpha(HBM)-subunit was preferentially associated with the alpha-subunit to form a heterotetramer, alpha(2)[betaalpha(HBM)(2)], just as is native beta-subunit. Deoxy-alpha(2)[betaalpha(HBM)(2)] tetramer exhibited the hyperfine-shifted NMR resonance from the proximal histidyl N(delta)H proton and the resonance Raman band from the Fe-His vibrational mode at the same positions as native hemoglobin. Also, NMR spectra of carbonmonoxy and cyanomet alpha(2)[betaalpha(HBM)(2)] tetramer were quite similar to those of native hemoglobin. Consequently, the heme environmental structure of the betaalpha(HBM)-subunit in tetrameric alpha(2)[betaalpha(HBM)(2)] was similar to that of the beta-subunit in native tetrameric Hb A, and the structural conversion by the module substitution was not clear in the hemoglobin subunits. The contrastive structural effects of the module substitution on myoglobin and hemoglobin subunits strongly suggest different regulation mechanisms of the heme proximal structure between these two globins. Whereas the heme proximal structure of monomeric myoglobin is simply determined by the amino acid sequence of the heme binding module, that of tetrameric hemoglobin appears to be closely coupled to the subunit interactions.

Preparation and kinetic characterization of a series of betaW37 variants of human hemoglobin A: evidence for high-affinity T quaternary structures

Four variants of human beta globin in which the Trp at position 37 has been replaced with a Tyr, Ala, Gly, or Glu have been expressed in Escherichia coli. These globins have been combined with normal human alpha chains and heme to form tetrameric hemoglobin molecules. A technique for the preparation of alpha chain dimers, which are cross-linked between their alpha99 lysine residues, has been developed, and these alpha dimers were combined with two of the beta globins, betaW37G and betaW37E, to form the corresponding cross-linked variants. The kinetics of CO binding to the deoxygenated derivatives following rapid mixing and of CO rebinding following flash photolysis have been examined as functions of pH in the presence and absence of the organic phosphate inositol hexaphosphate, IHP. The kinetic measurements indicate that replacement of the tryptophan with other residues destabilizes the hemoglobin tetramer, resulting in considerable dissociation of even the deoxygenated hemoglobins into alphabeta dimers at micromolar protein concentrations. Substitutions at beta37 also alter the properties of the deoxygenated hemoglobin tetramer. The alteration of the functional properties of the T states of these variants as well as the tendency of the deoxygenated derivatives to dissociate into alphabeta dimers increases in the order HbA < betaW37Y < betaW37A < betaW37G < betaW37E. Stabilizing the betaW37G or betaW37E tetramers by addition of IHP or by cross-linking does not restore the normal functional properties of the T state. Measurements of the geminate rebinding of CO establish a kinetic difference between the normal R state tetramer and the alphabeta dimer consistent with quaternary enhancement, the greater affinity of oxygen for the R state tetramer than for the alphabeta dimer. Kinetics of geminate rebinding also suggest that quaternary enhancement may be altered by substitutions at the beta37 position.

Thermodynamic studies on the equilibrium properties of a series of recombinant betaW37 hemoglobin mutants

In human hemoglobin (Hb) the beta37 tryptophan residue (betaW37), located at the hinge region of the alpha1beta2 interface, forms many contacts with alpha subunit residues of the opposite dimer, in both the T and R quaternary structures. We have carried out equilibrium O2 binding studies on a series of recombinant Hbs that have mutations at this residue site: betaW37Y, betaW37A, betaW37G, and betaW37E. Binding isotherms measured at high concentrations of these mutants were found to be shifted toward increased affinity and decreased cooperativity from that of the normal HbA0 tetramer. Analysis of these binding isotherms indicated that amino acid substitutions at the beta37 position could both destabilize the tetrameric form of the mutants relative to their constituent dimers and also alter cooperativity of the intact tetrameric species. These alterations from wild-type function are dependent on the particular side chain substituted, with the magnitude of change increasing as Trp is substituted by Tyr, Ala, Gly, and Glu. The dimer to tetramer assembly free energy of deoxy-betaW37E, the most perturbed mutant in the series, was measured using analytical gel chromatography to be 9 kcal/tetramer less favorable than that of deoxy HbA0. Stabilizing the betaW37E tetramer by addition of IHP, or by cross-linking at the alphaK99 positions, does not restore normal O2 binding behavior. Thermodynamic parameters of all the mutants were found to correlate with their CO binding rates and with their high-resolution X-ray crystal structures (see accompanying papers: Kwiatkowski et al. (1998) Biochemistry 37, 4325-4335; Peterson & Friedman (1998) Biochemistry 37, 4346-4357; Kavanaugh et al. (1998) Biochemistry 37, 4358-4373].

High-resolution crystal structures of human hemoglobin with mutations at tryptophan 37beta: structural basis for a high-affinity T-state,

The high-resolution X-ray structures of the deoxy forms of four recombinant hemoglobins in which Trp37(C3)beta is replaced with Tyr (betaW37Y), Ala (betaW37A), Glu (betaW37E), or Gly (betaW37G) have been refined and analyzed with superposition methods that partition mutation-induced perturbations into quaternary structure changes and tertiary structure changes. In addition, a new cross-validation statistic that is sensitive to local changes in structure (a "local Rfree" parameter) was used as an objective measure of the significance of the tertiary structure changes. No significant mutation-induced changes in tertiary structure are detected at the mutation site itself for any of the four mutants studied. Instead, disruption of the intersubunit contacts associated with Trp37(C3)beta results in (1) a change in quaternary structure at the alpha1beta2 interface, (2) alpha subunit tertiary structure changes that are centered at Asp94(G1)alpha-Pro95(G2)alpha, (3) beta subunit tertiary structure changes that are located between residues Asp99(G1)beta and Asn102(G4)beta, (4) increased mobility of the alpha subunit COOH-terminal dipeptide, and (5) shortening of the Fe-Nepsilon2His(F8) bond in the alpha and beta subunits of the betaW37G and betaW37E mutants. In each case, the magnitude of the change in a particular structural parameter increases in the order betaW37Y < betaW37A < betaW37E approximately betaW37G, which corresponds closely to the degree of functional disruption documented in the preceding papers.

Probing the alpha 1 beta 2 interface of human hemoglobin by mutagenesis. Role of the FG-C contact regions

The allosteric transition of hemoglobin involves an extensive reorganization of the alpha 1 beta 2 interface, in which two contact regions have been identified. This paper concerns at the effect of two mutations located in the "switch" (alpha C3 Thr --> Trp) and the "flexible joint" (beta C3 Trp --> Thr). We have expressed and characterized one double and two single mutants: Hb alpha T38W/beta W37T, Hb beta W37T, and Hb alpha T38W, whose structure has been determined by crystallography. We present data on: (i) the interface structure in the contact regions, (ii) oxygen and CO binding kinetics and cooperativity, (iii) dissociation rates of deoxy tetramers and association rates of deoxy dimers, and (iv) the effect of NaI on deoxy tetramer dissociation rate constant. All the mutants are tetrameric and T-state in the deoxygenated derivative. Reassociation of deoxygenated dimers is not modified by interface mutations. DeoxyHb alpha T38W/beta W37T dissociate much faster. We propose a binding site for I- at the switch region. The single mutants binds O2 cooperatively; the double one is almost non-cooperative, a feature confirmed by CO binding. The functional data, analyzed with the two-state model, indicate that these mutations reduce the value of the allosteric constant LO.

Ultraviolet resonance Raman studies of quaternary structure of hemoglobin using a tryptophan beta 37 mutant

Environmental changes of tyrosine and tryptophan residues of hemoglobin (Hb) upon its T to R transition of quaternary structure were investigated with ultraviolet resonance Raman (UVRR) spectroscopy excited at 235 nm. DeoxyHb A (T-form) showed a UVRR spectrum distinctly different from those of the ligated Hbs (R-form) including oxyHb, COHb, and metHb A, whereas the ligated Hbs exhibited similar UVRR spectra irrespective of the ligand species and the oxidation state of the heme. To characterize the spectral change of Trp-beta 37 at the alpha 1 beta 2 interface due to the quaternary structure transition, the UVRR spectra of Hb A were compared with the corresponding spectra of Hb Hirose (Trp-beta 37-->Ser). A difference spectrum between deoxyHb A and deoxyHb Hirose showed only Trp resonance Raman (RR) bands, which were reasonably ascribed to Trp-beta 37 in deoxyHb A. RR bands at 873 cm-1 (W17) and at 1360 and 1343 cm-1 (W7, Fermi doublet) indicated that the indole ring of Trp-beta 37 in deoxyHb A formed a strong hydrogen bond at the N1H site in hydrophobic environments. Tyr residues in deoxyHb Hirose seemed to be in the same environments as those of deoxyHb A. In contrast, the difference spectrum between Hb A and Hb Hirose in the ligated state displayed peaks for RR bands of both Trp and Tyr. The difference spectra were unaltered by the addition of 5 mM inositol hexaphosphate. This means that the differences were not caused by the tetramer to dimer dissociation but by a conformation change within a tetramer. Comparison of the Hb A-Hb Hirose difference spectra in the oxy and deoxy states revealed that the oxygenation-induced changes of Trp RR bands arose mostly from Trp-beta 37 with the small portion of remaining changes coming from Trp-beta 15, demonstrating that Trp-beta 37 plays a pivotal role in the quaternary structural change in Hb A.

Site-directed mutagenesis in hemoglobin: functional and structural study of the intersubunit hydrogen bond of threonine-38(C3)alpha at the alpha 1-beta 2 interface in human hemoglobin

To clarify the functional and structural roles of Thr-38 alpha at the alpha 1-beta 2 interface, two artificial alpha-chain mutants, in which Thr-38 alpha is replaced by Ser (Hb T38 alpha S) or Val (Hb T38 alpha V), were prepared. Thr-38 alpha is one of the highly conserved amino acid residues in hemoglobins and forms a hydrogen bond to Asp-99 beta, which is a crucial residue to stabilize the T state, via a water molecule in the deoxygenated form. We investigated their oxygen binding properties together with structural consequences of the mutations by using various spectroscopic probes. Their oxygen equilibrium curves showed small changes in the oxygen binding properties. Structural probes such as ultraviolet-region derivative and oxy-minus-deoxy difference spectra, resonance Raman scattering, and 1H-NMR spectra also indicated that the oxy and deoxy forms of these mutants show spectra characteristic of the R and T states, respectively, and the R-T transition is not very disturbed. The present structural and functional data of the mutants imply that the hydrogen bond between Thr-38 alpha and Asp-99 beta does not play a key role in stabilizing the deoxy T structure, which is in sharp contrast to the role of the hydrogen bond between Tyr-42 alpha and Asp-99 beta, and suggest that the interactions via the intersubunit hydrogen bonds are highly site-specific, depending on the amino acid residue which participates in them.

Double mixing stopped-flow method for the study of equilibria and kinetics of dimer-tetramer association of hemoglobins: studies on Hb Carp, Hb A, and Hb Rothschild

A double mixing stopped-flow method is described for studying the dimer-tetramer equilibria of oxyhemoglobins and the kinetics of association of unliganded dimers. The three hemoglobins studied were: Hb Carp, Hb A, and Hb Rothschild (Trp beta 37 (C3)----Arg). The new method reproduces the data obtained for oxyHb A by other established methods. In agreement with previous studies, the new method indicates little, if any, dissociation of oxyHb carp into dimers even in 2 M urea solutions (0.1 M Bis-Tris pH 7.0). OxyHb Rothschild, on the other hand, is extensively dissociated into dimers (K(Hb4L4 in equilibrium with 2Hb2) = 37.3 x 10(-6) M) and the rate constant for the association of deoxy dimers of Hb Rothschild is about one-tenth of the value for Hb A indicating that the deoxy tetramer of Hb Rothschild is at least 10 times more dissociated into dimers than deoxyHb A.

Unstable hemoglobins

About 70 variants of Hb A with associated hemolytic disorders have been reported during the past 30 years. I have classified them according to four grades of severity of chronic hemolysis. Acute episodes of severe hemolysis may be seen in all classes. In addition, some 80 variants without overt hemolysis have given positive results with in vitro hemoglobin instability tests. The stereochemical bases for instability can be conjectured in most cases, although few unstable hemoglobins have actually been studied by X-ray crystallography. The mechanisms for denaturation of normal Hb A and its acceleration in unstable hemoglobins were proposed some 15 years ago. The alterations of membrane lipids and proteins leading to red cell senescence and the relevance of hemoglobin denaturation to this process are presently being investigated. Several "hyperunstable" variants are clinically silent, or equivalent to a thalassemia, probably because of very efficient degradation of the abnormal chains.

Structural and functional studies of hemoglobin Wayne: an elongated alpha-chain variant

Hemoglobin Wayne (Hb Wayne) is a frame-shift, elongated alpha-chain variant that exists in two forms, with either asparagine or aspartic acid as residue 139. Oxygen equilibrium studies showed that stripped Hb Wayne Asn and Hb Wayne Asp possessed high oxygen affinity (P 1/2 = 0.60 and 0.23 mmHg at pH 7, respectively), were non-co-operative and have a markedly reduced Bohr effect (-delta log P 1/2/pH (7 to 8) = 0.34 and 0.10, respectively). Adding organic phosphate results in a decreased oxygen affinity and increased Bohr effect for both Hbs Wayne. The overall rate of carbon monoxide binding at pH 7 (l' = 5.6 X 10(6) M-1 S-1) was similar for both stripped Hbs Wayne and was 25-fold more rapid than that of stripped Hb A. When organic phosphate was added, Hb Wayne Asn exhibited a homogeneous slower rate of carbon monoxide binding (l' = 2.6 X 10(6) M-1 S-1), whereas Hb Wayne Asp showed heterogeneous binding (l' = 6.1 X 10(6) and 2.6 X 10(6) M-1 S-1 for fast and slow phases, respectively). The rates of overall oxygen dissociation and oxygen dissociation with carbon monoxide replacement for both Hbs Wayne were found to be slow compared to Hb A and uniquely different from each other. Similarly, sedimentation velocity experiments indicated that, although Hb Wayne Asn and Hb Wayne Asp were both less tetrameric than Hb A, each hemoglobin exhibited a distinct degree of oxygen-linked subunit dissociation. These observed differences in the allosteric properties of Hb Wayne Asn and Hb Wayne Asp appeared to be directly attributable to residue 139. The equilibrium and kinetic data are consistent with the X-ray diffraction analysis of Hb Wayne Asp, which shows that the C terminus of the deoxytetramers are severely disordered, a condition that results in major destabilization of the T conformation and disruption of normal hemoglobin function.

Relationship between tetramer-dimer assembly and the stability of Hb Malmö (alpha 2 beta 2 97Gln)

The effects of the mutation of the alpha 1 beta 2 contact in Hb Malmö (alpha 2 beta 2 97(FG4)His----Gln) on oxygen-binding properties, ease of dissociation into dimeric hemoglobin and stability were studied. The P50 value of Hb Malmö in the absence of organic phosphates was 1.9 mmHg, in contrast to 8.8 mmHg of Hb A. The n-value of Hb Malmö was 1.6. The overall free energy of interaction of oxygen with Hb Malmö was about 25% that of Hb A. The Adair constant, K1, of Hb Malmö was about 10-times larger than that of Hb A, but the K4 of Hb Malmö was similar to that of Hb A. The liganded form of Hb Malmö was found to dissociate into dimers more readily than Hb A by gel filtration on Sephadex G-100. Dissociation into dimeric hemoglobin was enhanced in dilute solutions. Increased instability during mechanical agitation of diluted samples was greater for Hb Malmö than for Hb A. The denaturation rate constants of tetramers of the oxyform of Hb A and Hb Malmö were about 20-times greater than those of dimers of these hemoglobins. The instability of Hb Malmö depends on a greater alpha 1 beta 2 dissociation constant compared with that of Hb A. These findings allow an examination of the role of the intersubunit contact in determining the functional properties and the stability of the hemoglobin molecule.

Spectral changes upon subunit association in valency hybrid hemoglobins

The absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of valency hybrid hemoglobins and their constituents (alpha + and beta chains for alpha 2+beta 2, alpha and beta + chains for alpha 2 beta 2+: + denotes ferric heme) were measured in the Soret region for F-, H2O, N3- and CN- derivatives. Absorption and MCD spectra of valency hybrid hemoglobins were very similar to the arithmetic mean of respective spectra of their corresponding component chains in all derivatives. The Soret MCD intensity around 408 nm for various complexes of valency hybrid hemoglobins seems to reflect the spin state of ferric chains. Upon ferric and deoxy ferrous subunit association to make the deoxy valency hybrid hemoglobins, only the high-spin forms bound with F- and H2O of alpha 2+beta 2 displayed a blue shift in the peak position around 430 nm and those of alpha 2 beta 2+ an increase in intensity around 430 nm. The blue shift and the increase in intensity were considered to be caused by the structural changes in deoxy beta chains of alpha 2+beta 2 and deoxy alpha chains of alpha beta 2+, respectively. These spectral changes were interpreted on the basis of their oxygen-equilibrium properties. In contrast to absorption and MCD spectra, the CD spectra of valency hybrid hemoglobins were markedly different from the simple addition of those of their component chains in all derivatives examined. The large part of CD spectral changes upon subunit association were interpreted as changes in the heme vicinity accompanied by formation of the alpha 1 beta 1 subunit contact.

Different effects of subunit association upon absorption and circular dichroism spectra of methemoglobin

Absorption and circular dichroism (CD) spectra of the Soret band, assigned as a pi-pi transition of the porphyrin pi-electron system, showed a great difference between alpha and beta subunits in the ferric state (alpha +, beta +). The nonequivalence of the spectra between alpha + and beta + subunits partly originates from the difference in the strength of the bond between heme iron and the proximal histidine. The peak positions for absorption and CD spectra of the ferric derivatives associated with F-, H2O, N-3 and CN- of the isolated subunits qualitatively correlate with the spin state of the ferric heme. The Soret absorption spectra obtained by simple addition of those for alpha + and beta + subunits are very similar to those for methemoglobin A (metHb A). On the other hand, the arithmetic means for the Soret CD spectra of alpha + and beta + subunits are different from those for metHb A. These differences were not observed between the Soret CD spectra of alpha 1 beta 1 dimer, which is predominantly present in metHb Hirose (beta 37Trp-Ser), and those of tetrameric metHb A. Therefore, the interaction between alpha 1 and beta 1 subunits to make the alpha 1 beta 1 dimer may strongly affect the CD spectral properties of alpha + and beta + subunits. The effect of the interaction between two homogeneous dimers, alpha 1 beta 1 and alpha 2 beta 2, forming a tetramer, on the Soret CD spectral properties, if any, is very small compared with that between alpha 1 and beta 1 subunits.

Urea polyacrylamide gel electrophoresis of PCMB precipitate as a sensitive test for the detection of the unstable hemoglobin subunit

A sensitive test for the detection of the abnormal subunit of unstable hemoglobins is described. The unstable hemoglobin subunit was selectively precipitated by paramercuribenzoic acid treatment of the carboxy hemolysate, and the precipitate was studied by urea polyacrylamide gel electrophoresis for globin composition. The test enabled the detection of an electrophoretically silent, alpha-chain unstable variant comprising only 1% of total hemoglobin. Other applications included the demonstration of Hb Köln in a cord blood, and the detection and purification of very slightly unstable hemoglobin variants.

Frequency and distribution of structural variants of hemoglobin and thalassemic states in Western Japan

Hemolysates from 100,000 people who visited the Kyushu University Hospital and affiliated hospitals during the past 15 years were screened for hemoglobinopathies using electrophoresis on thin-layer starch gel; those exhibiting an abnormality were characterized further on clinical, biochemical, and genetic grounds. Of about 97,000 adult and 3,140 cord blood samples, 29 contained electrophoretically detectable abnormalities in the heterozygous condition. Another 17 samples had quantitative changes in the levels of the minor hemoglobin components. Of the thalassemic conditions, 12 involved beta-thalassemia, 3 alpha-thalassemia, 1 delta beta-thalassemia, and 1 delta-thalassemia. Among 45 carriers of beta-thalassemia from 12 families, 5 were noted to have thalassemia intermedia since they exhibited much more severe hemolytic syndromes than those with typical beta-thalassemia minor. The frequency with which we could detect a structural variant of Hb A in the adults by electrophoresis was one in 3,800 samples. About one in 8,000 carried a beta-thalassemia gene.