The haemoglobin molecule is insensitive to replacements of most amino-acid residues on its surface, but extremely sensitive to even quite small alterations of internal non-polar contacts, especially those near the haems. Replacements at the contacts between α and β-subunits affect respiratory function.

Molecular Pathology of Human Haemoglobin

The accumulation of newly synthesized polypeptide chains expressed from cloned genes as non native aggregates has become an important factor in the recovery of such proteins. Studies of both the refolding of denatured proteins in vitro, and of in vivo folding and maturation pathways, indicate that aggregates derive from specific partially folded intermediates and not from mature native, or fully unfolded proteins. The aggregation process in both homologous and heterologous cytoplasms may be driven by partial intracellular denaturation of intermediates, for example by high temperature, or by the absence of a critical factor—prosthetic group, sub-unit, chaperone—during the maturation process. All of these processes appear to be highly specific and subject to modification by genetic engineering of the intermediates, or alteration of their environment. This requires appreciation of the properties of such intermediates as distinct from the native states.

Protein folding intermediates and inclusion body formation

Protein folding: local structures, domains, subunits, and assemblies.

Separation of tryptic peptides of normal and abnormal a, β, γ, and δ hemoglobin chains by high-performance liquid chromatography

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.

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Hemoglobin Variants: Biochemical Properties and Clinical Correlates

Three members of a family who have erythrocytosis and a new hemoglobin, designated hemoglobin Yakima, are described. The abnormal hemoglobin is characterized by the substitution of histidine for aspartic acid at residue 99 in the 8l-chain. Of three possible structure-function relations which would account for the increased oxygen affinity of hemoglobin Yakima, only two seem likely. These are: (a) an intrachain shift in the normal relations between the F and G helices and the heme group, or (b) an effect of the substituted side chain at a region of contact between nonpolar residues of the a- and fl-chains which favors the oxyhemoglobin quarternary structure.

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Hemoglobin Yakima: I. Clinical and Biochemical Studies

Hemoglobin Sabine (α2β2 91 leu → pro) comprises 8 per cent of the hemoglobin of a 16-year-old Scotch-English-German girl who has suffered from hemolytic anemia since infancy. The spleen was removed at 18 months. She has about 9 gm of hemoglobin per 100 ml, of which 12 percent is fetal, and a red-cell count of 2,500,000, with many reticulocytes and erythrocytic inclusions readily demonstrable on fluorescence microscopy. The erythrocyte half-time measured with 51Cr is four days. Hemoglobin Sabine is deficient in heme-binding capacity, is easily converted to methemoglobin and precipitates readily on mild heating or storage. Replacement of the leucine by a proline residue at β 91 in the helical position F 7 evidently disrupts the helical sequence of the globin molecule at a point adjacent to its chief heme contact. Hemoglobin Sabine probably arose as a mutation in the proposita.

Hemoglobin sabine beta 91 (f 7) leu to pro. An unstable variant causing severe anemia with inclusion bodies.

The current Indochinese resettlement pro- Chinese, Laotian, and European origin. gram in the United States has resulted in an These studies showed uniform hematologic increase In the number of persons with manifestations in hemoglobin E trait, charachemoglobin E trait. American physicians terized by slight microcytosis, by morphoshould be aware of the hematologic expres- logic features resembling those of thalasslons of this innocuous condition. The hema- semia minor, and often by increased erythrotologic manifestations of 21 persons with cyte count. Hemoglobin instability also was hemoglobin E trait were evaluated. The confirmed. subjects were of Tai-dam, Vietnamese, A LTHOUGH HEMOGLOBIN E (HbE) is one of the most prevalent hemoglobinopathies of man, it has been virtually limited to Southeast Asia, where altogether it may be estimated to occur in about 30,000,000 people.’5 Only rarely had HbE been encountered in North America until the resettlement of large numbers of Southeast Asians within the United States. The hematologic features that we have observed in HbE trait contradict earlier observations. Microcytosis and, frequently, mild erythrocytosis are characteristic of HbE trait. These features must be recognized in order that persons with this innocuous condition not be subjected to unnecessary medical examinations and inappropriate treatment.

Bernadine Brimhall

Papers

Hemoglobin Yakima: I. Clinical and Biochemical Studies

Hemoglobin sabine beta 91 (f 7) leu to pro. An unstable variant causing severe anemia with inclusion bodies.

E Trait Reexamined: A Cause of Microcytosis and Erythrocytosis

Hemoglobin Little Rock (β143 (H21) His→Gln) EFFECTS OF AN AMINO ACID SUBSTITUTION AT THE 2,3-DIPHOSPHOGLYCERATE BINDING SITE

Structural characterization of hemoglobin Tacoma.