Kinetics of α-globin binding to α-hemoglobin stabilizing protein (AHSP) indicate preferential stabilization of hemichrome folding intermediate

Redox-Regulation of α-Globin in Vascular Physiology

Interest in the structure, function, and evolutionary relations of circulating and intracellular globins dates back more than 60 years to the first determination of the three-dimensional structure of these proteins. Non-erythrocytic globins have been implicated in circulatory control through reactions that couple nitric oxide (NO) signaling with cellular oxygen availability and redox status. Small artery endothelial cells (ECs) express free α-globin, which causes vasoconstriction by degrading NO. This reaction converts reduced (Fe2+) α-globin to the oxidized (Fe3+) form, which is unstable, cytotoxic, and unable to degrade NO. Therefore, (Fe3+) α-globin must be stabilized and recycled to (Fe2+) α-globin to reinitiate the catalytic cycle. The molecular chaperone α-hemoglobin-stabilizing protein (AHSP) binds (Fe3+) α-globin to inhibit its degradation and facilitate its reduction. The mechanisms that reduce (Fe3+) α-globin in ECs are unknown, although endothelial nitric oxide synthase (eNOS) and cytochrome b5 reductase (CyB5R3) with cytochrome b5 type A (CyB5a) can reduce (Fe3+) α-globin in solution. Here, we examine the expression and cellular localization of eNOS, CyB5a, and CyB5R3 in mouse arterial ECs and show that α-globin can be reduced by either of two independent redox systems, CyB5R3/CyB5a and eNOS. Together, our findings provide new insights into the regulation of blood vessel contractility.

Nrf2 expands the intracellular pool of the chaperone AHSP in a cellular model of β-thalassemia

In β-thalassemia, free α-globin chains are unstable and tend to aggregate or degrade, releasing toxic heme, porphyrins and iron, which produce reactive oxygen species (ROS). α-Hemoglobin-stabilizing protein (AHSP) is a potential modifier of β-thalassemia due to its ability to escort free α-globin and inhibit the cellular production of ROS. The influence of AHSP on the redox equilibrium raises the question of whether AHSP expression is regulated by components of ROS signaling pathways and/or canonical redox proteins. Here, we report that AHSP expression in K562 cells could be stimulated by NFE2-related factor 2 (Nrf2) and its agonist tert-butylhydroquinone (tBHQ). This tBHQ-induced increase in AHSP expression was also observed in Ter119+ mouse erythroblasts at each individual stage during terminal erythroid differentiation. We further report that the AHSP level was elevated in α-globin-overexpressing K562 cells and staged erythroblasts from β^IVS-2-654 thalassemic mice. tBHQ treatment partially alleviated, whereas Nrf2 or AHSP knockdown exacerbated, α-globin precipitation and ROS production in fetal liver-derived thalassemic erythroid cells. MafG and Nrf2 occupancy at the MARE-1 site downstream of the AHSP transcription start site was detected in K562 cells. Finally, we show that MafG facilitated the activation of the AHSP gene in K562 cells by Nrf2. Our results demonstrate Nrf2-mediated feedback regulation of AHSP in response to excess α-globin, as occurs in β-thalassemia.

The role of globins in cardiovascular physiology

Globin proteins exist in every cell type of the vasculature, from erythrocytes to endothelial cells, vascular smooth muscle cells, and peripheral nerve cells. Many globin subtypes are also expressed in muscle tissues (including cardiac and skeletal muscle), in other organ-specific cell types, and in cells of the central nervous system. The ability of each of these globins to interact with molecular oxygen (O₂) and nitric oxide (NO) is preserved across these contexts. Endothelial α-globin is an example of extra-erythrocytic globin expression. Other globins, including myoglobin, cytoglobin, and neuroglobin are observed in other vascular tissues. Myoglobin is observed primarily in skeletal muscle and smooth muscle cells surrounding the aorta or other large arteries. Cytoglobin is found in vascular smooth muscle but can also be expressed in non-vascular cell types, especially in oxidative stress conditions after ischemic insult. Neuroglobin was first observed in neuronal cells, and its expression appears to be restricted mainly to the central and peripheral nervous systems. Brain and central nervous system neurons expressing neuroglobin are positioned close to many arteries within the brain parenchyma and can control smooth muscle contraction and, thus, tissue perfusion and vascular reactivity. Overall, reactions between NO and globin heme-iron contribute to vascular homeostasis by regulating vasodilatory NO signals and scaveging reactive species in cells of the mammalian vascular system. Here, we discuss how globin proteins affect vascular physiology with a focus on NO biology, and offer perspectives for future study of these functions.

The Hsp70 chaperone system: distinct roles in erythrocyte formation and maintenance

Erythropoiesis is a tightly regulated cell differentiation process in which specialized oxygen- and carbon dioxide-carrying red blood cells are generated in vertebrates. Extensive reorganization and depletion of the erythroblast proteome leading to the deterioration of general cellular protein quality control pathways and rapid hemoglobin biogenesis rates could generate misfolded/aggregated proteins and trigger proteotoxic stresses during erythropoiesis. Such cytotoxic conditions could prevent proper cell differentiation resulting in premature apoptosis of erythroblasts (ineffective erythropoiesis). The heat shock protein 70 (Hsp70) molecular chaperone system supports a plethora of functions that help maintain cellular protein homeostasis (proteostasis) and promote red blood cell differentiation and survival. Recent findings show that abnormalities in the expression, localization and function of the members of this chaperone system are linked to ineffective erythropoiesis in multiple hematological diseases in humans. In this review, we present latest advances in our understanding of the distinct functions of this chaperone system in differentiating erythroblasts and terminally differentiated mature erythrocytes. We present new insights into the protein repair-only function(s) of the Hsp70 system, perhaps to minimize protein degradation in mature erythrocytes to warrant their optimal function and survival in the vasculature under healthy conditions. The work also discusses the modulatory roles of this chaperone system in a wide range of hematological diseases and the therapeutic gain of targeting Hsp70.

The Interplay between Molten Globules and Heme Disassociation Defines Human Hemoglobin Disassembly

Hemoglobin functions as a tetrameric oxygen transport protein, with each subunit containing a heme cofactor. Its denaturation, either in vivo or in vitro, involves autoxidation to methemoglobin, followed by cofactor loss and globin unfolding. We have proposed a global disassembly scheme for human methemoglobin, linking hemin (ferric protoporphyrin IX) disassociation and apoprotein unfolding pathways. The model is based on the evaluation of circular dichroism and visible absorbance measurements of guanidine-hydrochloride-induced disassembly of methemoglobin and previous measurements of apohemoglobin unfolding. The populations of holointermediates and equilibrium disassembly parameters were estimated quantitatively for adult and fetal hemoglobins. The key stages are characterized by hexacoordinated hemichrome intermediates, which are important for preventing hemin disassociation from partially unfolded, molten globular species during early disassembly and late-stage assembly events. Both unfolding experiments and independent small angle x-ray scattering measurements demonstrate that heme disassociation leads to the loss of tetrameric structural integrity. Our model predicts that after autoxidation, dimeric and monomeric hemichrome intermediates occur along the disassembly pathway inside red cells, where the hemoglobin concentration is very high. This prediction suggests why misassembled hemoglobins often get trapped as hemichromes that accumulate into insoluble Heinz bodies in the red cells of patients with unstable hemoglobinopathies. These Heinz bodies become deposited on the cell membranes and can lead to hemolysis. Alternatively, when acellular hemoglobin is diluted into blood plasma after red cell lysis, the disassembly pathway appears to be dominated by early hemin disassociation events, which leads to the generation of higher fractions of unfolded apo subunits and free hemin, which are known to damage the integrity of blood vessel walls. Thus, our model provides explanations of the pathophysiology of hemoglobinopathies and other disease states associated with unstable globins and red cell lysis and also insights into the factors governing hemoglobin assembly during erythropoiesis.

Alpha-hemoglobin-stabilizing protein (AHSP): a modulatory factor in β-thalassemia

Hemoglobin (Hb) is an iron-containing metalloprotein that transports oxygen molecules from the lungs to the rest of the human body. Among the different variants of Hb, HbA1 is the most common and is composed of two alpha (αHb) and two beta globin chains (βHb) constructing a heterotetrameric protein complex (α₂β₂). Due to the higher number of AHSP genes, there is a tendency to produce approximately twice as much of α subunit as β subunit. Therefore, there is a chance of presenting excess α subunit leftover in human blood plasma; excess subunits subsequently bind with each other and aggregates β-thalassemia occurs due to lack of or reduced numbers of βHb subunit. Alpha-hemoglobin-stabilizing protein (AHSP) is a scavenger protein which acts as a molecular chaperon by reversibly binding with free αHb forming a complex (AHSP-αHb) that prevents aggregation and precipitation preventing deleterious effects towards developing serious human diseases including β-thalassemia. Clinical severity worsens if mutations in AHSP gene co-occur in patients with β-thalassemia. Considering the mechanism of action of AHSP and its contribution to ameliorating β-thalassemia severity, it could potentially be used as a modulatory agent in the treatment of β-thalassemia.

Major seminal plasma proteome of rabbits and associations with sperm quality

The present study was conducted to describe the major seminal plasma proteome of rabbits and potential associations between seminal proteins and semen criteria. Semen samples were collected from 18 New Zealand adult rabbits, and seminal plasma proteins were analyzed by 2-D SDS-PAGE and tandem mass spectrometry. Sperm motility, vigor, concentration, morphology and membrane sperm viability were evaluated. Rabbits ejaculated 364 ± 70 million sperm/ml, with 81 ± 6.1% motile cells, 3.8 ± 0.2 vigor and 66.7 ± 2.5% sperm with normal morphology. Based on the viability and acrosome integrity assay, there were 65.8 ± 2.5% live sperm with intact acrosome and most spermatozoa had both intact acrosome and functional membrane. On average, 2-D gels of rabbit seminal plasma had 232 ± 69.5 spots, as determined by PDQuest software (Bio Rad, USA). Mass spectrometry allowed the identification of 137 different proteins. The most abundant proteins in rabbit seminal plasma were hemoglobin subunit zeta-like, annexins, lipocalin, FAM115 protein and albumin. The intensity of the spots associated with these five proteins represented 71.5% of the intensity of all spots detected in the master gel. Multiple regression models were estimated using sperm traits as dependent variables and seminal plasma proteins as independent ones. Also, sperm motility had positive association with beta-nerve growth factor and cysteine-rich secretory protein 1-like and a negative one with galectin-1. The percentage of rabbit sperm with intact membrane was related to seminal plasma protein FAM115 complex and tropomyosin. Then, the population of morphologically normal sperm in rabbit semen was positively linked to carcinoembryonic antigen-related cell adhesion molecule 6-like and down regulated by seminal plasma isocitrate dehydrogenase. Based on another regression model, the variation in the percentage of live sperm with intact acrosome was partially explained by the amount of leukocyte elastase inhibitor and the peptidyl-prolyl cis-trans isomerase A in the rabbit seminal fluid. The current study reports the identification of 137 proteins of rabbit seminal plasma. Major proteins of seminal secretion relate primarily to prevention of damages caused by lipid peroxide radicals and oxidative stress, membrane functionality, transport of lipids to the sperm membrane and temperature regulation. Moreover, finding seminal plasma proteins as indicators of semen parameters will improve assisted reproductive technologies.

Replacement of the Distal Histidine Reveals a Noncanonical Heme Binding Site in a 2-on-2 Hemoglobin

Heme ligation in hemoglobin is typically assumed by the "proximal" histidine. Hydrophobic contacts, ionic interactions, and the ligation bond secure the heme between two α-helices denoted E and F. Across the hemoglobin superfamily, several proteins also use a "distal" histidine, making the native state a bis-histidine complex. The group 1 truncated hemoglobin from Synechocystis sp. PCC 6803, GlbN, is one such bis-histidine protein. Ferric GlbN, in which the distal histidine (His46 or E10) has been replaced with a leucine, though expected to bind a water molecule and yield a high-spin iron complex at neutral pH, has low-spin spectral properties. Here, we applied nuclear magnetic resonance and electronic absorption spectroscopic methods to GlbN modified with heme and amino acid replacements to identify the distal ligand in H46L GlbN. We found that His117, a residue located in the C-terminal portion of the protein and on the proximal side of the heme, is responsible for the formation of an alternative bis-histidine complex. Simultaneous coordination by His70 and His117 situates the heme in a binding site different from the canonical site. This new holoprotein form is achieved with only local conformational changes. Heme affinity in the alternative site is weaker than in the normal site, likely because of strained coordination and a reduced number of specific heme-protein interactions. The observation of an unconventional heme binding site has important implications for the interpretation of mutagenesis results and globin homology modeling.

Endothelial cell α-globin and its molecular chaperone α-hemoglobin-stabilizing protein regulate arteriolar contractility

Arteriolar endothelial cell-expressed (EC-expressed) α-globin binds endothelial NOS (eNOS) and degrades its enzymatic product, NO, via dioxygenation, thereby lessening the vasodilatory effects of NO on nearby vascular smooth muscle. Although this reaction potentially affects vascular physiology, the mechanisms that regulate α-globin expression and dioxygenase activity in ECs are unknown. Without β-globin, α-globin is unstable and cytotoxic, particularly in its oxidized form, which is generated by dioxygenation and recycled via endogenous reductases. We show that the molecular chaperone α-hemoglobin-stabilizing protein (AHSP) promotes arteriolar α-globin expression in vivo and facilitates its reduction by eNOS. In Ahsp-/- mice, EC α-globin was decreased by 70%. Ahsp-/- and Hba1-/- mice exhibited similar evidence of increased vascular NO signaling, including arteriolar dilation, blunted α1-adrenergic vasoconstriction, and reduced blood pressure. Purified α-globin bound eNOS or AHSP, but not both together. In ECs in culture, eNOS or AHSP enhanced α-globin expression posttranscriptionally. However, only AHSP prevented oxidized α-globin precipitation in solution. Finally, eNOS reduced AHSP-bound α-globin approximately 6-fold faster than did the major erythrocyte hemoglobin reductases (cytochrome B5 reductase plus cytochrome B5). Our data support a model whereby redox-sensitive shuttling of EC α-globin between AHSP and eNOS regulates EC NO degradation and vascular tone.

Structure and function of haemoglobins

Haemoglobin (Hb) is widely known as the iron-containing protein in blood that is essential for O₂ transport in mammals. Less widely recognised is that erythrocyte Hb belongs to a large family of Hb proteins with members distributed across all three domains of life-bacteria, archaea and eukaryotes. This review, aimed chiefly at researchers new to the field, attempts a broad overview of the diversity, and common features, in Hb structure and function. Topics include structural and functional classification of Hbs; principles of O₂ binding affinity and selectivity between O₂/NO/CO and other small ligands; hexacoordinate (containing bis-imidazole coordinated haem) Hbs; bacterial truncated Hbs; flavohaemoglobins; enzymatic reactions of Hbs with bioactive gases, particularly NO, and protection from nitrosative stress; and, sensor Hbs. A final section sketches the evolution of work on the structural basis for allosteric O₂ binding by mammalian RBC Hb, including the development of newer kinetic models. Where possible, reference to historical works is included, in order to provide context for current advances in Hb research.

The structure of α-haemoglobin in complex with a haemoglobin-binding domain from Staphylococcus aureus reveals the elusive α-haemoglobin dimerization interface

Adult haemoglobin (Hb) is made up of two α and two β subunits. Mutations that reduce expression of the α- or β-globin genes lead to the conditions α- or β-thalassaemia, respectively. Whilst both conditions are characterized by anaemia of variable severity, other details of their pathophysiology are different, in part owing to the greater stability of the β chains that is conferred through β self-association. In contrast, α subunits interact weakly, and in the absence of stabilizing quaternary interactions the α chain (α) is prone to haem loss and denaturation. The molecular contacts that confer weak self-association of α have not been determined previously. Here, the first structure of an α2 homodimer is reported in complex with one domain of the Hb receptor from Staphylococcus aureus. The α2 dimer interface has a highly unusual, approximately linear, arrangement of four His side chains within hydrogen-bonding distance of each other. Some interactions present in the α1β1 dimer interface of native Hb are preserved in the α2 dimer. However, a marked asymmetry is observed in the α2 interface, suggesting that steric factors limit the number of stabilizing interactions that can form simultaneously across the interface.

α-Hemoglobin-stabilizing protein (AHSP) perturbs the proximal heme pocket of oxy-α-hemoglobin and weakens the iron-oxygen bond

α-Hemoglobin (αHb)-stabilizing protein (AHSP) is a molecular chaperone that assists hemoglobin assembly. AHSP induces changes in αHb heme coordination, but how these changes are facilitated by interactions at the αHb·AHSP interface is not well understood. To address this question we have used NMR, x-ray absorption spectroscopy, and ligand binding measurements to probe αHb conformational changes induced by AHSP binding. NMR chemical shift analyses of free CO-αHb and CO-αHb·AHSP indicated that the seven helical elements of the native αHb structure are retained and that the heme Fe(II) remains coordinated to the proximal His-87 side chain. However, chemical shift differences revealed alterations of the F, G, and H helices and the heme pocket of CO-αHb bound to AHSP. Comparisons of iron-ligand geometry using extended x-ray absorption fine structure spectroscopy showed that AHSP binding induces a small 0.03 Å lengthening of the Fe-O2 bond, explaining previous reports that AHSP decreases αHb O2 affinity roughly 4-fold and promotes autooxidation due primarily to a 3-4-fold increase in the rate of O2 dissociation. Pro-30 mutations diminished NMR chemical shift changes in the proximal heme pocket, restored normal O2 dissociation rate and equilibrium constants, and reduced O2-αHb autooxidation rates. Thus, the contacts mediated by Pro-30 in wild-type AHSP promote αHb autooxidation by introducing strain into the proximal heme pocket. As a chaperone, AHSP facilitates rapid assembly of αHb into Hb when βHb is abundant but diverts αHb to a redox resistant holding state when βHb is limiting.

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.

Classification of the disorders of hemoglobin

Over the years, study of the disorders of hemoglobin has served as a paradigm for gaining insights into the cellular and molecular biology, as well as the pathophysiology, of inherited genetic disorders. To date, more than 1000 disorders of hemoglobin synthesis and/or structure have been identified and characterized. Study of these disorders has established the principle of how a mutant genotype can alter the function of the encoded protein, which in turn can lead to a distinct clinical phenotype. Genotype/phenotype correlations have provided important understanding of pathophysiological mechanisms of disease. Before presenting a brief overview of these disorders, we provide a summary of the structure and function of hemoglobin, along with the mechanism of assembly of its subunits, as background for the rationale and basis of the different categories of disorders in the classification.

α-Hemoglobin stabilizing protein (AHSP) markedly decreases the redox potential and reactivity of α-subunits of human HbA with hydrogen peroxide

α-Hemoglobin stabilizing protein (AHSP) is a molecular chaperone that binds monomeric α-subunits of human hemoglobin A (HbA) and modulates heme iron oxidation and subunit folding states. Although AHSP·αHb complexes autoxidize more rapidly than HbA, the redox mechanisms appear to be similar. Both metHbA and isolated met-β-subunits undergo further oxidation in the presence of hydrogen peroxide (H(2)O(2)) to form ferryl heme species. Surprisingly, much lower levels of H(2)O(2)-induced ferryl heme are produced by free met-α-subunits as compared with met-β-subunits, and no ferryl heme is detected in H(2)O(2)-treated AHSP·met-α-complex at pH values from 5.0 to 9.0 at 23 °C. Ferryl heme species were similarly not detected in AHSP·met-α Pro-30 mutants known to exhibit different rates of autoxidation and hemin loss. EPR data suggest that protein-based radicals associated with the ferryl oxidation state exist within HbA α- and β-subunits. In contrast, treatment of free α-subunits with H(2)O(2) yields much smaller radical signals, and no radicals are detected when H(2)O(2) is added to AHSP·α-complexes. AHSP binding also dramatically reduces the redox potential of α-subunits, from +40 to -78 mV in 1 m glycine buffer, pH 6.0, at 8 °C, demonstrating independently that AHSP has a much higher affinity for Fe(III) versus Fe(II) α-subunits. Hexacoordination in the AHSP·met-α complex markedly decreases the rate of the initial H(2)O(2) reaction with iron and thus provides α-subunits protection against damaging oxidative reactions.

Do not super-excess me!

I have always been puzzled by the ability of normal erythroid cells to produce a stoichiometric amount of α- and β-globin chains in the absence of cross-talk mechanisms that would control and normalize the relative rate of expression of the genes encoding these proteins.

Insights into hemoglobin assembly through in vivo mutagenesis of α-hemoglobin stabilizing protein

α-Hemoglobin stabilizing protein (AHSP) is believed to facilitate adult Hemoglobin A assembly and protect against toxic free α-globin subunits. Recombinant AHSP binds multiple forms of free α-globin to stabilize their structures and inhibit precipitation. However, AHSP also stimulates autooxidation of αO(2) subunit and its rapid conversion to a partially unfolded bishistidyl hemichrome structure. To investigate these biochemical properties, we altered the evolutionarily conserved AHSP proline 30 in recombinantly expressed proteins and introduced identical mutations into the endogenous murine Ahsp gene. In vitro, the P30W AHSP variant bound oxygenated α chains with 30-fold increased affinity. Both P30W and P30A mutant proteins also caused decreased rates of αO(2) autooxidation as compared with wild-type AHSP. Despite these abnormalities, mice harboring P30A or P30W Ahsp mutations exhibited no detectable defects in erythropoiesis at steady state or during induced stresses. Further biochemical studies revealed that the AHSP P30A and P30W substitutions had minimal effects on AHSP interactions with ferric α subunits. Together, our findings indicate that the ability of AHSP to stabilize nascent α chain folding intermediates prior to hemin reduction and incorporation into adult Hemoglobin A is physiologically more important than AHSP interactions with ferrous αO(2) subunits.