Stability of the heme-globin linkage in alphabeta dimers and isolated chains of human hemoglobin. A study of the heme transfer reaction from the immobilized proteins to albumin

Hemoglobin β Expression Is Associated with Poor Prognosis in Clear Cell Renal Cell Carcinoma

BACKGROUND

The regulation of the redox balance in the tumor microenvironment is thought to be an adaptive response of tumor cells to hypoxic environments. In recent years, it has been reported that the hemoglobin β-chain (HBB), which is involved in scavenging reactive oxygen species (ROS), is expressed in several carcinomas. However, the relationship between HBB expression and the prognosis of renal cell carcinoma (RCC) remains unclear.

METHODS

HBB expression was immunohistochemically analyzed in 203 nonmetastatic clear cell RCC (ccRCC) cases. Cell proliferation, invasion, and ROS production were measured in ccRCC cell lines treated with HBB-specific siRNA.

RESULTS

The prognosis of HBB-positive patients was worse than that of HBB-negative patients. Cell proliferation and invasion were inhibited, and ROS production was increased by treatment with HBB-specific siRNA. Oxidative stress increased HBB expression in cells exposed to H₂O₂.

CONCLUSIONS

HBB expression in ccRCC contributes to cancer cell proliferation by suppressing ROS production under hypoxic conditions. Taken together with clinical results and in vitro experiments, HBB expression may serve as a new prognostic biomarker for RCC in the future.

Haptoglobin: From hemoglobin scavenging to human health

Haptoglobin (Hp) belongs to the family of acute-phase plasma proteins and represents the most important plasma detoxifier of hemoglobin (Hb). The basic Hp molecule is a tetrameric protein built by two α/β dimers. Each Hp α/β dimer is encoded by a single gene and is synthesized as a single polypeptide. Following post-translational protease-dependent cleavage of the Hp polypeptide, the α and β chains are linked by disulfide bridge(s) to generate the mature Hp protein. As human Hp gene is characterized by two common Hp1 and Hp2 alleles, three major genotypes can result (i.e., Hp1-1, Hp2-1, and Hp2-2). Hp regulates Hb clearance from circulation by the macrophage-specific receptor CD163, thus preventing Hb-mediated severe consequences for health. Indeed, the antioxidant and Hb binding properties of Hp as well as its ability to stimulate cells of the monocyte/macrophage lineage and to modulate the helper T-cell type 1 and type 2 balance significantly associate with a variety of pathogenic disorders (e.g., infectious diseases, diabetes, cardiovascular diseases, and cancer). Alternative functions of the variants Hp1 and Hp2 have been reported, particularly in the susceptibility and protection against infectious (e.g., pulmonary tuberculosis, HIV, and malaria) and non-infectious (e.g., diabetes, cardiovascular diseases and obesity) diseases. Both high and low levels of Hp are indicative of clinical conditions: Hp plasma levels increase during infections, inflammation, and various malignant diseases, and decrease during malnutrition, hemolysis, hepatic disease, allergic reactions, and seizure disorders. Of note, the Hp:Hb complexes display heme-based reactivity; in fact, they bind several ferrous and ferric ligands, including O₂, CO, and NO, and display (pseudo-)enzymatic properties (e.g., NO and peroxynitrite detoxification). Here, genetic, biochemical, biomedical, and biotechnological aspects of Hp are reviewed.

Mechanisms of haemolysis-induced kidney injury

Intravascular haemolysis is a fundamental feature of chronic hereditary and acquired haemolytic anaemias, including those associated with haemoglobinopathies, complement disorders and infectious diseases such as malaria. Destabilization of red blood cells (RBCs) within the vasculature results in systemic inflammation, vasomotor dysfunction, thrombophilia and proliferative vasculopathy. The haemoprotein scavengers haptoglobin and haemopexin act to limit circulating levels of free haemoglobin, haem and iron - potentially toxic species that are released from injured RBCs. However, these adaptive defence systems can fail owing to ongoing intravascular disintegration of RBCs. Induction of the haem-degrading enzyme haem oxygenase 1 (HO1) - and potentially HO2 - represents a response to, and endogenous defence against, large amounts of cellular haem; however, this system can also become saturated. A frequent adverse consequence of massive and/or chronic haemolysis is kidney injury, which contributes to the morbidity and mortality of chronic haemolytic diseases. Intravascular destruction of RBCs and the resulting accumulation of haemoproteins can induce kidney injury via a number of mechanisms, including oxidative stress and cytotoxicity pathways, through the formation of intratubular casts and through direct as well as indirect proinflammatory effects, the latter via the activation of neutrophils and monocytes. Understanding of the detailed pathophysiology of haemolysis-induced kidney injury offers opportunities for the design and implementation of new therapeutic strategies to counteract the unfavourable and potentially fatal effects of haemolysis on the kidney.

Energetics underlying hemin extraction from human hemoglobin by Staphylococcus aureus

Staphylococcus aureus is a leading cause of life-threatening infections in the United States. It actively acquires the essential nutrient iron from human hemoglobin (Hb) using the iron-regulated surface-determinant (Isd) system. This process is initiated when the closely related bacterial IsdB and IsdH receptors bind to Hb and extract its hemin through a conserved tri-domain unit that contains two NEAr iron Transporter (NEAT) domains that are connected by a helical linker domain. Previously, we demonstrated that the tri-domain unit within IsdH (IsdH^N2N3) triggers hemin release by distorting Hb's F-helix. Here, we report that IsdH^N2N3 promotes hemin release from both the α- and β-subunits. Using a receptor mutant that only binds to the α-subunit of Hb and a stopped-flow transfer assay, we determined the energetics and micro-rate constants of hemin extraction from tetrameric Hb. We found that at 37 °C, the receptor accelerates hemin release from Hb up to 13,400-fold, with an activation enthalpy of 19.5 ± 1.1 kcal/mol. We propose that hemin removal requires the rate-limiting hydrolytic cleavage of the axial HisF8 Nϵ-Fe³⁺ bond, which, based on molecular dynamics simulations, may be facilitated by receptor-induced bond hydration. Isothermal titration calorimetry experiments revealed that two distinct IsdH^N2N3·Hb protein·protein interfaces promote hemin release. A high-affinity receptor·Hb(A-helix) interface contributed ∼95% of the total binding standard free energy, enabling much weaker receptor interactions with Hb's F-helix that distort its hemin pocket and cause unfavorable changes in the binding enthalpy. We present a model indicating that receptor-introduced structural distortions and increased solvation underlie the IsdH-mediated hemin extraction mechanism.

Heme acquisition mechanisms of Porphyromonas gingivalis - strategies used in a polymicrobial community in a heme-limited host environment

Porphyromonas gingivalis, a main etiologic agent and key pathogen responsible for initiation and progression of chronic periodontitis requires heme as a source of iron and protoporphyrin IX for its survival and the ability to establish an infection. Porphyromonas gingivalis is able to accumulate a defensive cell-surface heme-containing pigment in the form of μ-oxo bisheme. The main sources of heme for P. gingivalis in vivo are hemoproteins present in saliva, gingival crevicular fluid, and erythrocytes. To acquire heme, P. gingivalis uses several mechanisms. Among them, the best characterized are those employing hemagglutinins, hemolysins, and gingipains (Kgp, RgpA, RgpB), TonB-dependent outer-membrane receptors (HmuR, HusB, IhtA), and hemophore-like proteins (HmuY, HusA). Proteins involved in intracellular heme transport, storage, and processing are less well characterized (e.g. PgDps). Importantly, P. gingivalis may also use the heme acquisition systems of other bacteria to fulfill its own heme requirements. Porphyromonas gingivalis displays a novel paradigm for heme acquisition from hemoglobin, whereby the Fe(II)-containing oxyhemoglobin molecule must first be oxidized to methemoglobin to facilitate heme release. This process not only involves P. gingivalis arginine- and lysine-specific gingipains, but other proteases (e.g. interpain A from Prevotella intermedia) or pyocyanin produced by Pseudomonas aeruginosa. Porphyromonas gingivalis is then able to fully proteolyze the more susceptible methemoglobin substrate to release free heme or to wrest heme from it directly through the use of the HmuY hemophore.

The free heme concentration in healthy human erythrocytes

Heme, the prosthetic group of hemoglobin, may be released from its host due to an intrinsic instability of hemoglobin and accumulate in the erythrocytes. Free heme is in the form of hematin (Fe(3+) protoporphyrin IX OH) and follows several pathways of biochemical toxicity to tissues, cells, and organelles since it catalyzes the production of reactive oxygen species. To determine concentration of soluble free heme in human erythrocytes, we develop a new method. We lyse the red blood cells and isolate free heme from hemoglobin by dialysis. We use the heme to reconstitute horseradish peroxidase (HRP) from an excess of the apoenzyme and determine the HRP reaction rate from the evolution of the emitted luminescence. We find that in a population of five healthy adults the average free heme concentration in the erythrocytes is 21±2μM, ca. 100× higher than previously determined. Tests suggest that the lower previous value was due to the use of elevated concentrations of NaCl, which drive hematin precipitation and re-association with apoglobin. We show that the found hematin concentration is significantly higher than estimates based on equilibrium release and the known hematin dimerization. The factors that lead to enhanced heme release remain an open question.

Cj1386, an atypical hemin-binding protein, mediates hemin trafficking to KatA in Campylobacter jejuni

Catalase enzymes detoxify H2O2 by the dismutation of H2O2 into O2 and H2O through the use of hemin cofactors. While the structure and biochemical properties of catalase enzymes have been well characterized over many decades of research, it remained unclear how catalases acquire hemin. We have previously reported that Cj1386 is essential for ensuring proper hemin content in Campylobacter jejuni catalase (KatA) (A. Flint, Y. Q. Sun, and A. Stintzi, J Bacteriol 194: 334-345, 2012). In this report, an in-depth molecular characterization of Cj1386 was performed to elucidate the mechanistic details of this association. Coimmunoprecipitation assays revealed that KatA-Cj1386 transiently interact in vivo, and UV-visible spectroscopy demonstrated that purified Cj1386 protein binds hemin. Furthermore, hemin titration experiments determined that hemin binds to Cj1386 in a 1:1 ratio with hexacoordinate hemin binding. Mutagenesis of potential hemin-coordinating residues in Cj1386 showed that tyrosine 57 was essential for hemin coordination when Cj1386 was overexpressed in Escherichia coli. The importance of tyrosine 57 in hemin trafficking in vivo was confirmed by introducing the cj1386(Y57A) allele into a C. jejuni Δcj1386 mutant background. The cj1386(Y57A) mutation resulted in increased sensitivity toward H2O2 relative to the wild type, suggesting that KatA was not functional in this strain. In support of this finding, KatA immunoprecipitated from the Δcj1386+cj1386(Y57A) mutant had significantly reduced hemin content compared to that of the cj1386(WT) background. Overall, these findings indicate that Cj1386 is involved in directly trafficking hemin to KatA and that tyrosine 57 plays a key role in this function.

Haptoglobin, hemopexin, and related defense pathways-basic science, clinical perspectives, and drug development

Hemolysis, which occurs in many disease states, can trigger a diverse pathophysiologic cascade that is related to the specific biochemical activities of free Hb and its porphyrin component heme. Normal erythropoiesis and concomitant removal of senescent red blood cells (RBC) from the circulation occurs at rates of approximately 2 × 10⁶ RBCs/second. Within this physiologic range of RBC turnover, a small fraction of hemoglobin (Hb) is released into plasma as free extracellular Hb. In humans, there is an efficient multicomponent system of Hb sequestration, oxidative neutralization and clearance. Haptoglobin (Hp) is the primary Hb-binding protein in human plasma, which attenuates the adverse biochemical and physiologic effects of extracellular Hb. The cellular receptor target of Hp is the monocyte/macrophage scavenger receptor, CD163. Following Hb-Hp binding to CD163, cellular internalization of the complex leads to globin and heme metabolism, which is followed by adaptive changes in antioxidant and iron metabolism pathways and macrophage phenotype polarization. When Hb is released from RBCs within the physiologic range of Hp, the potential deleterious effects of Hb are prevented. However, during hyper-hemolytic conditions or with chronic hemolysis, Hp is depleted and Hb readily distributes to tissues where it might be exposed to oxidative conditions. In such conditions, heme can be released from ferric Hb. The free heme can then accelerate tissue damage by promoting peroxidative reactions and activation of inflammatory cascades. Hemopexin (Hx) is another plasma glycoprotein able to bind heme with high affinity. Hx sequesters heme in an inert, non-toxic form and transports it to the liver for catabolism and excretion. In the present review we discuss the components of physiologic Hb/heme detoxification and their potential therapeutic application in a wide range of hemolytic conditions.

In vivo reduction of cell-free methemoglobin to oxyhemoglobin results in vasoconstriction in canines

BACKGROUND

Cell-free hemoglobin (Hb) in the vasculature leads to vasoconstriction and injury. Proposed mechanisms have been based on nitric oxide (NO) scavenging by oxyhemoglobin (oxyHb) or processes mediated by oxidative reactions of methemoglobin (metHb). To clarify this, we tested the vascular effect and fate of oxyHb or metHb infusions.

STUDY DESIGN AND METHODS

Twenty beagles were challenged with 1-hour similar infusions of (200 μmol/L) metHb (n = 5), oxyHb (n = 5), albumin (n = 5), or saline (n = 5). Measurements were taken over 3 hours.

RESULTS

Infusions of the two pure Hb species resulted in increases in mean arterial blood pressure (MAP), systemic vascular resistance index, and NO consumption capacity of plasma (all p < 0.05) with the effects of oxyHb being greater than that from metHb (MAP; increase 0 to 3 hr; 27 ± 6% vs. 7 ± 2%, respectively; all p < 0.05). The significant vasoconstrictive response of metHb (vs. albumin and saline controls) was related to in vivo autoreduction of metHb to oxyHb, and the vasoactive Hb species that significantly correlated with MAP was always oxyHb, either from direct infusion or after in vivo reduction from metHb. Clearance of total Hb from plasma was faster after metHb than oxyHb infusion (p < 0.0001).

CONCLUSION

These findings indicate that greater NO consumption capacity makes oxyHb more vasoactive than metHb. Additionally, metHb is reduced to oxyHb after infusion and cleared faster or is less stable than oxyHb. Although we found no direct evidence that metHb itself is involved in acute vascular effects, in aggregate, these studies suggest that metHb is not inert and its mechanism of vasoconstriction is due to its delayed conversion to oxyHb by plasma-reducing agents.

Human Hp1-1 and Hp2-2 phenotype-specific haptoglobin therapeutics are both effective in vitro and in guinea pigs to attenuate hemoglobin toxicity

AIMS

Infusion of purified haptoglobin (Hp) functions as an effective hemoglobin (Hb) scavenging therapeutic in animal models of hemolysis to prevent cardiovascular and renal injury. Epidemiologic studies demonstrate the phenotype heterogeneity of human Hp proteins and suggest differing vascular protective potential imparted by the dimeric Hp1-1 and the polymeric Hp2-2.

RESULTS

In vitro experiments and in vivo studies in guinea pigs were performed to evaluate phenotype-specific differences in Hp therapeutics. We found no differences between the two phenotypes in Hb binding and intravascular compartmentalization of Hb in vivo. Both Hp1-1 and Hp2-2 attenuate Hb-induced blood pressure response and renal iron deposition. These findings were consistent with equal prevention of Hb endothelial translocation. The modulation of oxidative Hb reactions by the two Hp phenotypes was not found to be different. Both phenotypes stabilize the ferryl (Fe(4+)) Hb transition state, provide heme retention within the complex, and prevent Hb-driven low-density lipoprotein (LDL) peroxidation. Hb-mediated peroxidation of LDL resulted in endothelial toxicity, which was equally blocked by the addition of Hp1-1 and Hp2-2.

INNOVATION AND CONCLUSION

The present data do not provide support for the concept that phenotype-specific Hp therapeutics offer differential efficacy in mitigating acute Hb toxicity.

Heme impairs the ball-and-chain inactivation of potassium channels

Fine-tuned regulation of K(+) channel inactivation enables excitable cells to adjust action potential firing. Fast inactivation present in some K(+) channels is mediated by the distal N-terminal structure (ball) occluding the ion permeation pathway. Here we show that Kv1.4 K(+) channels are potently regulated by intracellular free heme; heme binds to the N-terminal inactivation domain and thereby impairs the inactivation process, thus enhancing the K(+) current with an apparent EC50 value of ∼20 nM. Functional studies on channel mutants and structural investigations on recombinant inactivation ball domain peptides encompassing the first 61 residues of Kv1.4 revealed a heme-responsive binding motif involving Cys13:His16 and a secondary histidine at position 35. Heme binding to the N-terminal inactivation domain induces a conformational constraint that prevents it from reaching its receptor site at the vestibule of the channel pore.

A heme peroxidase with a functional role as an L-tyrosine hydroxylase in the biosynthesis of anthramycin

We report the first characterization and classification of Orf13 (S. refuineus) as a heme-dependent peroxidase catalyzing the ortho-hydroxylation of L-tyrosine to L-DOPA. The putative tyrosine hydroxylase coded by orf13 of the anthramycin biosynthesis gene cluster has been expressed and purified. Heme b has been identified as the required cofactor for catalysis, and maximal L-tyrosine conversion to L-DOPA is observed in the presence of hydrogen peroxide. Preincubation of L-tyrosine with Orf13 prior to the addition of hydrogen peroxide is required for L-DOPA production. However, the enzyme becomes inactivated by hydrogen peroxide during catalysis. Steady-state kinetic analysis of L-tyrosine hydroxylation revealed similar catalytic efficiency for both L-tyrosine and hydrogen peroxide. Spectroscopic data from a reduced-CO(g) UV-vis spectrum of Orf13 and electron paramagnetic resonance of ferric heme Orf13 are consistent with heme peroxidases that have a histidyl-ligated heme iron. Contrary to the classical heme peroxidase oxidation reaction with hydrogen peroxide that produces coupled aromatic products such as o,o'-dityrosine, Orf13 is novel in its ability to catalyze aromatic amino acid hydroxylation with hydrogen peroxide, in the substrate addition order and for its substrate specificity for L-tyrosine. Peroxygenase activity of Orf13 for the ortho-hydroxylation of L-tyrosine to L-DOPA by a molecular oxygen dependent pathway in the presence of dihydroxyfumaric acid is also observed. This reaction behavior is consistent with peroxygenase activity reported with horseradish peroxidase for the hydroxylation of phenol. Overall, the putative function of Orf13 as a tyrosine hydroxylase has been confirmed and establishes the first bacterial class of tyrosine hydroxylases.

Blood aging, safety, and transfusion: capturing the "radical" menace

Throughout their life span, circulating red blood cells (RBCs) transport oxygen (O(2)) primarily from the lungs to tissues and return with carbon dioxide (CO(2)) from respiring tissues for final elimination by lungs. This simplistic view of RBCs as O(2) transporter has changed in recent years as other gases, for example, nitric oxide (NO), and small molecules, such as adenosine triphosphate (ATP), have been shown to either be produced and/or carried by RBCs to perform other signaling and O(2) sensing functions. In spite of the numerous biochemical and metabolic changes occurring within RBCs during storage, prior to, and after transfusion, perturbations of RBC membrane are likely to affect blood flow in the microcirculation. Subsequent hemolysis due to storage conditions and/or hemolytic disorders may have some pathophysiological consequences as a result of the release of Hb. In this review, we show that evolution has provided a multitude of protection and intervention strategies against free Hb from "cradle" to "death"; from early biosynthesis to its final degradation and a lot more in between. Furthermore, some of the same naturally occurring protective mechanisms can potentially be employed to oxidatively inactivate this redox active protein and control its damaging side reactions when released outside of the RBC.

HmuY haemophore and gingipain proteases constitute a unique syntrophic system of haem acquisition by Porphyromonas gingivalis

Haem (iron protoporphyrin IX) is both an essential growth factor and virulence regulator for the periodontal pathogen Porphyromonas gingivalis, which acquires it mainly from haemoglobin via the sequential actions of the R- and K-specific gingipain proteases. The haem-binding lipoprotein haemophore HmuY and its cognate receptor HmuR of P. gingivalis, are responsible for capture and internalisation of haem. This study examined the role of the HmuY in acquisition of haem from haemoglobin and the cooperation between HmuY and gingipain proteases in this process. Using UV-visible spectroscopy and polyacrylamide gel electrophoresis, HmuY was demonstrated to wrest haem from immobilised methaemoglobin and deoxyhaemoglobin. Haem extraction from oxyhaemoglobin was facilitated after oxidation to methaemoglobin by pre-treatment with the P. gingivalis R-gingipain A (HRgpA). HmuY was also capable of scavenging haem from oxyhaemoglobin pre-treated with the K-gingipain (Kgp). This is the first demonstration of a haemophore working in conjunction with proteases to acquire haem from haemoglobin. In addition, HmuY was able to extract haem from methaemalbumin, and could bind haem, either free in solution or from methaemoglobin, even in the presence of serum albumin.

Mechanism of methaemoglobin breakdown by the lysine-specific gingipain of the periodontal pathogen Porphyromonas gingivalis

Abstract The R- and K-gingipain proteases of Porphyromonas gingivalis are involved in proteolysis of haemoglobin from which the defensive dimeric haem pigment is formed. Whilst oxyhaemoglobin is refractory towards K-gingipain, methaemoglobin is rapidly degraded. Ligation of methaemoglobin with N3-, which effectively blocks haem dissociation from the protein, prevented haemoglobin breakdown. Haem-free globin was rapidly degraded by K-gingipain. These data emphasise the need for haemoglobin oxidation which encourages haem dissociation and makes the haem-free globin susceptible to proteolytic attack.

Albumin–quercetin combination offers a therapeutic advantage in the prevention of reduced survival of erythrocytes in visceral leishmaniasis

Visceral leishmaniasis is associated with the reduced survival of erythrocytes, the cause of which remains to be fully explored. Here, we described the mechanism underlying the shortened lifespan of erythrocytes in visceral leishmaniasis and proposed a combination therapy with quercetin and hamster serum albumin towards its rectification. Decreased redox potential in erythrocytes followed by oxidative denaturation of hemoglobin and pathologic association of iron with the cell membrane facilitated premature hemolysis during leishmanial infection. Recently, we have reported the therapeutic efficacy of quercetin in arresting the enhanced destruction of erythrocytes in visceral leishmaniasis. Since serum albumin, the principal carrier protein for quercetin gets depleted in visceral leishmaniasis, the situation may compromise the efficacy of quercetin in this disease. We now report the use of quercetin-hamster serum albumin combination to increase the bioavailability of quercetin. The combination targeted hemoglobin oxidation and produced an effective attenuation of heme degradation. This led to decreased iron decompartmentalization, thereby increasing the life span of erythrocytes during leishmanial infection. Thus, we speculate that suppression of iron decompartmentalization, with the combination of quercetin and serum albumin might be a new approach in the prevention of reduced survival of erythrocytes in visceral leishmaniasis.

The mechanism of heme transfer from the cytoplasmic heme binding protein PhuS to the delta-regioselective heme oxygenase of Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa has evolved two outer membrane receptor-mediated uptake systems (encoded by the phu and has operons) by which it can utilize the hosts heme and hemeproteins as a source of iron. PhuS is a cytoplasmic heme binding protein encoded within the phu operon and has previously been shown to function in the trafficking of heme to the iron-regulated heme oxygenase (pa-HO). While the heme association rate for PhuS was similar to that of myoglobin, a markedly higher rate of heme dissociation (approximately 10(5) s(-1)) was observed, in keeping with a function in heme-trafficking. Additionally, the transfer of heme from PhuS to pa-HO was shown to be specific and unidirectional when compared to transfer to the non-iron regulated heme oxygenase (BphO), in which heme distribution between the two proteins merely reflects their relative intrinsic affinities for heme. Furthermore, the rate of transfer of heme from holo-PhuS to pa-HO of 0.11 +/- 0.01 s(-1) is 30-fold faster than that to apo-myoglobin, despite the significant higher binding affinity of apo-myoglobin for heme (kH = 1.3 x 10(-8) microM) than that of PhuS (0.2 microM). This data suggests that heme transfer to pa-HO is independent of heme affinity and is consistent with temperature dependence studies which indicate the reaction is driven by a negative entropic contribution, typical of an ordered transition state, and supports the notion that heme transfer from PhuS to pa-HO is mediated via a specific protein-protein interaction. In addition, pH studies, and reactions conducted in the presence of cyanide, suggest the involvement of spin transition during the heme transfer process, whereby the heme undergoes spin change from 6-c LS to 6-c HS either in PhuS or pa-HO. On the basis of the magnitudes of the activation parameters obtained in the presence of cyanide, whereby both complexes are maintained in a 6-c LS state, and the biphasic kinetics of heme transfer from holo-PhuS to pa-HO-wt, supports the notion that the spin-state crossover occur within holo-PhuS prior to the heme transfer step. Alternatively, the lack of the biphasic kinetic with pa-HO-G125V, 6-c LS, and with comparable rate of heme transfer as pa-HO is supportive of a mechanism in which the spin-change could occur within pa-HO. The present data suggests either or both of the two pathways proposed for heme transfer may occur under the present experimental conditions. The dissection of which pathway is physiologically relevant is the focus of ongoing studies.

The crystal structure of the secreted dimeric form of the hemophore HasA reveals a domain swapping with an exchanged heme ligand

To satisfy their iron needs, several Gram-negative bacteria use a heme uptake system involving an extracellular heme-binding protein called hemophore. The function of the hemophore is to acquire free or hemoprotein-bound heme and to transfer it to HasR, its specific outer membrane receptor, by protein-protein interaction. The hemophore HasA secreted by Serratia marcescens, an opportunistic pathogen, was the first to be identified and is now very well characterized. HasA is a monomer that binds one b heme with strong affinity. The heme in HasA is highly exposed to solvent and coordinated by an unusual pair of ligands, a histidine and a tyrosine. Here, we report the identification, the characterization and the X-ray structure of a dimeric form of HasA from S. marcescens: DHasA. We show that both monomeric and dimeric forms are secreted in iron deficient conditions by S. marcescens. The crystal structure of DHasA reveals that it is a domain swapped dimer. The overall structure of each monomeric subunit of DHasA is very similar to that of HasA but formed by parts coming from the two different polypeptide chains, involving one of the heme ligands. Consequently DHasA binds two heme molecules by residues coming from both polypeptide chains. We show here that, while DHasA can bind two heme molecules, it is not able to deliver them to the receptor HasR. However, DHasA can efficiently transfer its heme to the monomeric form that, in turn, delivers it to HasR. We assume that DHasA can function as a heme reservoir in the hemophore system.

Detection of coexisting protein conformations in capillary zone electrophoresis subsequent to transient contact with sodium dodecyl sulfate solutions

Non-native conformations of proteins were generated by temporary contact with aqueous solutions of sodium dodecyl sulfate (SDS) and separated from the native state with capillary zone electrophoresis (CZE) in alkaline borate buffer deficient of SDS. Nine proteins at concentrations of 2.0 or 3.0 mg.L(-1) were compared in terms of their susceptibility to SDS. For superoxide dismutase and ferritin the tendency of unfolding was modest with < 25% of the protein being transformed to the non-native state at 10 mmol.L(-1) SDS. Highest susceptibility was observed for albumin, myoglobin (Mb), and hemoglobin with > 75% in the non-native state even at 2.0 mmol.L(-1) SDS. The influence of varying SDS concentrations on the conformational state of Mb was tested. Increasing the SDS concentration, circular dichroism revealed a reduction in alpha-helix, an increase in random coil, and an introduction of beta-sheet, which is absent in native structure. Modifications in the secondary structure were in agreement with distinct changes in the shape of the non-native Mb peak in CZE and make a gradual unfolding/refolding process with several coexisting molten globules instead of two-state transition of conformations most plausible for Mb. CZE was found to contribute to a further understanding of holo-Mb transformation towards a population of non-native conformations (i) by means of calculated peak area ratios of native to non-native states, which showed sigmoid transition, (ii) by detecting the release of the prosthetic heme group, and (iii) by changes in the effective electrophoretic mobility of the Mb-SDS peaks. Reconstituted holo-Mb forms differed in the Soret band around 410 nm, indicating diversity in the conformation of the heme pocket.

The lipocalin α1-microglobulin binds heme in different species

The lipocalin alpha(1)-microglobulin (alpha(1)m), found in plasma and tissues of various vertebrates, is brown, forms complexes with other proteins and has immunomodulatory effects in vitro, but the physiological function is not yet established. Human alpha(1)m was recently shown to bind heme and, after cleavage of a C-terminal tetrapeptide, initiate heme degradation, thus suggesting a heme-scavenger function. In this work the heme-binding of alpha(1)m was characterized using heme immobilized on agarose beads, spectrophotometry, and electrophoresis. alpha(1)m, both in plasma and in purified form, displayed a concentration-dependent binding to heme-agarose. The apparent dissociation-constant was estimated to be around 2 x 10(-6)M for both free alpha(1)m and the IgA-alpha(1)m complex. Incubation with free heme resulted in two forms of alpha(1)m with different electrophoretic mobility. alpha(1)m, identified on Western blotting, was found in eluates from heme-agarose after incubation with human biological fluids as well as sera from non-human species, indicating evolutionary conservation of the heme-binding property. Heme-binding could be instrumental for isolating new alpha(1)m-homologues.

No scavenging and the hypertensive effect of hemoglobin-based blood substitutes

The major pathway for nitric oxide scavenging in red cells involves the direct reaction of the gas with HbO2 to form nitrate and the ferric form of the protein, metHb. Because both atoms of O2 are incorporated into nitrate, this process is called NO dioxygenation (NOD). The NOD reaction involves an initial, very rapid bimolecular addition of NO to bound O2 to form a transient Fe(III)-peroxynitrite complex, which can be observed spectrally at alkaline pH. This intermediate rapidly isomerizes at pH 7 (t1/2 <== 1 ms) to metHb and NO3-, which is nontoxic and readily transported out of red cells and excreted. The rate of NO consumption by intracellular HbO2 during normal blood flow is limited by diffusion up to and into the red cells and is too slow to interfere significantly with vasoregulation. In contrast, extracellular HbO2 is highly vasoconstrictive, and the resultant hypertension is a significant side effect of most hemoglobin-based blood substitutes. The major cause of this blood pressure effect seems to be the high rate of NO dioxygenation by cell-free HbO2, which can extravasate into the vessel walls and interfere directly with NO signaling between endothelial and smooth muscle cells. This interpretation is supported by a strong linear correlation between the magnitude of the blood pressure effect caused by infusion of cross-linked recombinant hemoglobin tetramers in vivo and the rate of NO dioxygenation by these proteins measured in vitro.

Thermodynamics of heme binding to the HasA(SM) hemophore: effect of mutations at three key residues for heme uptake

HasA(SM) secreted by the Gram-negative bacterium Serratia marcescens belongs to the hemophore family. Its role is to take up heme from host heme carriers and to shuttle it to specific receptors. Heme is linked to the HasA(SM) protein by an unusual axial ligand pair: His32 and Tyr75. The nucleophilic nature of the tyrosine is enhanced by the hydrogen bonding of the tyrosinate to a neighboring histidine in the binding site: His83. We used isothermal titration microcalorimetry to examine the thermodynamics of heme binding to HasA(SM) and showed that binding is strongly exothermic and enthalpy driven: DeltaH = -105.4 kJ x mol(-1) and TDeltaS = -44.3 kJ x mol(-1). We used displacement experiments to determine the affinity constant of HasA(SM) for heme (K(a) = 5.3 x 10(10) M(-1)). This is the first time that this has been reported for a hemophore. We also analyzed the thermodynamics of the interaction between heme and a panel of single, double, and triple mutants of the two axial ligands His32 and Tyr75 and of His83 to assess the implication of each of these three residues in heme binding. We demonstrated that, in contrast to His32, His83 is essential for the binding of heme to HasA(SM), even though it is not directly coordinated to iron, and that the Tyr75/His83 pair plays a key role in the interaction.

Introduction of a new regulatory mechanism into human hemoglobin

Previous studies on bovine hemoglobin (HbBv) have suggested amino acid substitutions, which might introduce into human hemoglobin (HbA) functional characteristics of HbBv, namely a low intrinsic oxygen affinity regulated by Cl(-). Accordingly, we have constructed and characterized a multiple mutant, PB5, [beta(V1M + H2 Delta + T4I + P5A + A76K)] replacing four amino acid residues of HbA with those present at structurally analogous positions in HbBv, plus an additional substitution, beta T4I, which does not occur in either HbBv or HbA. This 'pseudobovine' hemoglobin has oxygen binding properties very similar to those of HbBv: the P(50) of HbA, PB5 and HbBv in the absence of Cl(-) are 1.6, 4.6 and 4.8 torr, respectively, and in 100 mM Cl(-) are 3.7, 10.5 and 12 torr, respectively. Moreover, PB5 has 3-fold slower autoxidation rate compared to HbA and HbBv. These are desirable characteristics for a human hemoglobin to be considered for use as a clinical artificial oxygen carrier. Although the functional properties of PB5 and HbBv are similar, van't Hoff plots indicate that the two hemoglobins interact differently with water, suggesting that factors regulating the R to T equilibrium are not the same in the two proteins. A further indication that PB5 is not a functional mimic of HbBv derives from PB5(control), a human hemoglobin with the same substitutions as PB5, except the beta T4I replacement. PB5(control) has a high oxygen affinity (P(50)=2.3 torr) in the absence of Cl(-), but retains the Cl(-) effect of PB5. The Cl(-) regulation of oxygen affinity in PB5 involves lysine residues at beta 8 and beta 76. PB4, which has the same substitutions as PB5 except beta A76K, and PB6, which has all the substitutions of PB5 plus beta K8Q, both have a low intrinsic oxygen affinity, like HbBv and PB5, but exhibit a decreased sensitivity to Cl(-). Since HbBv has lysine residues at both beta 8 and beta 76, these results imply that Cl(-) regulation in HbBv likewise involves these two residues. The mechanism responsible for the low intrinsic oxygen affinity of HbBv remains unclear. It is suggested that residues peculiar to HbBv at the alpha(1)beta(1) interface may play a role.

Processing of the lipocalin alpha(1)-microglobulin by hemoglobin induces heme-binding and heme-degradation properties

Alpha(1)-microglobulin is a 26-kd protein, widespread in plasma and tissues and well-conserved among vertebrates. Alpha(1)-microglobulin belongs to the lipocalins, a protein superfamily with highly conserved 3-dimensional structures, forming an internal ligand binding pocket. The protein, isolated from urine, has a heterogeneous yellow-brown chromophore bound covalently to amino acid side groups around the entrance of the lipocalin pocket. Alpha(1)-microglobulin is found in blood both in free form and complex-bound to immunoglobulin A (IgA) via a half-cystine residue at position 34. It is shown here that an alpha(1)-microglobulin species, which we name t-alpha(1)-microglobulin (t = truncated), with a free Cys34 thiol group, lacking its C-terminal tetrapeptide, LIPR, and with a more polar environment around the entrance of the lipocalin pocket, is released from IgA-alpha(1)-microglobulin as well as from free alpha(1)-microglobulin when exposed to the cytosolic side of erythrocyte membranes or to purified oxyhemoglobin. The processed t-alpha(1)-microglobulin binds heme and the alpha(1)-microglobulin-heme complex shows a time-dependent spectral rearrangement, suggestive of degradation of heme concomitantly with formation of a heterogeneous chromophore associated with the protein. The processed t-alpha(1)-microglobulin is found in normal and pathologic human urine, indicating that the cleavage process occurs in vivo. The results suggest that alpha(1)-microglobulin is involved in extracellular heme catabolism.

Interactions of haemoglobin with the Neisseria meningitidis receptor HpuAB: the role of TonB and an intact proton motive force

We have characterized the interaction of the Neisseria meningitidis TonB-dependent receptor HpuAB with haemoglobin (Hb). Protease accessibility assays indicated that HpuA and HpuB are surface exposed, HpuB interacts physically with HpuA, and TonB energization affects the conformation of HpuAB. Binding assays using [125I]-Hb revealed that the bipartite receptor has a single binding site for Hb (Kd 150 nM). Competitive binding assays using heterologous Hbs revealed that HpuAB Hb recognition was not species specific. The binding kinetics of Hb to HpuAB were dramatically altered in a TonB- mutant and in wild-type meningococci treated with the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP), indicating that TonB and an intact proton motive force are required for normal Hb binding and release from HpuAB. Our results support a model in which both HpuA and HpuB are required to form a receptor complex in the outer membrane with a single binding site, whose structure and ligand interactions are significantly affected by the TonB-mediated energy state of the receptor.

The influence of electrostatic interactions on the detection of heme-globin complexes in ESI-MS

The heme-globin complexes of hemoglobin and myoglobin are investigated in positive-ion mode and negative-ion mode using a nano-ESI source coupled to a quadrupole ion trap MS and an orthogonal time-of-flight MS. The extent of dissociation of these noncovalent complexes upon collisional activation and thus their gas-phase stability is strongly dependent on the polarity of the ESI-MS experiment as well as on the charge of the prosthetic group (ferri-heme [Fe3+-heme]+ vs. ferro-heme [Fe2+-heme]+/-0). The results clearly point to the important role of electrostatic interactions on the gas phase stability of noncovalent complexes and therefore the ion signals observed in ESI-MS experiments.

Enzymatic properties of human hemalbumin

The binding of hemin to the primary site of human serum albumin (HSA) has been reinvestigated using UV-Vis, CD and NMR techniques. The major fraction of bound hemin contains a five-coordinated high-spin iron(III) center, but a minor fraction of the metal appears to be in a six-coordinated, low-spin state, where a 'distal' residue, possibly a second histidine residue, completes the coordination sphere. The reduced, iron(II) form of the adduct contains six-coordinated low-spin heme. The distal residue hinders the access to the iron(III) center of hemin-HSA to small anionic ligands like azide and cyanide and destabilizes the binding of neutral diatomics like dioxygen and carbon monoxide to the iron(II) form. In spite of these limitations, the hemin-HSA complex promotes hydrogen peroxide activation processes that bear the characteristics of enzymatic reactions and may have biological relevance. The complex is in fact capable of catalyzing peroxidative reactions on phenolic compounds related to tyrosine and hydrogen peroxide dismutation. Kinetic and mechanistic studies confirm that the low efficiency with which peroxidative processes occur depends on the limited rate of the reaction between hydrogen peroxide and the iron(III) center, to form the active species, and by the competitive peroxide degradation reaction.

The Heme–Globin and Dimerization Equilibria of Recombinant Human Hemoglobins Carrying Site-Specific β Chains Mutations

The heme-globin and dimer-tetramer equilibria of ferric recombinant human hemoglobins with site-specific beta chain mutations at the heme pocket or at either the a1beta1 or the alpha1beta2 interfaces have been determined. The heme pocket mutation V67T leads to a marked stabilization of the beta chain heme and does not affect the dimer-tetramer association constant, K2,4. In the C112 mutants, the intrinsic rate of beta chain heme loss with respect to recombinant HbA (HbA-wt) is significantly increased only in C112G with some heme released also from the alpha chains. Gel filtration experiments indicate that the K2,4 value is essentially unaltered in C112G and C112L, but is increased in C112V and decreased in C112N. Substitution of cysteine 93 with A or M leads to a slight decrease of the rate of beta chain heme release, whereas the obvserved K2,4 value is similar to that obtained for HbA-wt. Modifications in oxygen affinity were observed in all the mutant hemoglobins with the exception of V67T, C93A, and C112G. The data indicate that there is no correlation between tetramer stability, beta chain heme affinity, and hemoglobin functionality and therefore point to a separate regulation of these properties.

Emerging strategies in microbial haem capture

Gram-negative pathogenic bacteria have evolved novel strategies to obtain iron from host haem-sequestering proteins. These include the production of specific outer membrane receptors that bind directly to host haem-sequestering proteins, secreted haem-binding proteins (haemophores) that bind haem/haemoglobin/haemopexin and deliver the complex to a bacterial cell surface receptor and bacterial proteases that degrade haem-sequestering proteins. Once removed from haem-sequestering proteins, haem may be transported via the bacterial outer membrane receptor into the cell. Recent studies have begun to define the steps by which haem is removed from bacterial haem proteins and transported into the cell. This review describes recent work on the discovery and characterization of these systems. Reference is also made to the transport of haem in serum (via haemoglobin, haemoglobin/haptoglobin, haemopexin, albumin and lipoproteins) and to mechanisms of iron removal from the haem itself (probably via a haem oxygenase pathway in which the protoporphyrin ring is degraded). Haem protein-receptor interactions are discussed in terms of the criteria that govern protein-protein interactions in general, and connections between haem transport and the emerging field of metal transport via metallochaperones are outlined.

Immobilized human hemoglobin, a versatile matrix for analytical and biotechnological applications

The analytical and biotechnological applications of human hemoglobin immobilized covalently on CNBr-Sepharose 4B are reviewed. Hemoglobin is bound to the matrix as alphabeta dimers via either chain. The immobilized alphabeta dimers maintain the capacity to interact reversibly with soluble ones under conditions where the soluble protein is in self-association equilibrium. Under these conditions, therefore, immobilized dimers bind part of the soluble protein. In turn, the binding process can be used to assess the specific features of the equilibrium on solid-phase and to extract selectively hemoglobin from a variety of biological specimens of practical interest. A different application of immobilized alphabeta dimers concerns their use in the determination of the equilibrium and kinetic stability of the heme-globin linkage, a property that is directly correlated with the stability of the hemoglobin molecule. The advantages and limitations attendant the use of the immobilized protein relative to the soluble one are discussed.

Free energy of burying hydrophobic residues in the interface between protein subunits

We have obtained an experimental estimate of the free energy change associated with variations at the interface between protein subunits, a subject that has raised considerable interest since the concept of accessible surface area was introduced by Lee and Richards [Lee, B. & Richards, F. M. (1971) J. Mol. Biol. 55, 379-400]. We determined by analytical ultracentrifugation the dimer-tetramer equilibrium constant of five single and three double mutants of human Hb. One mutation is at the stationary alpha1 beta1 interface, and all of the others are at the sliding alpha1 beta2 interface where cleavage of the tetramer into dimers and ligand-linked allosteric changes are known to occur. A surprisingly good linear correlation between the change in the free energy of association of the mutants and the change in buried hydrophobic surface area was obtained, after corrections for the energetic cost of losing steric complementarity at the alphabeta dimer interface. The slope yields an interface stabilization free energy of -15 +/- 1.2 cal/mol upon burial of 1 A2 of hydrophobic surface, in very good agreement with the theoretical estimate given by Eisenberg and McLachlan [Eisenberg, D. & McLachlan, A. D. (1986) Nature (London) 319, 199-203].

Immobilized apo-myoglobin, a new stable reagent for measuring rates of heme dissociation from hemoglobin

Apo-myoglobin covalently linked on CNBr-activated Sepharose 4B is proposed as a new heme acceptor for investigating the heme transfer reaction from hemoproteins. Immobilized apo-myoglobin has the desirable properties of an ideal heme acceptor in that it is characterized by a high affinity for ferric heme, a high stability towards denaturation even at physiological temperatures and can be lyophilized for long-term storage. The study of heme release from myoglobin at pH 5.0 and 37 degrees C indicates that heme affinity is increased at least 10-fold relative to the soluble protein. Experiments with human hemoglobin allowed the estimation of the heme release rates from both alpha and beta chains and brought out the greater temperature sensitivity of the alpha chain heme-globin linkage.

Modification of alpha-chain or beta-chain heme pocket polarity by Val(E11) --> thr substitution has different effects on the steric, dynamic, and functional properties of human recombinant hemoglobin. Deoxy derivatives

The dynamic and functional properties of mutant deoxyhemoglobins in which either the beta-globin Val67(E11) or the alpha-globin Val62(E11) is replaced by threonine have been investigated through the thermal evolution of the Soret absorption band in the temperature range 300 to 20 K and through the kinetics of CO rebinding after flash photolysis at room temperature. The conformational properties of the modified alpha chain and beta chain distal heme pockets were also studied through x-ray crystallography and molecular modeling. The data obtained with the various techniques consistently indicate that the polar isosteric mutation in the distal side of the alpha chain heme pocket has a larger effect on the investigated properties than the analogous mutation on the beta chain. We attribute the observed differences to the presence of a water molecule in the distal heme pocket of the modified alpha chains, interacting with the hydroxyl of the threonine side chain. This is indicated by molecular modeling which showed that the water molecule present in the alpha chain distal heme pocket can bridge by H bonding between Thr62(E11) and His58(E7) without introducing any unfavorable steric interactions. Consistent with the dynamic and functional data, the presence of a water molecule in the distal heme pocket of the modified beta chains is not observed by x-ray crystallography.

Kinetics of nitric oxide dissociation from five- and six-coordinate nitrosyl hemes and heme proteins, including soluble guanylate cyclase

Kinetics of NO dissociation were characterized for three five-coordinate systems, heme-NO, HSA-heme-NO (human serum albumin), GC-NO (soluble guanylate cyclase), and for the six-coordinate system, Im-heme-NO. Nitrosyl myoglobin was redetermined for comparison. Previously known, six-coordinate R and T state nitrosyl hemoglobins are also included in the comparison. The data indicate that NO dissociates more than 1000 times faster from five-coordinate model heme than it does from the six-coordinate analog. Such a negative trans-effect between NO and a proximal base is in sharp contrast to carboxy heme derivatives, in which ligand dissociation rates are greatly slowed in when a trans base is present. As a result of opposite trans-effects, six-coordinate carboxy and nitrosyl derivatives have comparable dissociation rates, even though the five-coordinate species are very different. In proteins, five- and six-coordinate forms do not show a large difference in dissociation rates. Part of the reason may be due to different probabilities for geminate recombination in the different proteins, but this cannot explain all the facts. There must also be influences of the protein structure on bond-breaking rate constants themselves. With the exception of hemoglobin in the T state, nitrosyl guanylate cyclase shows the highest NO dissociation rate constant, k(obs) = 6 x 10(-4) s(-1). This would yield a half-life of about 2 min at 37 degrees C for dissociation of NO from GC-NO, a number that has implications for the mechanism of regulation of the activity of this key heme enzyme.

Dissociation of heme-globin complexes by blackbody infrared radiative dissociation: molecular specificity in the gas phase?

The temperature dependence of the unimolecular kinetics for dissociation of the heme group from holo-myoglobin (Mb) and holo-hemoglobin alpha-chain (Hb-alpha) was investigated with blackbody infrared radiative dissociation (BIRD). The rate constant for dissociation of the 9 + charge state of Mb formed by electrospray ionization from a "pseudo-native" solution is 60% lower than that of Hb-alpha at each of the temperatures investigated. In solutions of pH 5.5-8.0, the thermal dissociation rate for Mb is also lower than that of HB-alpha (Hargrove, M. S. et al. J. Biol. Chem.1994, 269, 4207-4214). Thus, Mb is thermally more stable with respect to heme loss than Hb-alpha both in the gas phase and in solution. The Arrhenius activation parameters for both dissociation processes are indistinguishable within the current experimental error (activation energy 0.9 eV and pre-exponential factor of 10(8-10) s(-1)). The 9+ to 12+ charge states of Mb have similar Arrhenius parameters when these ions are formed from pseudo-native solutions. In contrast, the activation energies and pre-exponential factors decrease from 0.8 to 0.3 eV and 10(7) to 10(2) s(-1), respectively, for the 9 + to 12 + charge states formed from acidified solutions in which at least 50% of the secondary structure is lost. These results demonstrate that gas-phase Mb ions retain clear memory of the composition of the solution from which they are formed and that these differences can be probed by BIRD.

Spectral demonstration of semihemoglobin formation during CN-hemin incorporation into human apohemoglobins

The incorporation of CN-hemin into three human adult apohemoglobin species (apohemoglobin, alpha-apohemoglobin, and apohemoglobin modified at its beta93 sulfhydryl with p-hydroxymercuribenzoate) has been monitored at micromolar concentrations in 0.05 M potassium phosphate buffer, pH 7.0, at 10 degrees C. In all cases, Soret spectral blue shifts accompanied CN-protohemoglobin but not CN-deuterohemoglobin formation. This finding in conjunction with isofocusing studies provided evidence of a CN-protosemi-alpha-hemoglobin intermediate, the formation of which appeared to be a direct consequence of CN-protohemin-alpha heme pocket interactions. The kinetics of full reconstitution of CN-protohemoglobin and CN-deuterohemoglobin revealed four distinct phases that apparently correlated with heme insertion (Phase I), local structural rearrangement (Phase II), global conformational response (Phase III), and irreversible histidine iron bond formation (Phase IV). These phases exhibited rates of 7.8-22 x 10(7) M(-1) s(-1), 0.19-0.23 s(-1), 0.085-0.12 s(-1), and 0.008-0.012 s(-1), respectively. Partial (50%) reconstitution with CN-protohemin, in contrast, revealed only three kinetic phases (with Phase III missing) of heme incorporation into native and p-hydroxymercuribenzoate-modified apohemoglobin. Furthermore, the absence of Phase III slowed the rate of proximal bond formation. These findings support the premise that irreversible assembly of CN-protosemi-alpha-hemoglobin is deterred by the presence of a heme-free beta partner, the consequence of which may be that intermolecular heme transfer is encouraged under conditions of heme deficiency in vivo.