Random chemical modification of the oxygen-linked chloride-binding sites of hemoglobin: those in the central dyad axis may influence the transition between deoxy- and oxy-hemoglobin

Chloride Ions Stabilize Human Adult Hemoglobin in the T-State, Competing with Allosteric Interaction of Oxygen Molecules

In the context of a molecular-level understanding of the allostery mechanisms, human adult hemoglobin (HbA) has been extensively studied for over half a century. Chloride ions (Cl^-) have been known as one of HbA allosteric effectors, which stabilizes the T-state preferable to release oxygen molecules. The functional mechanisms were individually proposed by Ueno and Perutz several decades ago. Ueno considered that the site-specific Cl^- binding is essential, while Perutz proposed the non-site-specific interaction between HbA and Cl^-. Each speculation explains the mechanism plausibly since each was tightly associated with its reasonable experimental observation. However, both mechanisms themselves still seem to make their speculations controversial. In the present study, we have theoretically reconsidered these apart from their approaches. Our atomistic molecular dynamics simulations then showed that the increase of Cl^- concentration suppresses the conformational conversion from the T-state. Interestingly, chloride ions loosely interact with the amino acid residues inside the HbA central cavity, suggesting that both Perutz's and Ueno's speculations are involved in understanding the microscopic roles of Cl^-. In conclusion, we theoretically certified that the effect of Cl^- competes against that of solvated O₂, i.e., the destabilization of T-state through the non-site-specific interaction, implying the concerted regulation of HbA under physiological conditions.

Modern cross-linking strategies for synthesizing acellular hemoglobin-based oxygen carriers

Unmodified cell-free hemoglobin (Hb) is structurally unstable when transfused into the blood stream (Valeri et al., Artif Cells Blood Substit Immobil Biotechnol. 2000;28:451-475; Chan et al., Toxicol Pathol. 2000;28:635-642; Eike, Dissertation, 2005; Eike and Palmer, Biotechnol Prog. 2004;20:946-952). This review examines some of the latest chemical strategies used over the last 5 years to intra- and intermolecularly cross-link Hb, thereby stabilizing its quaternary structure. Therefore, this work will address the following aspects: (1) site-specific chemical modifications of Hb and (2) non-site-specific chemical modifications of Hb, including, but not limited to, PolyHeme, Hemopure, Oxyglobin, and SOD-Hb. Current strategies for synthesizing PEGylated Hb is outside the scope of this review and will not be discussed herein. For a more thorough review of PEGylated Hb, the reader is directed to the following works: Cabrales and Friedman, Transfus Alternatives in Transfus Med. 2007;9:281-293 and Winslow, Biochim Biophys Acta, 2008;1784(10):1382-1386.

Structural and functional characterization of glutaraldehyde-polymerized bovine hemoglobin and its isolated fractions

Glutaraldehyde-polymerized bovine hemoglobin (PolyHbBv, trade name Oxyglobin), is a non-site-specific modified hemoglobin-based oxygen-carrying solution, developed for use in veterinary medicine. PolyHbBv was fractionated into four distinct tetrameric and multiple polytetrameric forms ranging in molecular mass (87.2-502.3 kDa) using size exclusion chromatography (SEC) and verified by laser light scattering. We evaluated the structural modification occurring in the fractionated mixture of PolyHbBv and assessed the functionality and redox stability of each fraction in relation to the mixture as a whole. Intramolecular cross-linking evaluation as performed by MALDI-MS and SEC under dissociating conditions revealed no-site-specific tetramer stabilization within the fractions; Intermolecular cross-linking was highly correlated with lysine and histidine modification as determined by amino acid composition analysis. While native unmodified hemoglobin, HbBv, PolyHbBv, and PolyHbBv fractions (F1-F4) revealed significant methionine oxidation, modification, or both, the critical betaMet55 located in the functionally plastic domains (alpha1-beta1 interface) of HbBv was unaltered. Moreover, neither of the two betaCys93 located in the highly plastic alpha1-beta2 interface were modified in PolyHbBv or in F1-F4. Our structural analysis also revealed that the reported loss in sensitivity to chloride in PolyHbBv could not be attributed to direct alteration of chloride ion binding amino acids. Structural modification imparted by glutaraldehyde resulted in nearly identical functional characteristics of PolyHbBv and its fractions with regard to oxygen equilibrium, ligand binding, and autoxidative kinetics.

Distinct domain responses of R-state human hemoglobins A, C, and S to anions

Anionic regulation of hemoglobin (Hb) is of increasing interest for the design of Hb-based oxygen carriers. Even "external" amino-acid substitutions can alter the nature and extent of anionic control. This was shown by evaluation of the anion sensitivities of liganded, R-state, forms of HbA, HbC (beta6 Glu --> Lys) and HbS (beta6 Glu --> Val). The beta6 mutants differ in the anion-sensitivity of their central cavities, alpha1beta2 interfaces, and heme and beta93 Cys environments. The mutant Hbs also exhibit increased anion-dependent oxidation and surface denaturation. Moreover, differential chloride effects on oxygen binding by Hbs C, S compared to HbA occur after R-state stabilization by fluoresceination of beta93 Cys. It is concluded that the "external" substitutions in the mutant Hbs have structural consequences that are propagated to varying extents to other domains as a result of anion binding, and that these anion-dependent changes may underlie mechanisms leading to the observed increase in oxidation propensity and surface denaturation.

Effects of glutaraldehyde polymerization on oxygen transport and redox properties of bovine hemoglobin

Crosslinking of bovine Hb (HbBv) with glutaraldehyde produces a mixture of low oxygen affinity (P(50)) tetrameric and polymeric Hb species (PolyHbBv). Under physiological conditions the P(50) of HbBv and PolyHbBv were 27 and 35 mmHg, respectively. The dependence of the P(50) on pH and chloride ions and the cooperativity (n(50)) of the protein were diminished as a result of glutaraldehyde modification. Rapid kinetic studies showed greater overall rates of oxygen dissociation (k(off)) with little or no change in the association of CO (k(on)) to the modified protein. The rate of nitric oxide (NO)-induced oxidation of the PolyHbBv was slightly lower than that of HbBv. Autoxidation rate of PolyHbBv was 1.4 times faster than that of HbBv. The reaction of hydrogen peroxide (H2O2) with the ferrous (Fe(2+)) and ferric (Fe(3+)) forms of the proteins led to the formation of a more stable ferrylHb (Fe(4+)) in the case of PolyHbBv. Glutaraldehyde polymerization of HbBv alters its normal allosteric mechanisms, autoxidation kinetics and other related redox properties, which may compromise its function and cause greater toxicity when used as an oxygen transport fluid.

Transposing sequences between fetal and adult hemoglobins indicates which subunits and regulatory molecule interfaces are functionally related

To correlate amino acid sequence changes with hemoglobin function we are carrying out a detailed recombinant analysis of the adult hemoglobin/fetal hemoglobin (HbA/HbF) systems. The important physiological differences between these two tetramers lie at unspecified sites in the 39 sequence substitutions of the 146 amino acids in their beta and gamma chains. In this paper, significant differences in the tetramer-dimer dissociation constants (referred to as tetramer "strength" or "stability") of adult (HbA) and fetal (HbF) hemoglobin tetramers have been used to probe the relationship between the allosteric, sliding interface and the effects of the allosteric regulator, 2,3-DPG, in promoting oxygen release. The single amino acid difference at the allosteric interfaces of these two hemoglobins, Glu-43(beta) --> Asp-43(gamma), which is not near the DPG binding site, leads to a significantly lower DPG response, approaching that of HbF. The results are inconsistent with the long-held idea that the replacement of His-143(beta) in HbA to Ser-143(gamma) in HbF is solely responsible for the lowered DPG response in HbF. On the other hand, the Val-1(beta) --> Gly-1(gamma) replacement near the DPG binding site has no effect on the DPG response. The replacement of His-116(beta) by the hydrophobic Ile-116(gamma) at the rigid alpha(1)beta(1) interface has a marginal yet detectable effect on the allosteric alpha(1)beta(2) interface. The results, overall, are interpreted using a model involving electrostatic coupling between certain side chains and extend the concept of a long-range relationship between some distant regions of the tetramer that are likely mediated through the central cavity.

Recombinant Sickle Hemoglobin Containing a Lysine Substitution at Asp-85(α): Expression in Yeast, Functional Properties, and Participation in Gel Formation

Clinical modalities based on inhibition of gelation of HbS are hindered by the lack of quantitative information on the extent of participation of different amino acid residues in the aggregation process. One such site is Asp-85(α), which is involved in a parallel interdouble strand ionic interaction with Lys-144(β) according to the crystal structure of HbS, but electron microscopy does not specifically show Asp-85(α) as a contact site for fiber formation. Using a yeast recombinant system, we have substituted this site by Lys to abolish ion pairing and to make a quantitative determination of its participation in aggregation. The purified double mutant was shown to have the expected pI, the calculated molecular weight, correct amino acid composition, and peptide map. The recombinant double mutant has an oxygen affinity of 10 mm Hg, which is identical to that for HbA and HbS under the same conditions; it also has high cooperativity with an average n value of 2.7. The change in P50 in response to chloride ions was about 25% less than that for HbA or HbS and is ascribed to the introduction of a new positive charge near one of the major oxygen-linked chloride binding sites of hemoglobin. The gelation concentration of the double mutant was measured by a new procedure (Bookchin et al, 1994); the maximal amount of soluble hemoglobin (Csat ) in the presence of dextran indicated a decreased tendency for gelation with a Csat of 53 mg/mL compared with 34 mg/mL for HbS. This inhibitory effect is smaller than that of the E6V(β)/L88A(β) (Csat , 67 mg/mL) and the E6V(β)/K95I(β) (Csat , 90 mg/mL) recombinant hemoglobins. Thus, we would classify Asp-85(α) as a moderate contributor to the strength of the HbS aggregate. This wide range of gelation values demonstrates that some sites are more important than others in promoting HbS aggregation.

A recombinant human hemoglobin with asparagine-102(beta) substituted by alanine has a limiting low oxygen affinity, reduced marginally by chloride

A recombinant (r) mutant hemoglobin (Hb) with Asn-102(beta) replaced by an Ala (N102A(beta)) has been prepared by PCR amplification of a mutagenic DNA fragment and expression of the recombinant protein in yeast. The side chain of Asn-102(beta) is part of an important region of the alpha 1 beta 2 interface that undergoes large structural changes in the transition between the deoxy and oxy conformations. Three natural mutant Hbs with neutral substitutions of Thr, Ser, or Tyr at this site have low oxygen affinities because a hydrogen bond between Asn-102(beta) and Asp-94(alpha) in normal HbA was considered to be absent in these mutants, thereby destabilizing the oxy conformation in favor of the deoxy conformation. This proposal has been tested by expression of an rHb containing alanine at position 102(beta); alanine was chosen because its methyl side chain cannot participate in hydrogen bond formation, yet it is small enough not to disrupt the subunit interface. The nature of the desired replacement was established by sequencing the entire mutated beta-globin gene as well as the tryptic peptide containing the substitution. Further characterization by SDS-PAGE, isoelectric focusing, HPLC analysis, mass spectrometry, amino acid analysis, and sequencing of the mutant tryptic peptide confirmed the purity of the rHb. Its oxygen binding curve (2.4 mM in heme) in the absence of chloride showed that it had a very low oxygen affinity with a P50 of 42 mm Hg.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of a recombinant human hemoglobin with aspartic acid 99(beta), an important intersubunit contact site, substituted by lysine

Site-directed mutagenesis of an important subunit contact site, Asp-99(beta), by a Lys residue (D99K(beta)) was proven by sequencing the entire beta-globin gene and the mutant tryptic peptide. Oxygen equilibrium curves of the mutant hemoglobin (Hb) (2-15 mM in heme) indicated that it had an increased oxygen affinity and a lowered but significant amount of cooperativity compared to native HbA. However, in contrast to normal HbA, oxygen binding of the recombinant mutant Hb was only marginally affected by the allosteric regulators 2,3-diphosphoglycerate or inositol hexaphosphate and was not at all responsive to chloride. The efficiency of oxygen binding by HbA in the presence of allosteric regulators was limited by the mutant Hb. At concentrations of 0.2 mM or lower in heme, the mutant D99K(beta) Hb was predominantly a dimer as demonstrated by gel filtration, haptoglobin binding, fluorescence quenching, and light scattering. The purified dimeric recombinant Hb mutant exists in 2 forms that are separable on isoelectric focusing by about 0.1 pH unit, in contrast to tetrameric hemoglobin, which shows 1 band. These mutant forms, which were present in a ratio of 60:40, had the same masses for their heme and globin moieties as determined by mass spectrometry. The elution positions of the alpha- and beta-globin subunits on HPLC were identical. Circular dichroism studies showed that one form of the mutant Hb had a negative ellipticity at 410 nm and the other had positive ellipticity at this wavelength. The findings suggest that the 2 D99K(beta) recombinant mutant forms have differences in their heme-protein environments.