Stoichiometry of subunits and heme content of hemoglobin from the earthworm Lumbricus terrestris

Structural Stability and Biophysical Properties of the Mega-Protein Erythrocruorin Are Regulated by Polyethylene Glycol Surface Coverage

A wide variety of hemoglobin-based oxygen carriers (HBOCs) have been designed for use as red blood cell (RBC) substitutes in transfusion medicine, ex vivo organ perfusion, oxygen delivery to hypoxic tissues, and a myriad of other applications. However, hemoglobin (Hb) derived from annelids (erythrocruorins [Ecs]) comprise a natural class of HBOC, since they are larger in size (30 nm in diameter) and contain more heme groups per molecule (144 heme groups) compared to human Hb (hHb; 5 nm in diameter and 4 heme groups). The larger size of Ec compared to hHb reduces tissue extravasation from the vascular space, thus, reducing vasoconstriction, systemic hypertension, and tissue oxidative injury when used as an RBC substitute. In addition, prior research has shown that Ecs possess slower auto-oxidation rates than hHb at physiological temperature, thus, making them attractive candidates for use as RBC substitutes. Unfortunately, it was also observed that Ecs have a much lower circulatory half-life in vivo compared to other HBOCs. Hence, conjugating polyethylene glycol (PEG) to the surface of Ec was proposed as a simple strategy to increase Ec circulatory half-life. Therefore, in order to inform future in vivo studies with PEGylated Ec, we decided to investigate the structural stability and biophysical properties of variable PEG surface coverage on Ec compared to native Ec. We observed an increase in PEG-Ec diameter and molecular weight (MW) and changes to the quaternary structure, secondary structure, and surface hydrophobicity after PEGylation. There was also an increase in oxygen binding affinity, reduction in oxygen offloading rate, and increase in auto-oxidation rate for increasing PEGylation ratios. Weak dissociation of Ec was also observed after dense PEGylation caused by steric repulsion of the conjugated PEG chains. Hence, we determined an optimum Ec PEGylation ratio that resulted in a substantial size and MW increase along with preservation of oxygen binding properties. In future studies, these materials will be tested in animal models to evaluate pharmacodynamics, pharmacokinetics, tissue oxygenation, microcirculatory responses, and overall safety.

Purification of Lumbricus terrestris Mega-Hemoglobin for Diverse Oxygen Therapeutic Applications

Oxygen therapeutics are being developed for a variety of applications in transfusion medicine. In order to reduce the side-effects (vasoconstriction, systemic hypertension, and oxidative tissue injury) associated with previous generations of oxygen therapeutics, new strategies are focused on increasing the molecular diameter of hemoglobin obtained from mammalian sources via polymerization and encapsulation. Another approach towards oxygen therapeutic design has centered on using naturally occurring large molecular diameter hemoglobins (i.e. erythrocruorins) derived from annelid sources. Therefore, the goal of this study was to purify erythrocruorin from the terrestrial worm Lumbricus terrestris for diverse oxygen therapeutic applications. Tangential flow filtration (TFF) was used as a scalable protein purification platform to obtain a >99% pure LtEc product, which was confirmed by size exclusion high performance liquid chromatography and SDS-PAGE analysis. In vitro characterization concluded that the ultra-pure LtEc product had oxygen equilibrium properties similar to human red blood cells, and a lower rate of auto-oxidation compared to human hemoglobin, both of which should enable efficient oxygen transport under physiological conditions. In vivo evaluation concluded that the ultra-pure product had positive effects on the microcirculation sustaining functional capillary density compared to a less pure product (~86% purity). In summary, we purified an LtEc product with favorable biophysical properties that performed well in an animal model using a reliable and scalable purification platform to eliminate undesirable proteins.

Environmental heme utilization by heme-auxotrophic bacteria

Heme, an iron-containing porphyrin, is the prosthetic group for numerous key cellular enzymatic and regulatory processes. Many bacteria encode the biosynthetic enzymes needed for autonomous heme production. Remarkably, however, numerous other bacteria lack a complete heme biosynthesis pathway, yet encode heme-requiring functions. For such heme-auxotrophic bacteria (HAB), heme or porphyrins must be captured from the environment. Functional studies, aided by genomic analyses, provide insight into the HAB lifestyle, how they acquire and manage heme, and the uses of heme that make it worthwhile, and sometimes necessary, to capture this bioactive molecule.

Characterization of a 12 kDa thermal-stable antigenic protein in bovine blood

We have previously developed an immunoassay based on monoclonal antibody (MAb) Bb1H9 for quantitative detection of ruminant blood in processed food and feedstuffs. The purpose of this study was to characterize the unknown 12 kDa thermal-stable ruminant-specific antigenic protein recognized by MAb Bb1H9 in order to better define the application scope of the developed assay. Extracts obtained from raw and heat-treated bovine blood-derived products were analyzed with indirect ELISA and Western blot. Target proteins resolved by 2D electrophoresis were subjected to N-terminal sequencing. Results indicated that the 12 kDa protein is a monomer of the tetrameric hemoglobin molecule (64.5 kDa) and that the heme group is not required for its binding with MAb Bb1H9. This MAb can be utilized as a probe for red blood cell derived products of ruminant origin in raw or processed food and feedstuffs to enforce labeling regulations and to address consumer concerns.

PRACTICAL APPLICATION

MAb Bb1H9 represents the first antibody with the capacity to recognize bovine hemoglobin both in the absence and presence of the heme group, regardless of the heat treatment. MAb Bb1H9 can therefore be utilized in immunoassays by manufacturers and regulators to detect any ingredients containing hemoglobin or globin (hemoglobin without the heme group) in both raw and processed food and feed materials for product quality control and labeling law enforcement.

Effect of heavy metal exposure on blood haemoglobin concentration and methemoglobin percentage in Lumbricus terrestris

The earthworm haemoglobin (Hb) is a large extracellular hemoprotein flowing in a closed circulatory system. In spite of the fundamental role of this respiratory pigment in earthworm physiology, little is known about its sensitivity to environmental pollutants. The aim of the present work was to investigate the possible effect of heavy metal (cadmium, copper, mercury) exposure on Hb concentration and oxidation state (methemoglobin formation) in the earthworm Lumbricus terrestris. In addition, the tissue concentration of metallothioneins, a well-known biomarker of heavy metal exposure, was determined as an indicator of metal uptake. The animals were exposed to increasing concentrations of Cd, Cu and Hg utilizing the standard acute toxicity test, "Filter paper test" for 48 h. Exposure to heavy metals (10(-5)-10(-3) M for Cd, 10(-4)-10(-3) M for Hg, and 10(-4)-10(-2) M for Cu) was found to increase haemoglobin concentration in L. terrestris, although the magnitude of such an increase was dependent on the metal. In addition, metal exposure led to the formation of methemoglobin. Compared to other known biological responses to heavy metals, such as metallothionein induction, methemoglobin increase showed a higher sensitivity and a higher percentage variation in exposed organisms, showing to be a possible suitable biomarker of exposure/effect to be included in a multi biomarker strategy in earthworm in soil monitoring assessment.

Linker chains of the gigantic hemoglobin of the earthworm Lumbricus terrestris: primary structures of linkers L2, L3, and L4 and analysis of the connectivity of the disulfide bonds in linker L1

The extracellular hemoglobin (Hb) of the earthworm, Lumbricus terrestris, has four major kinds of globin chains: a, b, c, and d, present in equimolar proportions, and additional non-heme, non-globin scaffolding chains called linkers that are required for the calcium-dependent assembly of the full-sized molecule. The amino acid sequences of all four of the globin chains and one of the linkers (L1) have previously been determined. The amino acid sequences via cDNA of each of the three remaining linkers, L2, L3, and L4, have been determined so that the sequences of all constituent polypeptides of the hemoglobin are now known. Each linker has a highly conserved cysteine-rich segment of approximately 40 residues that is homologous with the seven ligand-binding repeats of the human low-density lipoprotein receptor (LDLR). Analysis of linker L1 shows that the connectivity of the three disulfide bonds is exactly the same as in the LDLR ligand-binding repeats. The presence of a calcium-binding site comprising one glutamyl and three aspartyl residues in both the LDLR repeats and in the linkers supports the suggestion that calcium is required for the folding and disulfide connectivity of the linkers as in the LDLR repeats. Linker L2 is markedly heterogeneous and contains unusual glycine-rich sequences near the NH2-terminus and a polar zipper-like sequence with imperfect repeats of Asp-Asp-His at the carboxyl terminus. Similar Asp-Asp-His repeats have been found in a protein homologous to superoxide dismutase in the hemolymph of certain mussels. These repeats may function as metal-binding sites.

The stoichiometry of the four linker subunits of Lumbricus terrestris hemoglobin suggests an asymmetric distribution

The extracellular, giant ( approximately 3.6 MDa) hexagonal bilayer hemoglobin of the earthworm Lumbricus terrestris consists of 12 dodecamers of globin chains tethered to a central complex of 36 non-globin, linker chains (24-32 kDa). Four types of linker chains L1-L4 have been detected by electrospray ionization (ESI) and by matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) and isolated by reversed phase high pressure liquid chromatography (HPLC). Deconvolution of the HPLC elution profile and of the MS spectra provided the following individual linker contents, expressed as percent of the sum of the four linker peak areas: HPLC-21% L1, 37% L2, 23% L3 and 19% L4, MALDI-47% L1, 29% L2, 16% L3 and 8% L4; ESI-24% L1, 16% L2, 40% L3 and 20% L4; respectively. Comparison with electrophoretic results revealed a surprising lack of overall agreement between all the methods. The calculated mean values of the available linker contents were found to be 32+/-12% L1, 28+/-9% L2, 27+/-10% L3 and 13+/-7% L4, suggesting the following relative stoichiometry: L1: L2: L3: L4 approximately 1: 1: 1: 0.5. With a total of 36 linkers, a hexagonally symmetric distribution of each of the four linker chains is impossible. Thus, the asymmetric linker distribution provides an explanation for the existence of a large dipole moment of Lumbricus terrestris hemoglobin, 17,300+/-2300 Da (Takashima et al., 1999).

Towards a resolution of the long-standing controversy regarding the molecular mass of extracellular erythrocruorin of the earthworm Lumbricus terrestris

The published molecular mass of erythrocruorin of Lumbricus terrestris and related earthworm species covers a bewildering range of 3.23-4.5 MDa. A critical reexamination reveals that some mass determinations were underestimated and the results do cluster, not at one, but at two values of the molecular mass. One cluster corresponds to approximately 3.6 MDa, as predicted for a stoichiometry of 144 globin and 36 linker chains-the Vinogradov model for the hexagonal bilayer (HBL) assembly of Lumbricus erythrocruorin-and as estimated from the crystal structure of HBL at 5.5 A resolution [Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 7107]. The other cluster corresponds to approximately 4.4 MDa. In addition, a molecular mass of 4.1 MDa, determined by multiangle laser light scattering (MALLS), stands apart of the two clusters, separated from the masses obtained by other methods of molecular mass determination. We propose a stoichiometry of 192 globin and 36 linker chains for the 4.4-MDa molecule. The 36 linkers and 144 out of 192 globin chains are identified with the HBL and the remaining 48 globins are allotted equally to the two halves of the axial cavity above and below the central torus of the structure. The proposed model is supported by the occurrence in some annelid species of erythrocruorin with centrally placed subunits [Biochim. Biophys. Acta 359 (1974) 210], and by the oxidation-dependent shedding of subunits in Lumbricus erythrocruorin. We propose further that the 4.1 MDa determination represents the weight average molecular mass of a population of molecules resulting from a partial dissociation of 4.4-MDa erythrocruorin. This interpretation seems reasonable on the background of the very low protein concentrations ( approximately 100 microg/ml and lower) prevailing at the MALLS experiment.

Fluorescence properties of tryptophan residues in the monomeric d-chain of Glossoscolex paulistus hemoglobin: an interpretation based on a comparative molecular model

The primary structure of the 142 residue Glossoscolex paulistus d-chain hemoglobin has been determined from Edman degradation data of 11 endo-Glu-C peptides and 11 endo-Lys-C peptides, plus the results of Edman degradation of the intact globin. Tryptophan occupies positions 15, 33 and 129. Homology modeling allowed us to assign the positions of these Trp residues relative to the heme and its environment. The reference coordinates of the indole rings (average coordinates of the C(varepsilon2) and C(delta2) atoms) for W15 and W129 were 16.8 and 18.5 A, respectively, from the geometric center of the heme, and W33 was located in close proximity to the heme group at a distance which was approximately half of that for W15 and W129. It was possible to identify three rotamers of W33 on the basis of electrostatic and Van der Waals energy criteria. The calculated distances from the center of the heme were 8.3, 8.4 and 9.1 A for Rot1, Rot2 and Rot3, respectively. Radiationless energy transfer from the excited indole to the heme was calculated on the basis of Förster theory. For W33, the distance was more important than the orientation factor, kappa(2), due to its proximity to the heme. However, based on kappa(2), Rot2 (kappa(2)=0.945) was more favorable for the energy transfer than Rot1 (kappa(2)=0.433) or Rot3 (kappa(2)=0.125). In contrast, despite its greater distance from the heme, the kappa(2) of W129 (2.903) established it as a candidate to be more efficiently quenched by the heme than W15 (kappa(2)=0.191). Although the Förster approach is powerful for the evaluation of the relative efficiency of quenching, it can only explain pico- and sub-nanosecond lifetimes. With the average lifetime, <tau>=3 ns, measured for the apomonomer as the reference, the lifetimes calculated for each emitter were: W33-1 (1 ps), W33-2 (2 ps), W33-3 (18 ps), W129 (100 ps), and W15 (600 ps). Experimentally, there are four components for oxymonomers at pH 7: two long ones of 4.6 and 2.1 ns, which contribute approximately 90% of the total fluorescence, one of 300 ps (4%), and the last one of 33 ps (7.4%). It is clear that the equilibrium structure resulting from homology modeling explains the sub-nanosecond fluorescence lifetimes, while the nanosecond range lifetimes require more information about the protein in solution, since there is a significant contribution of lifetimes that resemble the apo molecule.

Observation of large, non-covalent globin subassemblies in the approximately 3600 kDa hexagonal bilayer hemoglobins by electrospray ionization time-of-flight mass spectrometry

A non-covalent globin subassembly comprising 12 globin chains (204 to 214 kDa) was observed directly by electrospray ionization time-of-flight mass spectrometry in the native hexagonal bilayer hemoglobins from the oligochaetes Lumbricus terrestris and Tubifex tubifex, the polychaetes Tylorrhynchus heterochaetus, Arenicola marina, Amphitrite ornata and Alvinella pompejana, the leeches Macrobdella decora, Haemopis grandis and Nephelopsis oscura and the chlorocruorin from the polychaete Myxicola infundibulum, over the pH range 3.5-7.0. The Hb from the deep-sea polychaete Alvinella exhibited in addition, peaks at approximately 107 kDa and at approximately 285 kDa, which were assigned to subassemblies of six globin chains and of 12 globin chains with three non-globin linker chains, respectively. The experimental masses decreased slightly with increased de-clustering potential (60 to 160 V) and were generally 0.1 to 0.2 % higher than the calculated masses, due probably to complexation with cations and water molecules.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.

Subunit distribution of calcium-binding sites in Lumbricus terrestris hemoglobin

The giant, approximately 3.6-MDa hexagonal bilayer hemoglobin (Hb) of Lumbricus terrestris consist of twelve 213-kDa globin subassemblies, each comprised of three disulfide-bonded trimers and three monomer globin chains, tethered to a central scaffolding of 36-42 linkers L1-L4 (24-32 kDa). It is known to contain 50-80 Ca and 2-4 Cu and Zn; the latter are thought to be responsible for the superoxide dismutase activity of the Hb. Total reflection X-ray fluorescence spectrometry was used to determine the Ca, Cu, and Zn contents of the Hb dissociated at pH approximately 2.2, the globin dodecamer subassembly, and linker subunits L2 and L4. Although the dissociated Hb retained 20 Ca2+ and all the Cu and Zn, the globin subassembly had 0.4 to approximately 3 Ca2+, depending on the method of isolation, and only traces of Cu and Zn. The linkers L2 and L4, isolated by reversed-phase high-pressure liquid chromatography at pH approximately 2.2, had 1 Ca per mole and very little Cu and Zn. Electrospray ionization mass spectrometry of linker L3 at pH approximately 2.2 and at neutral pH demonstrated avid binding of 1 Ca2+ and additional weaker binding of 7 Ca2+ in the presence of added Ca2+. Based on these and previous results which document the heterogeneous nature of the Ca2+-binding sites in Lumbricus Hb, we propose three classes of Ca2+-binding sites with affinities increasing in the following order: (i) a large number of sites (>100) with affinities lower than EDTA associated with linker L3 and dodecamer subassembly, (ii) approximately 30 sites with affinities higher than EDTA occurring within the cysteine-rich domains of linker L3 and dodecamer subassembly, and (iii) approximately 25 very high affinity sites associated with the linker subunits L1, L2, and L4. It is likely that the low-affinity type (i) sites are the ones involved in the effects of 1-100 mM Group IIA cations on Lumbricus Hb structure and function, namely increased stability of its quaternary structure and increased affinity and cooperativity of its oxygen binding.

Electrospray ionization mass spectrometric determination of the molecular mass of the approximately 200-kDa globin dodecamer subassemblies in hexagonal bilayer hemoglobins

Hexagonal bilayer hemoglobins (Hbs) are approximately 3.6-MDa complexes of approximately 17-kDa globin chains and 24-32-kDa, nonglobin linker chains in a approximately 2:1 mass ratio found in annelids and related species. Studies of the dissociation and reassembly of Lumbricus terrestris Hb have provided ample evidence for the presence of a approximately 200-kDa linker-free subassembly consisting of monomer (M) and disulfide-bonded trimer (T) subunits. Electrospray ionization mass spectrometry (ESI-MS) of the subassemblies obtained by gel filtration of partially dissociated L. terrestris and Arenicola marina Hbs showed the presence of noncovalent complexes of M and T subunits with masses in the 213. 3-215.4 and 204.6-205.6 kDa ranges, respectively. The observed mass of the L. terrestris subassembly decreased linearly with an increase in de-clustering voltage from approximately 215,400 Da at 60 V to approximately 213,300 Da at 200 V. In contrast, the mass of the A. marina complex decreased linearly from 60 to 120 V and reached an asymptote at approximately 204,600 Da (180-200 V). The decrease in mass was probably due to the progressive removal of complexed water and alkali metal cations. ESI-MS at an acidic pH showed both subassemblies to consist of only M and T subunits, and the experimental masses demonstrated them to have the composition M(3)T(3). Because there are three isoforms of M and four isoforms of T in Lumbricus and two isoforms of M and 5 isoforms of T in Arenicola, the masses of the M(3)T(3) subassemblies are not unique. A random assembly model was used to calculate the mass distributions of the subassemblies, using the known ESI-MS masses and relative intensities of the M and T subunit isforms. The expected mass of randomly assembled subassemblies was 213,436 Da for Lumbricus Hb and 204,342 Da for Arenicola Hb, in good agreement with the experimental values.

A re-evaluation of the molecular mass of earthworm extracellular hemoglobin from meniscus depletion sedimentation equilibrium. Nature of the 10 S dissociation species

Previous calculations from meniscus depletion sedimentation equilibrium earthworm hemoglobin from Lumbricus terrestris (E.J. Wood et al., Biochem. J. 153 (1976) 589-96) and from the related species Lumbricus sp. (L. sp.) (M.M. David and E. D Mol. Biol. 87 (1974) 89--101) were made on the assumption that the solutions behaved ideally. Re-examination of their results reveals, however, a dependence of the apparent molecular mass on concentration. Taking this effect into consideration, we have nowrecalculated from their data molecular masses of 4.4--4.5 MDa for the hemoglobin of both L. terrestris and L. sp. On the basis of the new determinations, we propose for the polypeptide chain composition of L. terrestris hemoglobin a model [(abcd )4L1L2L3]12 where a,b,c,d are the four globin and L1,L2,L3 are the three major linker chain constituents of the protein. The model is consistent with the D6 symmetry of the molecule. A 10 S intermediate product in the alkaline dissociation Lumbricus hemoglobin is viewed as a binary mixture of products resulting from a disproportionation reaction involving the structural unit. The present interpretation is shown to be consistent with observed relations between molecular masses and SDS gel electrophoretic band patterns of 10 S species and intact hemoglobin.

A two-exposure technique for ice-embedded samples successfully reconstructs the chlorocruorin pigment of Sabella spallanzanii at 2. 1 Nm resolution

A technique for reconstructing ice-embedded macromolecules from electron micrographs taken at two specimen tilts (+/-23 degrees ) has been used to determine the structure of chlorocruorin isolated from the Polychaete annelid Sabella spallanzanii. Images of individual molecules were extracted in couples from two micrographs of the same field of view so each couple consists of two projections of the same molecule. One couple was used as a fixed reference for alignment. Different references yielded reconstructions with different orientations. These were merged to give a model against which the orientation of 1624 first-exposure images was refined to give a final reconstruction at 2.1 nm resolution. The structure of this hematic pigment, essentially the same as that for Lumbricus terrestris, is a bilayer structure with overall symmetry D6, containing six hollow groups per layer. A hollow group is formed by six globular masses and has approximate threefold symmetry. Other structural elements connect the two layers and the hollow groups in a layer. This non-globin material occupies about 15% of the total molecular volume. The results show that the double-exposure strategy, previously described by some of the authors and tested in computer simulations, performs well in real experiments and could be used to obtain preliminary reconstructions in a semiautomatic way.

Analysis of a 3.6-MDa hexagonal bilayer hemoglobin from Lumbricus terrestris using a gas-phase electrophoretic mobility molecular analyzer

The recent successful use of electrospray gas-phase electrophoretic mobility molecular analysis (GEMMA) to separate globular proteins (mass 6 to 670 kDa) and the excellent correlation found between the electrophoretic mobility diameter (EMD), or Millikan diameter, and the protein mass (S. L. Kaufman et al., 1996, Anal. Chem. 68, 1895-1904; 1996, Anal. Chem. 68, 3703), prompted the examination of a large protein complex, the 3.6-MDa, heteromultimeric, hexagonal bilayer hemoglobin (Hb) and its subunits from the earthworm Lumbricus terrestris. The native Hb had an EMD of 25.7 nm and the products of its dissociation at pH >8 and <5 were resolved into peaks with EMDs of 10.5, 6.3, 5.0, and 4.2 nm, identified as a dodecamer of globin chains ([a+b+c]3d3, 213 kDa), the disulfide-bonded trimer of globin chains ([a+b+c], 52.7 kDa), all the linker chains (L1, 27.5 kDa; L2, 32.1 kDa; L3, 24.9 kDa; L4, 24. 1 kDa), and the monomer subunit (chain d, 17 kDa), respectively. Reassembly of the Hb complex was observed on restoring the pH from >8 to 7. The EMDs and the masses of the Hb and its subunits are in excellent agreement with the correlation found earlier, under the assumption of nearly spherical shape with an effective density around 0.7 g/cm3. GEMMA also provided a profile of the Hb completely dissociated in 0.1% SDS; its deconvolution permitted a quantitative determination of the subunit stoichiometry, providing a globin to linker ratio of 3 to 1.

Assembly of the gigantic hemoglobin of the earthworm Lumbricus terrestris. Roles of subunit equilibria, non-globin linker chains, and valence of the heme iron

The extracellular hemoglobin of the earthworm Lumbricus terrestris has four major kinds of O2-binding chains: a, b, and c (forming a disulfide-linked trimer), and chain d. Non-heme, non-globin structural chains, "linkers," are also present. Light-scattering techniques have been used to show that the ferrous CO-saturated abc trimer and chain d form an (abcd)4 complex of 285 kDa at neutral pH. Formation of the full-sized 4-MDa molecule requires the addition of linker chains in the proportion of two linkers per (abcd)4 and occurs much more rapidly in the presence of 10 mM calcium. This stoichiometry is supported not only by direct quantitative analysis of the intact hemoglobin but also by the fact that the addition of 50% of the proposed stoichiometric quantity of linkers results in the conversion of 50% of the (abcd)4 to full-sized molecules. Isolated CO-saturated abc trimers self-associate to (abc)2 and higher aggregates up to an apparent limit of (abc)10 approximately 550 kDa. The CO-saturated chain d forms dimers, (d)2, and tetramers, (d)4. Oxidation of the (abcd)4 complex with ferricyanide causes complete dissociation of chain d from the abc trimer, but addition of CN- maintains the (abcd)4 complex. Valence hybrids have also been studied. The ferrous CO-saturated abc trimer and met (ferric) chain d also associate to form (abcd)4, but the met abc trimer and ferrous CO-saturated chain d do not. Oxidation of the abc trimer and chain d to the ferric form causes the formation of a characteristic hemichrome spectrum with a maximum at 565 nm and a shoulder near 530 nm. These results show that interactions between the abc trimer and chain d are strongly dependent on the ligand and valence state of the heme iron. Light-scattering measurements reveal that oxidation of the intact Hb produces a significant drop in molecular mass from 4.1 to 3.6 MDa. Inclusion of CN- prevents this drop. These experiments indicate that oxidation causes the Hb to shed subunits. The observations provide an explanation for the wide variations in the molecular mass of L. terrestris Hb that have been observed previously.