The extracellular hemoglobin of the earthworm, Lumbricus terrestris. Determination of subunit stoichiometry

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.

Initial biophysical characterization of Amynthas gracilis giant extracellular hemoglobin (HbAg)

The aim of the present work was the biophysical characterization of the Amynthas gracilis hemoglobin (HbAg). The oxy-HbAg optical absorption data, with Soret and Q bands centered at 415, 540 and 575 nm, were stable and unchanged at pH 7.0. An increase in pH promotes decrease in the intensity in the optical absorption bands, suggesting an oligomeric dissociation and partial oxidation. Identical stability at pH 7.0 was observed in DLS results that presented a hydrodynamic diameter of 28 nm, characteristic of the whole oligomer. DLS shows that HbAg undergoes oligomeric dissociation and an aggregation/denaturation process that corroborates spectroscopic data. Our results showed that the monomer d presents four isoforms with molecular mass (MM) ranging from 16,244 to 16,855 Da; the trimer subunit presents two isoforms, (abc)₁ and (abc)₂, with MM of 51,415 ± 20 Da and 51,610 ± 14 Da, respectively, and a less intense species, at 67,793 Da, assigned to the tetramer abcd. Monomeric chains a, obtained from reduction of the disulfide-bonded trimer abc, present four isoforms with MM 17,015 Da, 17,061 Da, 17,138 Da and 17,259 Da. DLS and LSI revealed an isoeletric point (pI) of oxy-HbAg of 6.0 ± 0.3 and 5.5, respectively. Data analysis by IEF-SDS-PAGE revealed that the pI of oxy-HbAg is 6.11, correlating with DLS and LSI data. These studies indicate that oxy-HbAg is very stable, at pH 7.0, and has differing properties from orthologous giant hemoglobins.

The structure of the giant haemoglobin from Glossoscolex paulistus

The sequences of all seven polypeptide chains from the giant haemoglobin of the free-living earthworm Glossoscolex paulistus (HbGp) are reported together with the three-dimensional structure of the 3.6 MDa complex which they form. The refinement of the full particle, which has been solved at 3.2 Å resolution, the highest resolution reported to date for a hexagonal bilayer haemoglobin composed of 12 protomers, is reported. This has allowed a more detailed description of the contacts between subunits which are essential for particle stability. Interpretation of features in the electron-density maps suggests the presence of metal-binding sites (probably Zn(2+) and Ca(2+)) and glycosylation sites, some of which have not been reported previously. The former appear to be important for the integrity of the particle. The crystal structure of the isolated d chain (d-HbGp) at 2.1 Å resolution shows different interchain contacts between d monomers compared with those observed in the full particle. Instead of forming trimers, as seen in the complex, the isolated d chains associate to form dimers across a crystallographic twofold axis. These observations eliminate the possibility that trimers form spontaneously in solution as intermediates during the formation of the dodecameric globin cap and contribute to understanding of the possible ways in which the particle self-assembles.

Evaluating the capacity to generate and preserve nitric oxide bioactivity in highly purified earthworm erythrocruorin: a giant polymeric hemoglobin with potential blood substitute properties

The giant extracellular hemoglobin (erythrocruorin) from the earth worm (Lumbricus terrestris) has shown promise as a potential hemoglobin-based oxygen carrier (HBOC) in in vivo animal studies. An important beneficial characteristic of this hemoglobin (LtHb) is the large number of heme-based oxygen transport sites that helps overcome issues of osmotic stress when attempting to provide enough material for efficient oxygen delivery. A potentially important additional property is the capacity of the HBOC either to generate nitric oxide (NO) or to preserve NO bioactivity to compensate for decreased levels of NO in the circulation. The present study compares the NO-generating and NO bioactivity-preserving capability of LtHb with that of human adult hemoglobin (HbA) through several reactions including the nitrite reductase, reductive nitrosylation, and still controversial nitrite anhydrase reactions. An assignment of a heme-bound dinitrogen trioxide as the stable intermediate associated with the nitrite anhydrase reaction in both LtHb and HbA is supported based on functional and EPR spectroscopic studies. The role of the redox potential as a factor contributing to the NO-generating activity of these two proteins is evaluated. The results show that LtHb undergoes the same reactions as HbA and that the reduced efficacy for these reactions for LtHb relative to HbA is consistent with the much higher redox potential of LtHb. Evidence of functional heterogeneity in LtHb is explained in terms of the large difference in the redox potential of the isolated subunits.

The self-association of the giant hemoglobin from the earthworm, Lumbricus terrestris

BACKGROUND

The crystallographic structure of the gigantic hemoglobin (erythrocruorin) of the annelid worm, Lumbricus terrestris, provides a molar mass of 3.6MDa for the hexagonal bilayer structure. Prior to this determination, some light-scattering and ultracentrifugal measurements indicated higher masses: 4.1-4.4MDa. Values of 3.6MDa were attributed to dissociation or subunit loss. However, early electron microscopy of the giant hemoglobin from a related annelid, Eumenia crassa by Öster Levin, showed that the hexagonal bilayer molecules were present mostly as oligomers; few were monomeric.

METHODS

Measurements by light-scattering of solutions of Lumbricus hemoglobin resolved by size-exclusion chromatography have been used to determine the weight-average molar mass of self-associating proteins. The X-ray structure has been re-examined.

RESULTS

Our measurements show that both 3.6MDa monomers and self-association products are present as a mixture. Analysis of the X-ray structure indicates several different kinds of monomer-monomer interactions.

CONCLUSIONS

We propose that the measured masses of Lumbricus hemoglobin as high as 4.4MDa, result from oligomerization. These masses would result from the presence of an array of oligomers of various sizes together with monomers of 3.6MDa. Furthermore, several different kinds of monomer-monomer interactions are clearly evident in the X-ray structure as well as in solution.

GENERAL SIGNIFICANCE

The results demonstrate that self-association of monomers of the hemoglobin of Lumbricus terrestris explains the high molar masses of 4.1-4.4MDa previously observed.

Partial characterization of giant extracellular hemoglobin of Glossoscolex paulistus: A MALDI-TOF-MS study

In this work, MALDI-TOF-MS analysis was performed to obtain information on the molecular mass of the different subunits from the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp) in the oxy-form. Experiments were performed for the whole protein at pH 7.0, for the partially dissociated protein at pH 9.0, and for the fraction obtained from gel filtration in Sephadex G-200, at pH 9.0, corresponding to the isolated monomer d. Besides that, experiments were performed for the whole protein treated with 2-mercaptoethanol in order to monitor the effects of reduction of the disulfide bonds, which are expected to maintain the trimer (abc) in the native molecule. The results are compared to those reported for the homologous hemoglobin of Lumbricus terrestris (HbLt) and some tentative assignments are made for the observed polypeptides. The monomer d is found to exist in, at least, two major forms of identical proportions with masses of 16,355+/-25 and 16,428+/-24 Da, respectively. Two minor forms were also observed around 16 kDa for the monomers. Upon disulfide bonds reduction the peak associated to the trimer is absent in the mass spectrum, and new peaks assigned tentatively to the monomers a, b and c on the basis of comparison with Lumbricus terrestris hemoglobin literature data are observed. Their molecular masses were 18,258+/-30, 16,492+/-24 and 17,363+/-17 Da, respectively. Two linker chains for HbGp were also observed at 25,817+/-50 and 26,761+/-16 Da, and this result is different from HbLt, where four linker chains were reported in the range 24-32 kDa. Finally, trimers (abc) were observed at 51-52 kDa. This partial characterization, performed for the first time, is an important step in the characterization of subunits of this giant extracellular hemoglobin.

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.

Small angle X-ray scattering studies and modeling of Eudistylia vancouverii chlorocruorin and Macrobdella decora hemoglobin

Annelids possess giant extracellular oxygen carriers that exhibit a hexagonal bilayer appearance and have molecular masses of approximately 3.5 MDa. By small angle x-ray scattering (SAXS), Eudistylia vancouverii chlorocruorin and Macrobdella decora hemoglobin were investigated in solution. On the basis of the experimental SAXS data, three-dimensional models were established in a two-step approach (trial and error and averaging). The main differences between the complexes concern the structure of their central part and the subunit architecture. Usage of our SAXS models as templates for automated model generation (program DAMMIN) led to refined models that fit perfectly the experimental data. Special attention was paid to the inhomogeneous density distribution observed within the complexes. DAMMIN models without a priori information could not reproducibly locate low-density areas. The usage of templates, however, improved the results considerably, in particular by applying electron microscopy-based templates. Biologically relevant information on the presence of low-density areas and hints for their presumable location could be drawn from SAXS and sophisticated modeling approaches. Provided that different models are analyzed carefully, this obviously opens a way to gain additional biologically relevant structural information from SAXS data.

Determination of the formal reduction potential of Lumbricus terrestris hemoglobin using thin layer spectroelectrochemistry

The formal reduction potential (Eo') of Lumbricus terrestris hemoglobin was determined using thin layer spectroelectrochemistry as 0.073 (+/-0.005) V vs Ag/AgCl (0.281 V vs SHE, standard hydrogen electrode). Nernst plots of Lumbricus terrestris hemoglobin with tris-bipyridinecobalt(II) as a mediator titrant have similar linear slopes as Nernst plots of horse heart myoglobin with hexaamineruthenium(II) as a mediator titrant.

On the molecular mass of Lumbricus erythrocruorin

A critical examination of the published molecular mass of erythrocruorin (Ec) from Lumbricus and related earthworm species reveals that 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 from the Vinogradov model for the hexagonal bilayer (HBL) assembly of Lumbricus terrestris EC and as estimated from the crystal structure of HBL at 5.5 A resolution. The other cluster corresponds to approximately 4.4 MDa. However, in contrast to the controversy over the molecular mass, there is a consensus that the sedimentation coefficient of intact L. terrestris Ec is approximately 60S. Drawing on the occurrence of central subunits in Ec of Oenone fulgida and few other annelid species, we propose for the 4.4 MDa molecule a model of HBL supplemented by a central mass. The proposed model abides by D6 symmetry and is a suitable candidate to represent 60S Lumbricus terrestris Ec.

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).

Mass distributions of a macromolecular assembly based on electrospray ionization mass spectrometric masses of the constituent subunits

Macromolecular assemblies containing multiple protein subunits and having masses in the megadalton (MDa) range are involved in most of the functions of a living cell. Because of variation in the number and masses of subunits, macromolecular assemblies do not have a unique mass, but rather a mass distribution. The giant extracellular erythrocruorins (Ers), approximately 3.5 MDa, comprised of at least 180 polypeptide chains, are one of the best characterized assemblies. Three-dimensional reconstructions from cryoelectron microscopic images show them to be hexagonal bilayer complexes of 12 subassemblies, each comprised of 12 globin chains, anchored to a subassembly of 36 nonglobin linker chains. We have calculated the most probable mass distributions for Lumbricus and Riftia assemblies and their globin and linker subassemblies, based on the Lumbricus Er stoichiometry and using accurate subunit masses obtained by electrospray ionization mass spectrometry. The expected masses of Lumbricus and Riftia Ers are 3.517 MDa and 3.284 MDa, respectively, with a possible variation of approximately 9% due to the breadth of the mass distributions. The Lumbricus Er mass is in astonishingly good agreement with the mean of 23 known masses, 3.524 +/- 0.481 MDa.

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.

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.

Lumbricus terrestris hemoglobin--the architecture of linker chains and structural variation of the central toroid

The extracellular giant hemoglobin from the earthworm Lumbricus terrestris was reconstructed at 14.9-A resolution from cryo-electron microscope images, using a new procedure for estimating parameters of the contrast transfer (CTF) function. In this approach, two important CTF parameters, defocus and amplitude contrast ratio, can be refined iteratively within the framework of 3D projection alignment procedure, using minimization of sign disagreement between theoretical CTF and cross-resolution curves. The 3D cryo-EM map is in overall good agreement with the recent X-ray crystallography map of Royer et al. (2000, Proc. Natl. Acad. Sci. USA 97, 7107-7111), and it reveals the local threefold arrangement of the three linker chains present within each 1/12 of the complex. The 144 globin chains and 36 linker chains within the complex are clearly visible, and the interdigitation of the 12 coiled-coil helical spokes forming the central toroidal piece is confirmed. Based on these findings, two mechanisms of the dodecameric unit assembly are proposed and termed "zigzag" and "pairwise" polymerizations. However, the detection by cryo-EM of 12 additional rod-like bodies within the toroid raises the possibility that the architecture of the toroid is more complex than previously thought or that yet unknown ligands or allosteric effectors for this oxygen carrier are present.

Occurrence of two architectural types of hexagonal bilayer hemoglobin in annelids: comparison of 3D reconstruction volumes of Arenicola marina and Lumbricus terrestris hemoglobins

A 3D reconstruction at 25 A resolution of native hemoglobin of the polychaete worm Arenicola marina was carried out from frozen-hydrated specimens examined in the electron microscope. The reconstruction volume of this large extracellular multimeric respiratory pigment appears as a hexagonal bilayer structure with eclipsed vertices in its upper and lower hexagonal layers. Conversely, in hemoglobins of oligochaetes, achaetes, and vestimentiferans and in chlorocruorins of the Sabellidae (polychaete) family, the vertices of the upper layer are 16 degrees clockwise rotated with respect to those of the lower layer. The fact that two other polychaete hemoglobins (Alvinella pompejana and Tylorrhynchus heterochaetus) have the same architecture as Arenicola led us to define two types of hexagonal bilayer hemoglobins/chlorocruorins: (i) type-I present in oligochaete, achaete, and vestimentiferan hemoglobins and in Sabellidae chlorocruorins; and (ii) type-II present in polychaete hemoglobins. A comparative study of the hemoglobins of Lumbricus terrestris (type-I) and Arenicola marina (type-II) showed that only two small differences located in the c4 and c5 linking units are responsible of the important architectural difference present in oligomers. A likely scheme proposed to explain the phylogenic distribution of the two types suggests that Clitellata, Sabellida (polychaete), and vestimentiferan hemoglobins and chlorocruorins derive from a type-I ancestral molecule, while Terebellida (Alvinella), Phyllodocida (Tylorrhynchus), and Scolecida (Arenicola) and possibly other polychaetes derive from an ancestor molecule with type-II hemoglobin. The architectures of the hollow globular substructures are highly similar in Arenicola and Lumbricus hemoglobins, with 12 globin chains and three linking units (c3a, c3b, and c4). The central piece of Arenicola hemoglobin is an ellipsoid while that of Lumbricus is a toroid. No phylogenic correlation could be found between the structure of the central pieces and the architecture type.

Reassembly of Lumbricus terrestris hemoglobin: a study by matrix-assisted laser desorption/ionization mass spectrometry and 3D reconstruction from frozen-hydrated specimens

Dodecamers and four types of linker chains (L1-L4) were purified from dissociated hemoglobin of the earthworm Lumbricus terrestris. Various preparations comprising dodecamer of globin chains and linker chains were allowed to reassemble at neutral pH. They produced various oligomers that were purified by gel filtration, analyzed in matrix-assisted laser desorption/ionization mass spectrometry and submitted to 3D reconstruction from isolated particles observed in cryoelectron microscopy. Despite the impossibility to completely free the L2, L3, and L4 preparations from L1, the following conclusions were obtained. First, hemoglobin molecules indistinguishable from native hemoglobin at 25 A resolution were obtained in the absence of linker chains L2, L3, or L4. Second, the 3D reconstruction volumes of reassembled hemoglobins containing dodecamers and L1+L3 or dodecamers and L1+L4 demonstrate that reassembly of native-like structures can be obtained from at most two linker chains and dodecamers. Third, the 3D reconstruction volumes of native and reassembled hemoglobins containing dodecamers and (1) L1, L2, and L4, (2) L1, L3, and L4, (3) L1 and L4, and (4) L1 and L3 were highly similar. Since these structures comprise two types of substructures (one involved in the c3a, c3b, and c4 linking units of the hollow globular substructure and the other in the c5 connection and the toroid), it seems highly probable that the minimal number of linker chains required to reassemble native-like hemoglobin is at most two.

Matrix-assisted laser desorption/ionization mass spectrometry of carbohydrates

This review describes the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to carbohydrate analysis and covers the period 1991-1998. The technique is particularly valuable for carbohydrates because it enables underivatised, as well as derivatised compounds to be examined. The various MALDI matrices that have been used for carbohydrate analysis are described, and the use of derivatization for improving mass spectral detection limits is also discussed. Methods for sample preparation and for extracting carbohydrates from biological media prior to mass spectrometric analysis are compared with emphasis on highly sensitive mass spectrometric methods. Quantitative aspects of MALDI are covered with respect to the relationship between signal strength and both mass and compound structure. The value of mass measurements by MALDI to provide a carbohydrate composition is stressed, together with the ability of the technique to provide fragmentation spectra. The use of in-source and post-source decay and collision-induced fragmentation in this context is described with emphasis on ions that provide information on the linkage and branching patterns of carbohydrates. The use of MALDI mass spectrometry, linked with exoglycosidase sequencing, is described for N-linked glycans derived from glycoproteins, and methods for the analysis of O-linked glycans are also covered. The review ends with a description of various applications of the technique to carbohydrates found as constituents of glycoproteins, bacterial glycolipids, sphingolipids, and glycolipid anchors.

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.

Three-dimensional reconstruction of Lumbricus terrestris hemoglobin at 22 A resolution: intramolecular localization of the globin and linker chains

A 3D reconstruction of the hemoglobin (Hb) of the earthworm Lumbricus terrestris was carried out by the 3D projection alignment method from electron microscopy images of a frozen-hydrated specimen at 22 A resolution. The results were analyzed by a new approach taking into account the evolution of the 210 densities forming the 3D volume as a function of the threshold of surface representation. The whole oligomer with D6point-group symmetry is comprised of 12 hollow globular substructures (HGS) with local 3-fold symmetry tethered to a complex network of linking subunits (linker complex). The 12 globin subunits of each HGS are distributed around local 3-fold axis in four layers of three subunits. The first layer, the most external, contains monomeric globin chains 2A, 3A, and 5A. The three trimers corresponding to the nine remaining subunits have one subunit in each of the second (2B, 3B, 5B), third (1A, 4A, 6A), and fourth (1B, 4B, 6B) layer. The distances between the centers of the globin chains forming the trimers are in the ranges 20-32 A and 45-52 A. The linker complex is made up of two types of linking units. The first type forms three loops connecting globin chains of the second, third and fourth layers. The average molecular mass (Mm) of these subunits was 25 kDa. The second type forms the central structure, termed hexagonal toroid, and its 12 connections to the HGS. This structure corresponds to a hexamer of a single linking unit with a Mm (31.2 kDa), size and a shape different from those of the HGS loops. A careful study of 3D volume architecture shows that each toroid linking unit is bound to the three loops of a HGS pair located in the upper and lower hexagonal layers, respectively. As shown in a model of architecture, hexagonal bilayered (HBL) Hbs can be built very simply from 144 globin chains and 42 linker chains belonging to two different types. We also propose a simple assembly sequence for the construction of HBL Hbs based on the architecture model.

Hexagonal bilayer structuring activity of linker chains of an annelid giant hemoglobin from the polychaete Perinereis aibuhitensis

Preferential activity of linker chains to clamp submultiples to form hexagonal bilayer (HBL) assembly of the multisubunit hemoglobin (Hb) of the polychaete Perinereis aibuhitensis (approximately 3.4 MDa) was demonstrated. To understand the HBL assembly that should rely on structuring activity of each subunit, reassociation in response to combining isolated subunits was monitored using gel filtration, SDS-PAGE, and transmission electron microscopy. The isolation of each subunit L, T, and M (L, linker chains; T, disulfide-bonded trimer A-b-B; M, monomeric chain a) of Perinereis Hb was made simply by exposing Hb to pH 10.5, where Hb was completely dissociated into its subunits L, T, and M. As a result, it was concluded that (i) subunits T and M have strong affinity to form an intermediate complex, submultiple D, which is a dodecamer of globin chains, 3[a. A-b-B], (ii) addition of subunit L to submultiple D brings about the formation of whole molecule, similarly (iii) addition of subunit M to T+L forms the whole molecule, and (iv) addition of subunit T to M+L brings about the formation of the whole molecule, too. The results obtained lead us to conclude that linkers do function to clamp 12 submultiples D up to a whole molecule at the final step of formation of Perinereis Hb. In summary, linkers appeared to have high affinity for submultiple D, a little affinity for subunit T, but no affinity for subunit M at all. Thus linker chains were demonstrated to preferentially clamp submultiples D together to form the HBL disc of the whole molecule.

Identification of myoglobin in human smooth muscle

Myoglobin (Mb) has been believed to be absent generally from mammalian smooth muscle tissue. Examination of human rectal, uterine, bladder, colon, small intestine, arterial, and venous smooth muscle by immunohistochemical techniques shows that each of these tissues is immunopositive for both smooth muscle myosin and human Mb. Mb-specific primers were used for the polymerase chain reaction to generate cDNA from smooth muscle tissues. Southern hybridization with a Mb-specific probe gave a very strong signal with the cDNA from rectum, weaker signals from small intestine and uterus, a faint signal from colon, and no signal from bladder tissue. High performance liquid chromatography analysis coupled with sequence determination has shown that contaminating heme-binding serum albumin as well as hemoglobin in extracts of smooth muscle seriously compromise any heme-based or spectrophotometric assay of Mb. Combined affinity and size exclusion chromatography, however, provide the necessary resolution. The cDNA-derived amino acid sequence of human smooth muscle Mb was found to be identical to that of Mb from striated muscle.

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.

Lumbricus terrestris hemoglobin: a comparison of small-angle x-ray scattering and cryoelectron microscopy data

The quaternary structure of Lumbricus terrestris hemoglobin was investigated by small-angle x-ray scattering (SAXS). Based on the SAXS data from several independent experiments, a three-dimensional (3D) consensus model was established to simulate the solution structure of this complex protein at low resolution (about 3 nm) and to yield the particle dimensions. The model is built up from a large number of small spheres of different weights, a result of the two-step procedure used to calculate the SAXS model. It accounts for the arrangement of 12 subunits in a hexagonal bilayer structure and for an additional central unit of clylinder-like shape. This model provides an excellent fit of the experimental scattering curve of the protein up to h = 1 nm-1 and a nearly perfect fit of the experimental distance distribution function p(r) in the whole range. Scattering curves and p(r) functions were also calculated for low-resolution models based on 3D reconstructions obtained by cryoelectron microscopy (EM). The calculated functions of these models also provide a very good fit of the experimental scattering curve (even at h > 1 nm-1) and p(r) function, if hydration is taken into account and the original model coordinates are slightly rescaled. The comparison of models reveals that both the SAXS-based and the EM-based model lead to a similar simulation of the protein structure and to similar particle dimensions. The essential differences between the models concern the hexagonal bilayer arrangement (eclipsed in the SAXS model, one layer slightly rotated in the EM model), and the mass distribution, mainly on the surface and in the central part of the protein complex.

A molecular model for the d chain of the giant haemoglobin from Lumbricus terrestris and its implications for subunit assembly

A structural model for the monomeric d chain of the giant haemoglobin from Lumbricus terrestris is described. Based on the crystal structures of other globins, the model provides evidence for the existence of a novel tryptophan-haem interaction. The observation that all three tryptophans are buried within the hydrophobic core is consistent with fluorescence data on the isolated monomer and the intact molecule. The model has also been used to predict the probable arrangement of the abcd tetramer as being similar to that observed in the clam Hb II structure. Such predictions allow the identification of four residues of particular importance in stabilising one of the subunit-subunit interfaces: Arg48, Arg97, His89 and Gln93. The latter two may be of special importance in the mediation of cooperative effects within the tetramer and indeed the intact molecule.

Investigation by electrospray ionization mass spectrometry of the extracellular hemoglobin from the polychaete annelid Alvinella pompejana: an unusual hexagonal bilayer hemoglobin

Alvinella pompejana inhabits deep-sea hydrothermal vent sites along the East-Pacific Rise, where it colonizes the walls of actively venting high-temperature chimneys. This worm is the most thermophilic metazoan known to date. In Alvinella, as in other alvinellids, oxygen transport is mainly achieved by an extracellular Hb dissolved in the vascular blood. This Hb has a molecular mass of 3833 +/- 14 kDa as revealed by multiangle laser light scattering (MALLS). Native and derivative Hb (reduced, carbamidomethylated, and deglycosylated) were analyzed by electrospray ionization mass spectrometry (ESI-MS). The data were processed by the maximum entropy deconvolution system (MaxEnt). We identified three groups of peaks for Alvinella Hb, at ca. 16, 23-26, and 50 kDa corresponding to (i) four monomeric globin chains, a1 (16 633.4), a2(16 532.4), a3 (16 419.6), and a4(16 348.9); (ii) four linker chains, L1-L4 (22 887. 1, 24 230.5, 26 233.6, and 25 974.4); and (iii) one disulfide-bonded trimer T (51 431.9) composed of globin chains b (16 477.5), c (16 916.1), and d (18 048.8). These Hbs were also subjected to SDS-PAGE analysis for comparative purposes. In addition, using the ESI-MS data we propose two alternative models for the quaternary structure of Alvinella's Hb.

A first evaluation of the natural high molecular weight polymeric Lumbricus terrestris hemoglobin as an oxygen carrier

Lumbricus terrestris hemoglobin (LtHb), an unusually stable Hb (MW approximately 4x10(6) Da) with respect to dissociation and oxidation, circulates extracellularly in the earthworm and at neutral pH exhibits oxygen affinity and cooperativity similar to that of human HbA. Results suggest that LtHb may serve as a model for a high molecular weight extracellular oxygen carrier. Mice and a rat model partially exchanged with LtHb showed no apparent behavioral and physical changes. 31P NMR spectroscopy of perfused guinea pig hearts, used to assess phosphocreatine levels as an indication of the ability of LtHb to serve as an oxygen carrier to the heart, demonstrated that LtHb provides oxygen to the tissue and maintains the energy metabolism significantly better than the control non-Hb perfusion media. One day after infusion, video enhanced microscopy imaging of the mice cremaster muscle vasculature reveals temporal adhesion of leukocytes to the endothelial walls with temporal infiltration of leukocytes to the surrounding tissue, correlated with dosage. Exchanged mice rechallenged with LtHb show no overt allergic response or death. Further evaluation of this natural extracellular Hb as a potential polymeric Hb blood substitute/perfusion agent is warranted.

Three-dimensional reconstruction of native and reassembled Lumbricus terrestris extracellular hemoglobin. Localization of the monomeric globin chains

The approximately 3.5 MDa hexagonal bilayer (HBL) hemoglobin (Hb) of the earthworm Lumbricus terrestris is composed of monomers and disulfide-bonded trimers (T) of globin chains and of four types of heme-deficient linker chains (L). Cryoelectron microscopic images of native Hb and of HBL reassembled from the constituent subunits depleted in monomer subunit (HBL[T+L]) were subjected to three-dimensional reconstructions by the random conical tilt series method. Native Hb has an architecture very similar to those of other annelid and vestimentiferan Hbs, consisting of 12 hollow globular substructures (HGS). Each HGS is comprised of six dense masses, has a 3-fold symmetry, and is organized in two hexagonally symmetric layers, with the vertices of the upper layer rotated 16 degrees clockwise relative to those of the lower layer. The layers are tethered to a central linker complex, consisting of two bracelets of connections perpendicular to the 6-fold axis and a set of six vertical connections linked to a flat hexagonal mass. HBL[T+L] shared all these features with the native Hb, except for a large hole around the 3-fold symmetry axis in each HGS, indicating the probable location of the missing monomer subunit.

Spectroscopic studies of the met form of the extracellular hemoglobin from Glossoscolex paulistus

Sephadex G-200 chromatography of the extracellular hemoglobin from the giant earthworm G. paulistus in the met form presents a single peak at pH 7.0 and two peaks at pH 9.0 as a result of alkaline dissociation. SDS-PAGE shows that the polypeptide chains are very similar to those observed for the oxy form and the two peaks at pH 9.0 correspond to the trimer contaminated by linkers and monomers which seems to be quite pure. The aquomet acid form is stable as an oligomer of molecular mass 3.1 x 10(6) Da only in a narrow pH range around neutrality. Increasing the pH above 7.5 leads to an irreversible transition from aquomet to hemichrome I which is the low-spin bis-imidazole complex. At pHs above 9.5-10.0 a second reversible transition takes place from hemichrome I to hemichrome II, a high-spin complex which is associated with the weakening and possible disruption of the proximal Fe--N histidine bond. Thus, increase in pH above 8.0 induces changes in the heme pocket that involve both the distal and proximal sides of the heme. EPR measurements show a very sharp decrease of the aquomet high-spin signal in the range of pH 7.0-8.0 and a very small low-spin signal even at liquid helium temperatures. The transition to hemichrome I is also accompanied by the loss of heme optical activity monitored by CD, which is consistent with the weakening of heme--globin interaction. Hemichrome I in the presence of cyanide gives the typical cyanometHb derivative which has a transition to a hemichrome at much higher pHs. This observation suggests that the dissociation of the oligomer in alkaline medium as well as the stability of the heme on the proximal side, depend both upon the ligand present at the sixth coordination position on the distal side. Hence, we believe that hemi(hemo)chrome formation in G. paulistus Hb and other invertebrate hemoglobins is a common phenomenon, not associated with protein denaturation, which may provide a fine tuning mechanism to control subunit interactions through changes in the distal side of the heme pocket.

A comparative study on earthworm hemoglobins: an amino acid sequence comparison of monomer globin chains of two species, Pontodrilus matsushimensis and Pheretima communissima that belong to the family Megascolecidae

The monomer subunits of giant extracellular hemoglobins from earthworms Pontodrilus matsushimensis and Pheretima communissima that belong to the family Megascolecidae, Oligochaeta, were purified by a reversed-phase column, Resource RPC, and sequenced. The complete amino acid sequences of the two monomeric globin chains were determined: 141 amino acid residues with a molecular weight of 16,366 Da for Pontodrilus matsushimensis and 140 amino acid residues with a molecular weight of 16,000 for Pheretima communissima, respectively. The Pontodrilus matsushimensis monomer globin has three cysteine residues, and the two located at positions 2 and 131 are conserved as those observed in all annelids and contribute to form a disulfide-bonded interchain. The third cysteine residue at position 73 is the first evidence for the annelid monomer globin subunits. The physiological functions of the third cysteine residue, however, are still unknown. The monomer sequences of the two species were aligned with those of five known sequences from annelids, including a polychaete, Tylorrhynchus heterochaetus, and four oligochaetes, Pheretima hilgendorfi, Pheretima sieboldi, Lumbricus terrestris and Tubifex tubifex. Using computer analysis, a 87.9% identity of the amino acid sequences between two monomeric subunits of Pheretima communissima and Pheretima hilgendorfi hemoglobins showed the highest degree of sequence similarity. A molecular phylogenetic tree of seven species of annelids has constructed, suggesting that the divergence times among the three species of Pheretima and between Pheretima and Pontodrilus were 50 to 100 and about 209 million years ago, respectively.

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

The extracellular hemoglobin (Hb) of the earthworm, Lumbricus terrestris, has four major O2-binding chains, a, b, c (forming a disulfide-linked trimer), and d ("monomer"). Additional structural chains, "linkers," are required for the assembly of the approximately 200-polypeptide molecule. The proportion of linker chains had been reported to be one-third of the total mass on the basis of densitometry of Coomassie Blue-stained SDS-gels. Reverse-phase high performance liquid chromatography (HPLC), however, gave 16.3% linkers on the basis of both 220-nm absorbance and amino acid analysis (Ownby, D. W., Zhu, H., Schneider, K., Beavis, R. C., Chait, B. C., and Riggs, A. F. (1993) J. Biol. Chem. 268, 13539-13547). The subunit proportions have now been redetermined by SDS capillary electrophoresis as a test of the HPLC results. The electrophoresis, monitored at 214 nm, avoided the use of Coomassie Blue and provided results identical with those obtained by HPLC. Capillary electrophoresis monitored at both 214 and 415 nm was used to show that linker chains do not bind heme. Heme content has been found to be 2.9% by determination of hemin, amino acid analysis and dry weight. Measurement of the rate of hemin loss from oxidized L. terrestris Hb shows that high rates of loss can account for values of heme content significantly below 2.9% (or 0.26% iron).

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.

Three-dimensional reconstruction of the hexagonal bilayer hemoglobin of the hydrothermal vent tube worm Riftia pachyptila by cryoelectron microscopy

A frozen-hydrated specimen of the V1 hemoglobin of the hydrothermal vent tube worm Riftia pachyptila was observed in the electron microscope and subjected to three-dimensional reconstruction by the method of random conical tilt series. The 3D volume possesses a D6 point-group symmetry. When viewed along its 6-fold axis the vertices of its upper hexagonal layer are 16 degrees clockwise rotated compared to those of the lower layer. A central linker complex is decorated by 12 hollow globular substructures. The linker complex comprises (i) a central hexagonal toroid, (ii) two internal bracelets onto which the hollow globular substructures are built, and (iii) six structures connecting the two hexagonal layers. The hollow globular substructures, related to the dodecamers of globin chains resulting from the dissociation of the hexagonal bilayer hemoglobin, have a local pseudo 3-fold symmetry and are composed each of three elongated structures visible when the volume is displayed at high threshold. At a resolution of 36 A, the 3D volumes of the hexagonal bilayer hemoglobins of Riftia pachyptyla and of the leech Macrobdella decora look almost perfectly identical.

The role of the dodecamer subunit in the dissociation and reassembly of the hexagonal bilayer structure of Lumbricus terrestris hemoglobin

The dissociation of the approximately 3500-kDa hexagonal bilayer (HBL) hemoglobin (Hb) of Lumbricus terrestris upon exposure to Gdm salts, urea and the heteropolytungstates [SiW11O39]8- (SiW), [NaSb9W21O86]18- (SbW) and [BaAs4W40O140]27- (AsW) at neutral pH was followed by gel filtration, SDS-polyacrylamide gel electrophoresis, and scanning transmission electron microscopy. Elution curves were fitted to sums of exponentially modified gaussians to represent the peaks due to undissociated oxyHb, D (approximately 200 kDa), T+L (approximately 50 kDa), and M (approximately 25 kDa) (T = disulfide-bonded trimer of chains a c, M = chain d, and L = linker chains). OxyHb dissociation decreased in the order Gdm*SCN > Gdm.Cl > urea > Gdm.OAc and AsW > SbW > SiW. Scanning transmission electron microscopy mass mapping of D showed approximately 10-nm particles with masses of approximately 200 kDa, suggesting them to be dodecamers (a+b+c)3d3. OxyHb dissociations in urea and Gdm.Cl and at alkaline pH could be fitted only as sums of 3 exponentials. The time course of D was bell-shaped, indicating it was an intermediate. Dissociations in SiW and upon conversion to metHb showed only two phases. The kinetic heterogeneity may be due to oxyHb structural heterogeneity. Formation of D was spontaneous during HBL reassembly, which was minimal (</= 10%) without Group IIA cations. During reassembly, maximal (approximately 60%) at 10 mM cation, D occurs at constant levels (approximately 15%), implying the dodecamer to be an intermediate.

The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila. II. Complete polypeptide chain composition investigated by maximum entropy analysis of mass spectra

The deep-sea tube worm Riftia pachyptila Jones possesses a complex of three extracellular Hbs: two in the vascular compartment, V1 (approximately 3500 kDa) and V2 (approximately 400 kDa), and one in the coelomic cavity, C1 (approximately 400 kDa). These native Hbs, their dissociation products and derivatives were subjected to electrospray ionization mass spectrometry (ESI-MS). The data were analyzed by the maximum entropy deconvolution system. We identified three groups of peaks for V1 Hb, at approximately 16, 23 27, and 30 kDa, corresponding to (i) two monomeric globin chains, b (Mr 16,133.5) and c (Mr 16,805.9); (ii) four linker subunits, L1 L4 (Mr 23,505.2, 23,851.4, 26,342.4, and 27,425.8, respectively); and (iii) one disulfide-bonded dimer D1 (Mr 31,720.7) composed of globin chains d (Mr 15,578.5) and e (Mr 16, 148.3). V2 and C1 Hbs had no linkers and contained a glycosylated monomeric globin chain, a (Mr 15,933.4) and a second dimer D2 (Mr 32,511.7) composed of chains e and f (Mr 16,368.1). The dimer D1 was absent from C1 Hb, clearly differentiating V2 and C1 Hbs. These Hbs were also subjected to SDS-PAGE analysis for comparative purposes. The following models are proposed ((cD1)(bD1)3) for the one-twelfth protomer of V1 Hb, ((cD)(bD)6(aD)) (D corresponding to either D1 or D2) for V2 and C1 Hbs. HBL V1 Hb would be composed of 180 polypeptide chains with 144 globin chains and 36 linker chains, each twelfth being in contact with three linker subunits, providing a total molecular mass = 3285 kDa. V2 and C1 would be composed of 24 globin chains providing a total molecular mass = 403 kDa and 406 kDa, respectively. These results are in excellent agreement with experimental Mr determined by STEM mass mapping and MALLS.

Characterization of the constituent polypeptides of the extracellular hemoglobin from Lumbricus terrestris: heterogeneity and discovery of a new linker chain L4

The extracellular hemoglobin of Lumbricus terrestris comprises four oxygen binding chains, a, b, c, d, and three linker chains L1, L2, L3 as major components. A stoichiometry of the whole molecule has been proposed on the basis of these chains, with a total number of 216 chains: forty-eight chains of each oxygen binding chain and eight molecules of each linker chain. We have isolated additional minor components by HPLC and two-dimensional gel electrophoresis. The following biochemical characterizations have been made. (i) All components so far reported, the heme-containing chains a, b, c, d, and linker chains L1, L2, L3 and a new minor polypeptide, L4, were mapped on a two-dimensional gel. Their estimated isoelectric points were between 4.7 and 5.9. (ii) The sequences of several peptides including the unique N-terminal peptide from linker L4 show that it can be considered as a duplicated gene product with a similar mass. (iii) Chain d2 was isolated and found to correspond to the minor chain previously pointed out by Shishikura et al. (J. Biol. Chem. 262 (1987) 3123-3131). (iv) The major chain d1 has serine at position 7 from the N-terminus. This is not consistent with previously reported glycine (Shishikura et al., J. Biol. Chem. 262 (1987) 3123-3131).

Alteration of tryptophan fluorescence properties upon dissociation of Lumbricus terrestris hemoglobin

Fluorescence analysis has been used to study dissociation of the dodecameric 3.8 kDa Lumbricus terrestris hemoglobin. Since tryptophan intrinsic fluorescence has been used as a reporter group to study Lumbricus hemoglobin, it is of interest to study dissociation perturbed properties of the tryptophan residues. Shifts in the fluorescence emission maximum to longer wavelengths upon dissociation at pH 9.2 suggest that tryptophans buried at the subunit interface(s) become more exposed. Fluorescence lifetime and quenching studies are employed in this present investigation as a means to confirm the location of tryptophan residues at the subunit interfaces. Acrylamide titration (to 2.5 M) indicate only a fraction of the residues can be quenched at either pH. At pH 7.0, the Stern-Volmer plot has downward curvature, while at pH 9.2 there is slight upward curvature, again indicating a change in environment. The intrinsic fluorescence decay requires at least four exponentials at both pHs. The mean fluorescence lifetime of CO Lumbricus hemoglobin increases from 1.1 ns at pH 7 to 3.3 ns at pH 9.2. The lifetime data can be further interpreted as a decrease in the quenching of residues with a approximately 30 ps lifetime, and a concomitant increase in the longer lifetime components. This is consistent with interface tryptophans becoming exposed to solvent upon dissociation, and loss of quenching by intersubunit hemes. The overall results suggest that in the dodecamer, most of the tryptophans are located in a hydrophobic environment, not all of which are located at the subunit interface.