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).

An electrospray ionization mass spectrometric study of the subunit structure of the giant hemoglobin from the leech Nephelopsis oscura

The subunit structure of the giant, extracellular hexagonal bilayer (HBL) hemoglobin (Hb) from the leech Nephelopsis oscura was investigated by electrospray ionization mass spectrometry (ESI-MS) employing a maximum entropy deconvolution of its complex, multiply charged ESI spectra. The denatured unreduced Hb consisted of three monomer globin chains (M), a1 = 16535 Da, a2 = 17171 Da and a3 = 17315 Da, five nonglobin linker chains, L1 = 24512 Da, L2 = 24586 Da, L3 = 24979 Da, L4 = 25006 Da, and L5 = 25566 Da and two subunits of 32950 Da and 33125 Da. ESI-MS of the denatured, reduced Hb showed that the latter were disulfide-bonded heterodimers (D) of globin chains b1 = 16322 Da and b2 = 16499 Da with chain c = 16632 Da. Time-of-flight ESI-MS of the Hb at pH 3.8, 4.5, 5.0, 5.8 and 7.0 revealed a distribution of charge states from 32(+) to 37(+) with masses decreasing from 211 to 208.5 kDa with increase in cone voltage from 60 to 160 V, indicating the presence of a subassembly comprising 12 globin chains. The subunit composition 6M + 3D + 12h, where M = 16993 Da and D = 33004 Da are the weighted masses and h = 616.5 Da, provides a calculated mass, 208.37 kDa that is closest to 208.5 kDa. Our experimental findings are consistent with the bracelet model of HBL Hbs, verified by the recent low-resolution crystal structure of Lumbricus Hb, wherein an HBL arrangement of 12 globin dodecamer subassemblies is tethered to a central complex of 36 linker chains for a total mass of 208.37 x 12 + 24.94 x 36 = 3398 kDa.

[Effect of normobaric hypoxia and physical load on the functional indices of cardiorespiratory system in overweight people]

Changes in the cardiorespiratory function in obese persons exposed to different combinations of normobaric hypoxia and physical exercise were studied. The results show a positive effect of combined use of normobaric hypoxic exercise on functional reserves of cardiorespiratory system, physical performance and aerobic potentialities in obese patients. The highest effect is achieved in combination of physical exercise with hypoxic gas mixtures breathing.

Globin and linker sequences of the giant extracellular hemoglobin from the leech Macrobdella decora

A detailed electrospray ionization mass spectrometric study of the approximately 3.5-MDa hexagonal bilayer hemoglobin (HBL Hb) from the pond leech Macrobdella decora has shown it to consist of at least six approximately 17-kDa globin chains, of which two are monomeric and the remaining four occur as disulfide-bonded heterodimers, and three approximately 24-kDa nonglobin linker chains (Weber et al., J. Mol. Biol. 251: 703-720, 1995). The cDNA sequences of the five major constituent chains, globin chains IIA, IIB, B, and C and linker chain L1, are reported here. The globins and linkers share 30%-50% and 20%-30% identity, respectively, with other annelid sequences. Furthermore, IIB and C align with strain A of annelid sequences, whereas IIA and B align with the strain B sequences. Although chains B and C are monomeric, chains IIA and IIB form the main disulfide-bonded dimer. They also have some unusual features: the distal His (E7) is replaced by Phe in IIA, and the highly conserved CD1Phe is replaced by Leu in IIB. In spite of these unusual features, the functional properties of Macrobdella Hb are comparable to those of other HBL Hbs. A phylogenetic analysis of the globin sequences from Macrobdella, the polychaete Tylorrhynchus, the oligochaete Lumbricus, and the vestimentiferan Lamellibrachia, indicates that the two strains originated by gene duplication followed by additional duplication of each of the two strains. The mutation rate of the linkers appeared to be faster than that of the globin chains. The phylogenetic trees constructed using the Maximum Likelihood, Neighbor-Joining and Fitch methods showed the Macrobdella globin sequences to be closest to Lumbricus, in agreement with a view of annelid evolution in which the divergence of the polychaetes occurred before the divergence of the leeches from oligochaetes.

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.

Polypeptide chain composition diversity of hexagonal-bilayer haemoglobins within a single family of annelids, the alvinellidae

Following previous analysis of the structure of Alvinella pompejana heaxagonal-bilayer haemoglobin (HBL Hb) [1], we report in this paper the structure of three other HBL Hbs belonging to Alvinella caudata, Paralvinella grasslei and Paralvinella palmiformis, members of the Alvinellidae, annelid family strictly endemic to deep-sea hydrothermal vents located on the ridge crests in the Pacific ocean. The multi-angle laser light scattering (MALLS) and fast protein liquid chromatography (FPLC) analysis revealed a broad range of molecular masses for the extracellular Hb molecules, 3517 +/- 14 kDa (A. caudata), 3822 +/- 28 kDa (P. grasslei) and 3750 +/- 150 kDa (P. palmiformis). Native and derivative Hbs (reduced, carbamidomethylated and deglycosylated) were analysed by electrospray ionization mass spectroscopy (ESI-MS) and the data was processed by the maximum entropy deconvolution system (MaxEnt). The most important difference between alvinellid HBL Hbs was the variation in their composition, from two to four monomeric globin chains, and from one to four linker chains. Therefore, despite the fact that all these species belong to a single family, notable differences in the polypeptide chain composition of their HBL Hbs were observed, probably accounting for their different functional properties as previously reported by this group Toulmond, A., El Idrissi Slitine, F., De Frescheville, J. & Jouin, C. (1990) Biol. Bull. 179, 366-373.

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.

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.

Combinatorics of giant hexagonal bilayer hemoglobins

The paper discusses combinatorial and probabilistic models allowing to characterize various aspects of spacial symmetry and structural heterogeneity of the giant hexagonal bilayer hemoglobins (HBL Hb). Linker-dodecamer configurations of HBL are described for two and four linker types (occurring in the two most studied HBL Hb of Arenicola and Lumbricus, respectively), and the most probable configurations are found. It is shown that, for HBL with marked dodecamers, the number of 'normal-marked' pairs of dodecamers in homological position follows a binomial distribution. The group of symmetries of the dodecamer substructure of HBL is identified with the dihedral group D6. Under natural symmetry assumptions, the total dipole moment of the dodecamer substructure of HBL is shown to be zero. Biological implications of the mathematical findings are discussed.

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.

The role of linkers in the reassembly of the 3.6 MDa hexagonal bilayer hemoglobin from Lumbricus terrestris

The extent and kinetics of reassembly of the four groups of linkers L1-L4 with 213 kDa subassemblies of twelve globin chains D, (bac)3(d)3, isolated from the approximately 3.6 MDa hexagonal bilayer (HBL) hemoglobin (Hb) of Lumbricus terrestris, was investigated using gel filtration. The reassembled HBL's were characterized by scanning transmission electron microscopic (STEM) mass mapping and their subunit content determined by reversed-phase chromatography. In reassembly by method (A), the linkers isolated by RP-HPLC at pH approximately 2.2 were added to D at neutral pH; in method (B), the linkers were renatured at neutral pH and then added to D. With method (A) the percentage of HBL reassembly varied from >/=13% in the absence of Ca(II) to </=75% in 1-10 mM Ca(II). Reassembly to HBL structures whose linker contents, STEM images and masses were similar to the native Hb was observed with all the linkers (>/=75%), with ternary and binary linker combinations (40-50%) and with individual linkers producing yields increasing in the following order: L1=1-3%, L2 approximately L3=10-20% and L4=35-55%. The yield was two- to eightfold lower with method (B), except in the case of linkers L1-L3. Although the reassembly kinetics were always biphasic, with t1/2=0.3-3.3 hours and 10-480 hours, the ratio of the amplitudes fast:slow was 1:0.6 with method (A) and 1:2.5 with method (B). These results are consistent with a scheme in which the slow HBL reassembly is dependent on a slow conversion of linker conformation at neutral pH from a reassembly incompetent to a reassembly competent conformation. Although all the linkers self-associate extensively at neutral pH, forming complexes ranging from dimers to >18-mers, the size of the complex does not affect the extent or rate of reassembly. The oxygen binding affinity of reassembled HBLs was similar to that of the native Hb, but their cooperativity was lower. A model of HBL reassembly was proposed which postulates that binding of linker dimers to two of the three T subunits of D causes conformational alterations resulting in the formation of complementary binding sites which permit lateral self-association of D subassemblies, and thus dictate the formation of a hexagonal structure due to the 3-fold symmetry of D.

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.

The apparently symmetrical hexagonal bilayer hemoglobin from Lumbricus terrestris has a large dipole moment

The giant approximately 3.6 MDa hexagonal bilayer hemoglobin (HBL Hb) from Lumbricus terrestris consists of 12 213-kDa dodecamers of four globin chains ([b + a + c]3[d]3) tethered to a central scaffold of approximately 36 non-globin, linker subunits L1-L4 (24-32 kDa). Three-dimensional reconstructions obtained by electron cryomicroscopy showed it to have a D6 point-group symmetry, with the two layers rotated approximately 16 degrees relative to each other. Measurement of the dielectric constants of the Hb and the dodecamer over the frequency range 5-100 kHz indicated relaxation frequencies occurring at 20-40 and 300 kHz, respectively, substantially lower than the 700-800 kHz in HbA. The dipole moments calculated using Oncley's equation were 17,300 +/- 2300 D and 1400 D for the Hb and dodecamer, respectively. The approximately threefold higher dipole moment of the dodecamer relative to HbA is consistent with an asymmetric shape in solution suggested by small-angle X-ray scattering. Although a two-term Debye equation and a prolate ellipsoid of revolution model provided a good fit to the experimental dielectric dispersion of the dodecamer, a three-term Debye equation based on an oblate ellipsoid of revolution model was required to fit the asymmetric dielectric dispersion curve of the Hb: the required additional term may represent either an induced dipole moment or a substructure which rotates independently of the main permanent dipole component of the Hb. The D6 point-group symmetry implies that the dipole moments of the dodecamers cancel out. Thus, in addition to a possible contribution from fluctuations of the proton distribution, the large dipole moment of the Hb may be due to an asymmetric distribution of the heterogeneous linker subunits.

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.

A hierarchy of disulfide-bonded subunits: the quaternary structure of Eudistylia chlorocruorin

The quaternary structure of the cysteine-rich, approximately 3500-kDa chlorocruorin (Chl) from the marine polychaete Eudistylia vancouverii was investigated using maximum entropy deconvolution of the electrospray ionization mass spectra (ESIMS). The native Chl provided two groups of peaks, at approximately 25 and approximately 33 kDa, and one peak at approximately 66 kDa. ESIMS of the reduced and reduced and carbamidomethylated Chl and of its subunits obtained by HPLC provided the complete subunit composition of the Chl. Two groups of nonglobin linker chains were observed: L1a-f (25 000.4, 25 017.9, 25 039.6, 25 057.0, 25 074.4 and 25 096.8 Da) and L2a-d (25 402.7, 25 446.0, 25 461.6 and 25 478.3Da) (+/-2.5 Da), with relative intensities L1:L2 = 5:2. Six globin chains were found, a1, a2, and b1-4, with reduced masses of 16 051.5, 16 172.4, 16 853.5, 17 088.9, 17 161.2 and 17 103.6 (+/-1.0 Da) and relative intensities of 8:4:1:4:2:1, respectively. Disulfide-bonded dimers and a tetramer of globin chains were identified: D1 = a1 + b3 at 33 207.1; D2 at 33 374.1, which had a cysteinylated Cys (a2 + b2 + Cys); and D3 = a1 + b4 at 33 149.4 Da (+/-3.0 Da), with relative intensities D1:D2:D3 = 5:4:1 and T = a1 + a2 + b1 + b2 at 66 154.8 +/- 4.0 Da. A 206-kDa dodecamer subunit obtained by dissociation of the Chl in 4 M urea [Qabar, A. N., et al. (1991) J. Mol. Biol. 222, 1109-1129], was found to consist only of tetramers T. A model was proposed for the Chl, based on a dimer:tetramer ratio of 2:1: four 206-kDa dodecamers (trimer of tetramers) and 48 dimers tethered to a framework of 30 L1 and 12 L2 linker chains. The 144 globin chains (2480 kDa) and 42 linker chains (1059 kDa) provide a total mass of 3539 kDa, in good agreement with the 3480 +/- 225 kDa determined previously by STEM mass mapping. The hierarchy of disulfide-bonded globin subunits observed for Eudistylia Chl provides a built-in heterogeneity of hexagonal bilayer structures.

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.

An electrospray ionization mass spectrometric study of the extracellular hemoglobins from Chironomus thummi thummi

The aquatic larvae of the dipteran, Chironomus thummi thummi contain extracellular hemoglobins which exhibit stage-specific expression. We have used maximum entropy-based deconvolution of the complex, multiply charged electrospray ionization mass spectra, to demonstrate the presence of more than 20 components, ranging in mass from 14,417.3 Da to 17,356.5 Da in the 4th instar larvae. Of the 15 major peaks with intensities > 10 relative to 100 for the 14,417.3 Da-component (CTT-IV), only the 15,528.2-Da peak does not correspond to a known amino acid sequence. Since the number of C. thummi thummi globin genes now stands at 27, including one cDNA and not counting three that must encode known globins, our results suggest that only a limited number of the globin genes are expressed in the 4th instar larvae.

Electrospray ionization mass spectrometric study of the multiple intracellular monomeric and polymeric hemoglobins of Glycera dibranchiata

The intracellular hemoglobin (Hb) of the marine polychaete Glycera dibranchiata is comprised of two groups of globins differing in their primary structures and state of aggregation. About six electrophoretically and chromatographically distinct monomeric Hbs which have Leu as the distal residue, and an equal number of polymeric Hbs which have the usual distal His, have been identified to date. Deconvolution of the electrospray ionization mass spectra (ESI-MS) of the Hbs and of their carbamidomethylated, reduced, and reduced/carbamidomethylated forms, using a maximum entropy-based approach (MaxEnt), showed the presence of at least 18 peaks attributable to monomer Hbs (14,500-15,200 Da) and an approximately equal number of polymer Hb peaks (15,500-16,400 Da). Although the ratio of the monomer to polymer components in pooled Hb preparations remained constant at 60:40, Hb from individuals had generally less than 6 monomer and 6 polymer components; -2 of the 19 individuals appeared to be deficient in polymer Hbs. Taking into account possible fragmentations of the known monomeric and polymeric globin sequences, we estimate conservatively that there are 10 monomeric and an equal number of polymeric Hbs, the majority comprising a single free Cys. Surprisingly, the calculated mass of the sequence deduced from the high-resolution monomer Hb crystal structures does not correspond to any of the observed masses. ESI-MS of the monomer Hb crystal revealed 11 components, of which 5, accounting for 67% of total, were related to the three major sequences GMG2-4. These findings underline the need for routine mass spectrometric characterization of all protein preparations. The complete resolution of the Glycera Hb ESI-MS using MaxEnt processing illustrates the power of this method to resolve complex protein mixtures.

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.

Trematode myoglobins, functional molecules with a distal tyrosine

The myoglobins of two trematodes, Paramphistomum epiclitum and Isoparorchis hypselobagri, were isolated to homogeneity. The native molecules are monomeric with Mr 16,000-17,000 and pI 6.5-7.5. In each species, at least four different globin isoforms occur. Primary structure was determined at the protein level. The globin chains contain 147 amino acid residues. Although major determinants of the globin fold are conserved, characteristic substitutions are present. A Tyr residue occurs at the helical positions B10 and E7 (distal position). This is confirmed by NMR measurements (Zhang, W., Rashid, K. A., Haque, M., Siddiqi, A. H., Vinogradov, S. N., Moens, L. & La Mar, G. N. (1997) J. Biol. Chem. 272, 3000-3006). A distal Tyr normally provokes oxidation of the iron atom and the inability to bind oxygen, whereas a Tyr-B10 is indicative for a high oxygen affinity. In contrast, trematode myoglobins are functional molecules with a high oxygen affinity. Molecular modeling predicts two possible positions for the aromatic ring of Tyr-E7: one being outside the heme pocket making it freely accessible to the ligand and one within the heme pocket potentially able to form a second hydrogen bond with the iron-bound oxygen. A hydrogen bond between Tyr-B10 and the bound oxygen as in the Ascaris hemoglobin is predicted as well. The predicted structure may explain the high oxygen affinity of the trematode myoglobins.

Solution of 1H NMR structure of the heme cavity in the oxygen-avid myoglobin from the trematode Paramphistomum epiclitum

A two-dimensional 1H NMR study has been carried out on the heme cavity of the extreme oxygen-avid and autoxidation-resistant oxy-myoglobin complex from the trematode Paramphistomum epiclitum, and the residues were identified which potentially provide hydrogen bond stabilization for the bound oxygen. Complete assignment of the heme core resonances allows the identification of 10 key heme pocket residues, 4 Phe, 4 Tyr, and 2 upfield ring current aliphatic side chains. Based solely on the conserved myoglobin folding topology that places the E helix-heme crossover and the completely conserved Phe(CD1)-heme contact at opposing meso positions, the heme orientation in the cavity and the E helix alignment were unambiguously established that place Tyr66 at position E7. Moreover, all eight aromatic and the two aliphatic side chains were shown to occupy the positions in the heme cavity predicted by amino acid sequence alignment with globins of known tertiary structure. The dipolar contacts for the Tyr32(B10) and Tyr66(E7) rings indicate that both residues are oriented into the heme cavity, which is unprecedented in globins. The ring hydroxyl protons for both Tyr are close to each other and in a position to provide hydrogen bonds to the coordinated oxygen, as supported by strong retardation of their exchange rate with bulk solvent. A more crowded and compact structure increases the dynamic stability of the distal pocket and may contribute to the autoxidation resistance of this myoglobin.