Hemocyanin--a current perspective

Transcriptomic analysis of Pacific white shrimp (Litopenaeus vannamei, Boone 1931) in response to acute hepatopancreatic necrosis disease caused by Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND), caused by marine bacteria Vibrio Parahaemolyticus, is a huge problem in shrimp farms. The V. parahaemolyticus infecting material is contained in a plasmid which encodes for the lethal toxins PirABVp, whose primary target tissue is the hepatopancreas, causing sloughing of epithelial cells, necrosis, and massive hemocyte infiltration. To get a better understanding of the hepatopancreas response during AHPND, juvenile shrimp Litopenaeus vannamei were infected by immersion with V. parahaemolyticus. We performed transcriptomic mRNA sequencing of infected shrimp hepatopancreas, at 24 hours post-infection, to identify novel differentially expressed genes a total of 174,098 transcripts were examined of which 915 transcripts were found differentially expressed after comparative transcriptomic analysis: 442 up-regulated and 473 down-regulated transcripts. Gene Ontology term enrichment analysis for up-regulated transcripts includes metabolic process, regulation of programmed cell death, carbohydrate metabolic process, and biological adhesion, whereas for down-regulated transcripts include, microtubule-based process, cell activation, and chitin metabolic process. The analysis of protein- protein network between up and down-regulated genes indicates that the first gene interactions are connected to oxidation-processes and sarcomere organization. Additionally, protein-protein networks analysis identified 20-top highly connected hub nodes. Based on their immunological or metabolic function, ten candidate transcripts were selected to measure their mRNA relative expression levels in AHPND infected shrimp hepatopancreas by RT-qPCR. Our results indicate a close connection between the immune and metabolism systems during AHPND infection. Our RNA-Seq and RT-qPCR data provide the possible immunological and physiological scenario as well as the molecular pathways that take place in the shrimp hepatopancreas in response to an infectious disease.

Cross-linking with bifunctional reagents and its application to the study of the molecular symmetry and the arrangement of subunits in hexameric protein oligomers

Cross-linking with a bifunctional reagent and subsequent SDS gel electrophoresis is a simple but effective method to study the symmetry and arrangement of subunits in oligomeric proteins. In this study, theoretical expressions for the description of cross-linking patterns were derived for protein homohexamers through extension of the method used for tetramers by Hajdu et al. (1976). The derived equations were used for the analysis of cross-linking by glutardialdehyde of four protein hexamers: beef liver glutamate dehydrogenase (GDH), jack bean urease, hemocyanin from the spiny lobster Panulirus pencillatus (PpHc), Escherichia coli glutamate decarboxylase (GDC) and for analysis of published data on the cross-linking of hexameric E. coli rho by dimethyl suberimidate. Best fit models showed that the subunits in the first four proteins are arranged according to D(3) symmetry in two layers, each subunit able to cross-link to three neighboring subunits for GDH and urease, or to four for PpHc and GDC. The findings indicate a dimer-of-trimers eclipsed arrangement of subunits for GDH and urease and a trimer-of-dimers staggered one for PpHc and GDC. In rho, the subunits are arranged according to D(3) symmetry in a trimer-of-dimers ring. The conclusions from cross-linking of GDH and GDC, PpHc and rho are consistent with results from X-ray crystal structure, those for urease with findings from electron microscopy.

Functional properties of hemocyanin from Oncomelania hupensis, the intermediate host of Schistosoma japonicum

The gastropod mollusc, Oncomelania hupensis is a unique intermediate host for the human parasite Schistosoma japonicum. It is a primary factor for the epidemic of schistosomiasis and its distribution is consistent with the epidemic area of schistosomiasis. Here we report the functional properties of hemocyanin of O. hupensis (OhH), a copper-containing respiratory protein which was isolated from its hemolymph and purified by ammonium sulfate fractionation and ultracentrifugation. We identified the protein characters including UV absorption at 340 nm, copper content and quaternary structure. Furthermore, by induction of phenoloxidase and enzyme-linked immunosorbent assay we show that OhH exhibited o-diphenoloxidase activity after limited proteolysis, and shared carbohydrate epitopes with glycoconjugates of S. japonicum.

The molecular heterogeneity of hemocyanin: Structural and functional properties of the 4×6-meric protein of Upogebia pusilla (Crustacea)

The structural properties of the hemocyanin isolated from the Mediterranean mud shrimp, Upogebia pusilla (Decapoda: Thalassinidea), were investigated. Our intent was to make use of the U. pusilla case to perform a structural comparison between crustacean and chelicerate 4x6-meric hemocyanins. The thalassinidean hemocyanin appears similar in size but different in structural organization compared to the chelicerate 4x6-mer. Ultracentrifuge analyses on the purified protein revealed a sedimentation coefficient of 39S, typical of 4x6 hemocyanins. Electron micrographs are in agreement with a model in which four 2x6-meric building blocks are arranged in a tetrahedron-like quaternary structure and not in the quasi-square-planar orientation characteristic of the chelicerate protein. Size-exclusion chromatography-fast protein chromatography analysis showed elevated instability of the protein in absence of divalent ions or at pH values higher than 8.0. This analysis also shows that the dissociation of the U. pusilla 4x6-meric hemocyanin into hexamers occurs without any intermediate 2x6-meric state, in contrast with the dissociation profile of the chelicerate protein exhibiting several dissociation intermediates. The oxygen-binding properties of U. pusilla hemocyanin were studied to disclose possible effects by the typical allosteric effectors that modulate the functional properties of crustacean hemocyanin. A marked Bohr and lactate effect, but no significant influence of urate, on the oxygen affinity of U. pusilla hemocyanin were found.

Quaternary structure and functional properties of Penaeus monodon hemocyanin

The hemocyanin of the tiger shrimp, Penaeus monodon, was investigated with respect to stability and oxygen binding. While hexamers occur as a major component, dodecamers and traces of higher aggregates are also found. Both the hexamers and dodecamers were found to be extremely stable against dissociation at high pH, independently of the presence of calcium ions, in contrast to the known crustacean hemocyanins. This could be caused by only a few additional noncovalent interactions between amino acids located at the subunit-subunit interfaces. Based on X-ray structures and sequence alignments of related hemocyanins, the particular amino acids are identified. At all pH values, the p50 and Bohr coefficients of the hexamers are twice as high as those of dodecamers. While the oxygen binding of hexamers from crustaceans can normally be described by a simple two-state model, an additional conformational state is needed to describe the oxygen-binding behaviour of Penaeus monodon hemocyanin within the pH range of 7.0 to 8.5. The dodecamers bind oxygen according to the nested Monod-Whyman-Changeaux (MWC) model, as observed for the same aggregation states of other hemocyanins. The oxygen-binding properties of both the hexameric and dodecameric hemocyanins guarantee an efficient supply of the animal with oxygen, with respect to the ratio between their concentrations. It seems that under normoxic conditions, hexamers play the major role. Under hypoxic conditions, the hexamers are expected not to be completely loaded with oxygen. Here, the dodecamers are supposed to be responsible for the oxygen supply.

Bohr-effect and buffering capacity of hemocyanin from the tarantula E. californicum

A previous report showed that binding of oxygen to the 24-meric hemocyanin from E. californicum does not correlate linearly with the release of protons as known from hemoglobin. However, this unusual complex phenomenological observation could not be explained at that time. Here, I present a full analysis of the thermodynamic coupling between protons and oxygen for the 24-meric tarantula hemocyanin in Ringer-solution based on the Nested-MWC-model. A strategy is presented which allows to reduce the number of free parameters when fitting the model to the data by including explicitly the equilibrium constants for binding of protons to the different conformations. The results show that the Nested-MWC-model presents a good description of the cooperative and allosteric properties of spider hemocyanin also under physiological conditions and additional constraints imposed by proton-binding data. The analysis allowed to estimate the average number of allosteric proton-binding sites per subunit and the corresponding pK for each conformation. Furthermore, an estimate of the number and affinity of proton buffering binding sites could be given. Approximately 80% of all proton-binding sites are non-allosteric buffering binding sites. The results obtained in this study allow to predict the relative contribution of the four different conformations under conditions found in vivo. The conformational distribution indicates that the conformation with the highest proton affinity (tR) might be important for the proton transport in the hemolymph.

Small-angle X-ray Scattering-based Three-dimensional Reconstruction of the Immunogen KLH1 Reveals Different Oxygen-dependent Conformations

For decades the respiratory protein keyhole limpet hemocyanin (KLH1) from the marine gastropod Megathura crenulata has been used widely as a potent immunostimulant, useful hapten carrier, and valuable agent in the treatment of bladder carcinoma. Although much information on the immunological properties of KLH1 is available, biochemical and structural data are still incomplete. Small-angle x-ray scattering revealed the existence of two conformations, an oxy state being slightly more compact than the deoxy state. Based on small-angle scattering curves, a newly developed Monte Carlo algorithm delivered a surface representation of proteins. The massive changes of the surfaces of reconstructed didecameric KLH1 molecules are explained as a twist of the two non-covalently associated decameric half-molecules. Upon oxygenation, the KLH1 molecule becomes longer and skinnier. This study provides the first real evidence how a molluscan hemocyanin changes conformation during an allosteric transition.

A hemocyanin from the Onychophora and the emergence of respiratory proteins

The velvet worms (Onychophora) are considered living fossils and are closely related to the Euarthropoda. Onychophora possess a tracheal system for respiratory function, but oxygen-transport proteins have been considered unnecessary. Here, we show that the hemolymph of the Epiperipatus sp. (Onychophora: Peripatidae) contains an arthropod-type hemocyanin, demonstrating that such protein exists outside the Euarthropoda. Thus, the evolution of oxygen carriers preceded the divergence of the Onychophora and Euarthropoda and was most likely linked to the evolution of an efficient circulatory system in a low-oxygen environment. The cDNA of the Epiperipatus hemocyanin subunit comprises 2,287 bp and encodes for a protein of 641 aa (73.6 kDa). Phylogenetic analyses of the arthropod hemocyanin sequences show that the Onychophora form a robust sister-group of the Euarthropoda, whereas the monophyly of the Tracheata is not supported.

The archaeogastropod mollusc Haliotis iris: tissue and blood metabolites and allosteric regulation of haemocyanin function

We investigated divalent cation and anaerobic end-product concentrations and the interactive effects of these substances and pH on haemocyanin oxygen-binding (Hc-O2) in the New Zealand abalone Haliotis iris. During 24 h of environmental hypoxia (emersion), d-lactate and tauropine accumulated in the foot and shell adductor muscles and in the haemolymph of the aorta, the pedal sinus and adductor muscle lacunae, whereas l-lactate was not detected. Intramuscular and haemolymph d-lactate concentrations were similar, but tauropine accumulated to much higher levels in muscle tissues. Repeated disturbance and short-term exposure to air over 3 h induced no accumulation of d- or l-lactate and no change in [Ca2+], [Mg2+], pH and O2-binding properties of the native haemolymph.

The haemolymph showed a low Hc-O2 affinity, a large reverse Bohr effect and marked cooperativity. Dialysis increased Hc-O2 affinity, obliterated cooperativity and decreased the pH-sensitivity of O2 binding. Replacing Mg2+ and Ca2+ restored the native O2-binding properties and the reverse Bohr shift. l- and d-lactate exerted minor modulatory effects on O2-affinity. At in vivo concentrations of Mg2+ and Ca2+, the cooperativity is dependent largely on Mg2+, which modulates the O2 association equilibrium constants of both the high-affinity (KR) and the low-affinity (KT) states (increasing and decreasing, respectively). This allosteric mechanism contrasts with that encountered in other haemocyanins and haemoglobins. The functional properties of H. iris haemocyanin suggest that high rates of O2 delivery to the tissues are not a priority but are consistent with the provision of a large O2 reserve for facultatively anaerobic tissues during internal hypoxia associated with clamping to the substratum.

Effects of molting and environmental factors on trace metal body-burdens and hemocyanin concentrations in the American lobster, Homarus americanus

Hemocyanin concentrations in the hemolymph of marine crustacea are dependent on the molt cycle and on environmental conditions. Studies in our laboratories have found that hemocyanin levels in blue crabs are reduced after ecdysis and under conditions of environmental stress (Engel, Brouwer, & McKenna, 1993. Hemocyanin concentrations in marine crustaceans as a function of environmental conditions. Marine Ecology Progress Series, 93, 233-244). We have extended those studies to include the American lobster, Homarus americanus. Hemolymph and digestive gland tissues from Long Island Sound lobsters were analyzed for hemocyanin, copper, and zinc during different stages of the molt cycle. Hemocyanin, copper and zinc in the hemolymph were highest in premolt stages (D1-D4), and lowest in the postecdysal papershell stages (B1-B2). Concomitantly, copper in digestive glands decreased significantly following ecdysis, but no significant changes in the metals bound to metallothionein (MT) were observed. Copper-MT was the predominant form throughout the molt cycle, presumably because lobsters were obtained from copper-contaminated areas. To examine the effects of environmental factors, intermolt lobsters were collected from locations of different environmental quality along the Atlantic coast, and were analyzed for hemocyanin and trace metals. In general, animals from areas with a history of contamination showed the highest hemocyanin concentrations.

The allosteric effector l-lactate induces a conformational change of 2x6-meric lobster hemocyanin in the oxy state as revealed by small angle x-ray scattering

Hemocyanins are multisubunit respiratory proteins found in many invertebrates. They bind oxygen highly cooperatively. However, not much is known about the structural basis of this behavior. We studied the influence of the physiological allosteric effector l-lactate on the oxygenated quaternary structure of the 2x6-meric hemocyanin from the lobster Homarus americanus employing small angle x-ray scattering (SAXS). The presence of 20 mm l-lactate resulted in different scattering curves compared with those obtained in the absence of l-lactate. The distance distribution functions p(r) indicated a more compact molecule in presence of l-lactate, which is also reflected in a reduction of the radius of gyration by about 0.2 nm (3%). Thus, we show for the first time on a structural basis that a hemocyanin in the oxy state can adopt two different conformations. This is as predicted from the analysis of oxygen binding curves according to the "nesting" model. A comparison of the distance distribution functions p(r) obtained from SAXS with those deduced from electron microscopy revealed large differences. The distance between the two hexamers as deduced from electron microscopy has to be shortened by up to 1.1 nm to agree well with the small angle x-ray curves.

Two types of urate binding sites on hemocyanin from the crayfish Astacus leptodactylus: an ITC study

The oxygen binding behaviour of hemocyanins from Crustacea is regulated by small organic compounds such as urate and L-lactate. We investigated the binding characteristics of urate and the related compound caffeine to the 2 x 6-meric hemocyanin of A. leptodactylus under fully oxygenated conditions employing isothermal titration calorimetry (ITC). An analysis of urate and caffeine binding based on a model of n identical binding sites resulted in approximately four binding sites for caffeine and eight for urate. This result suggests that the binding process for these effectors is more complex than this most simple model. Therefore, we introduced a number of alternative models. Displacement experiments helped to select the appropriate model. Based on these experiments, at least two different types of binding sites for urate and caffeine exist on the 2 x 6-meric hemocyanin of A. leptodactylus. The two binding sites differ strongly in their specificity towards the two analogues. It can be hypothesized that two different subunit types (beta and gamma) are responsible for the two types of binding sites.

Molecular heterogeneity of the hemocyanin isolated from the king crab Paralithodes camtschaticae

Native Paralithodes camtschaticae hemocyanin is found as a mixture of dodecamers (24S; 80%) and hexamers (16S; 20%). Removal of Ca2+ ions by dialysis against EDTA-containing buffer solution at neutral pH induces complete dissociation of the 24S form into the 16S form. Under these conditions, a further increase in pH to 9.2 produces complete dissociation of the hexamers into monomers (5S). In both cases, the dissociation process is reversible. The dodecamer (24S) is composed of two different hexamers which can be discriminated only by ion-exchange chromatography in the presence of Ca2+ ions. At alkaline pH and in the presence of EDTA, two major monomeric fractions can be separated by ion-exchange chromatography: ParcI (60%) and ParcII (40%). The reassociation properties of the two fractions were studied separately to define their ability to form hexamers and dodecamers. The oxygen-binding properties of the different aggregation states were investigated. Native hemocyanin binds O2 co-operatively (nH = 3) and with low affinity (p50 approximately 103 Torr). The two monomeric fractions, ParcI and ParcII, are not co-operative and the affinity is twice that of the native protein (p50 approximately 65 and 52 Torr). Oxygen-binding measurements of native hemocyanin carried out at different pH values indicate a strong positive Bohr effect within the pH range 6.5-8.0 and an increase in oxygen affinity at pH below 6.5.

Temperature adaptation influences the aggregation state of hemocyanin from Astacus leptodactylus

When Astacus leptodactylus were kept at various temperatures for several weeks, different ratios between di-hexameric and hexameric hemocyanins were observed in their hemolymph. The higher the temperature the more hexamers were present. This long-term adaptation to different temperatures or/and to temperature-induced pH-shifts as observed in the hemolymph has different effects on the expression of subunit types building up hexamers and those which covalently link two hexamers within the di-hexamers. The oxygen binding behaviour of di-hexameric hemocyanins from cold and warm adapted animals do not show differences with respect to affinity, Bohr effect and cooperativity.

Oxygen transport proteins: III. Structural studies of the scorpion (Buthus sindicus) hemocyanin, partial primary structure of its subunit Bsin1

The hemocyanin (Hc) from Buthus sindicus, studied in the native state, demonstrated to be an aggregate of eight different types of subunits arranged in four cubic hexamers. Both, the 'top' and the 'side' views of the native molecule have been identified from the negatively stained specimens using transmission electron microscopy. Out of these, eight different polypeptide chains, the partial primary structure (68%) of a subunit Bsin1 (Mr = 72422.7 Da) was established using a combination of automated Edman degradation and mass spectrometry. A multiple sequence alignment with other closely related cheliceratan Hc subunits revealed average identities of ca. 60%. Most of the structurally important residues, i.e. copper and calcium-binding ligands, as well as the residues involved in the presumed oxygen entrance pathway, proved to be strictly conserved in Bsin1. Sequence variations have been observed around the functionally important chloride-binding site, not only for the B. sindicus subunit Bsin1, but also for the subunit Aaus-6 of the scorpion A. australis and the subunit Ecal-a from the spider Eurypelma californicum Hcs. Deviation in the primary structure related to the chloride-binding site suggest that the effect of chloride ions may vary in different hemocyanins. Furthermore, the secondary structural contents of the Hc subunit Bsin1 were determined by circular dichroism revealing ca. 33% alpha-helix, 18%, beta-sheet, 19% beta-turn, and 30% random coil composition. These values are in good agreement with the crystal structure of the closely related Hc subunit Lpol-II from horseshoe crab L. polyphemus. Electron microscopic studies of the purified Hc subunit under native conditions revealed that Bsin1 has self aggregation properties. Results of these studies are discussed.

Fluorescence properties and stability of dioxygen--binding functional units from the Rapana thomasiana hemocyanin subunit RHSS2

Molecular aggregates of the Rapana thomasiana hemocyanin are composed of two structural subunits, RHSS1 and RHSS2, each of which contains eight functional units (FUs) reversibly binding dioxygen. Multiunit fragments and individual 50-60 kDa FUs from RHSS2 were isolated and characterized by electron and fluorescence spectroscopy. The units have similar fluorescence parameters demonstrating that the tryptophyl side chains are located in the hydrophobic core of the globular folded regions. The copper-dioxygen system at the binuclear active site stabilizes considerably the native protein structure and quenches the indole emission. The removal of this system decreased the 'melting points' drastically Tm by 13-20 degrees C and increased 2-4 times the fluorescence quantum yields. The individual FUs differ considerably in their thermostability. The activation energy for the thermal deactivation of the excited tryptophyl residues of the apo-FUs is lower compared to that of the whole molluscan apo-Hcs.

Low-resolution structure of the proteolytic fragments of the Rapana venosa hemocyanin in solution

Rapana venosa hemocyanin (Hc) is a giant oxygen-binding protein consisting of different subunits assembled in a hollow cylinder. The polypeptide chain of each subunit is believed to be folded in several oxygen-binding functional units of molecular mass 50 kDa, each containing a binuclear copper active site. Limited proteolysis with alpha-chymotrypsin of native R. venosa hemocyanin allows the separation of three functional proteolytic fragments with molecular masses of approximately 150, 100, and 50 kDa. The functional fragments, purified by combining gel filtration chromatography and ion-exchange FPLC, were analyzed by means of small-angle X-ray scattering (SAXS). The gyration radius of the 50-kDa Rapana Hc fraction (2.4 nm) agrees well with that calculated on the basis of the dimensions determined by X-ray crystallography for one functional unit of Octopus Hc (2.1 nm). Independent shape determination of the 50- and 100-kDa proteolytic fragments yields consistent low-resolution models. Simultaneous fitting of the SAXS data from these fragments provides a higher-resolution model of the 100-kDa species made of two functional units tilted with respect to each other. The model of the 150-kDa proteolytic fragment consistent with the SAXS data displays a linear chain-like aggregation of the 50-kDa functional units. These observations provide valuable information for the reconstruction of the three-dimensional structure of the minimal functional subunit of gastropod hemocyanin in solution. Furthermore, the spatial relationships among the different functional units within the subunit will help in elucidation of the overall quaternary structure of the oligomeric native protein.

X-ray Absorption Spectroscopic Studies of the Copper(I) Complexes of Crown Ether Appended Bis{(2-pyridyl)ethyl}amines and Their Dioxygen Adducts

An X-ray absorption spectroscopic study of the Cu complexes of the bis{(2-pyridyl)ethyl}-appended monoaza crown ether 1, diaza crown ether 2, and diphenylglycoluril diaza basket 3 is reported. Following detailed analysis of the spectra of the crystallographically characterized model compound tetrapyridyl Cu(II) bis(nitrato pyridine) (4), the contributions of the ring atoms of the coordinating pyridine to the EXAFS were simulated using a multiple-scattering approach and the final parameters obtained by restrained refinement. Oxygenation of the Cu(I) complexes resulted in a large increase of the intensity of the major peak in the phase-corrected Fourier transform. This was interpreted as evidence for a &mgr;-eta(2):eta(2)-peroxo coordination mode of the oxygen between the copper ions, which had changed valence from Cu(I) to Cu(II) as judged from the edge position. This oxygen binding mode is reminiscent of that of hemocyanin, but the Cu-Cu distances are significantly shorter in the model than in the enzyme and vary with solvent. The EXAFS of the oxygenated complexes was simulated in a new approach in which, besides the parameters of the pyridine unit, those of an additional multiple scattering unit describing the geometry of the Cu-O(2)-Cu moiety were also refined.

Mass spectrometric characterization of Limulus polyphemus hemocyanin

Electrospray ionization-mass spectrometry of ion-exchange and reverse-phase purified hemocyanin from Limulus polyphemus revealed six predominant isoforms with molecular weights ranging from 71,730 to 72,695 Da. The heaviest of these agreed closely with the molecular weight calculated for the previously determined Edman sequence with substitution of isoleucine for valine at position 9 and inclusion of three internal disulfide bonds and one copper atom. Proposed structures for the other isoforms were made on the basis of the molecular weight measurements. Reverse-phase chromatography can be used in addition to the traditional ion-exchange step to produce hemocyanin preparations of greater purity that might be valuable for further detailed investigations of the physicochemical properties of these important proteins. The results reflect yet again the value of mass spectrometry for recognizing molecular microheterogeneity in biological macromolecules and for following protein purification.

Crystal structure of a functional unit from Octopus hemocyanin

Hemocyanins are giant oxygen transport proteins found in many arthropods and molluscs. Freely dissolved in the hemolymph, they are multisubunit proteins that contain many copies of the active site, a copper atom pair that reversibly binds oxygen. Octopus hemocyanin is composed of ten subunits, each of which contain seven oxygen-binding "functional units". The carboxyl-terminal 47 kDa functional unit, Odg, is a proteolytic isolate that binds oxygen reversibly while exhibiting slight Bohr and magnesium ion effects. In this work we present the X-ray structure determination and analysis of Odg at 2.3 A resolution. Odg has two structural domains: a largely alpha-helical copper binding domain, and a five-stranded anti-parallel beta-sandwich with the jelly roll topology found in many viruses. Six histidine residues ligate the copper atoms, one of which is involved in a thioether bridge. The results show that the hemocyanin from the mollusc and that from the arthropod have distinct tertiary folds in addition to the long recognized differences in their quaternary structures. Nonetheless, a comparison of Octopus and horseshoe crab hemocyanin reveals a similar active site, in a striking example of perhaps both convergent and divergent evolution.

On the stability of the 24-meric hemocyanin from Eurypelma californicum

The stability of the 24-meric hemocyanin from Eurypelma californicum towards various denaturants (GdnHCl, urea, urea derivatives and salts of the Hofmeister series) indicates that the quaternary structure is stabilized by hydrophilic and polar forces. Thus, the interaction between the seven different subunit types of this cheliceratan hemocyanin is comparable with that of the closely related crustacean hemocyanins. In contrast, no significant influence of divalent ions such as Ca2+ and Mg2+ on the stability is observed at pH 8.0 and pH 8.5 but not at pH 7.0. Studies, both in the presence of urea and GdnHCl indicate that the denaturation process consists of a dissociation of the oligomeric structure into intact subunits at lower concentrations of denaturants followed by denaturation of the subunits at higher concentrations of denaturants. No intermediates such as hexamers or dodecamers were detected after 24 h of incubation. This study also reveals that oligomerization has a stabilizing effect on the heterogeneous subunits. In addition, differences in the primary structures result in different stabilities of the seven different subunit types.

Cross-reactivity between terrestrial snails (Helix species) and house-dust mite (Dermatophagoides pteronyssinus). II. In vitro study

Epidemiologic and in vitro data have shown that the association of house-dust mite (HDM) allergy and snail allergy in the same patients was due to cross-reactivity between HDM and snail allergenic components. However, the cross-reacting allergen(s) have not yet been identified. In vitro reactivity of seven patients' sera to the various extracts and hemolymph of four different Helix snail species was analyzed by IgE detection and immunodots and Western blots. Cross-reactivity between snails and Dermatophagoides pteronyssinus was assessed by immunodot and ELISA inhibition in two patients. Heterologous inhibition of the snail immunodot and ELISA was observed in one serum. Western blotting showed a specific binding on all four snail species extracts; molecular weights of snail allergens ranged from < 21 to 200 kDa. Marked individual differences were observed in the seven sera under study; most sera demonstrated IgE recognition of multiple bands, illustrating that no single allergen is responsible for cross-reactivity between snail and mite. These results confirm that cross-reactivity exists between snails of the Helix genus and HDM. This cross-reactivity, involving more than a single allergen, may be of clinical significance in atopic patients allergic to D. pteronyssinus. The identity of the cross-reacting allergens remains to be determined. Potential candidates include the thermostable minor allergens of D. pteronyssinus, tropomyosin and hemocyanin.

Cross-reactivity between terrestrial snails (Helix species) and house-dust mite (Dermatophagoides pteronyssinus). I. In vivo study

Clinical reports have suggested an unusual frequency in the number of patients with food allergy to snails who are also allergic to the house-dust mite (HDM). As allergy to HDM is one of the most frequent sensitizations in atopic patients of Western countries, evaluation of the relevance of the concomitant sensitization to Dermatophagoides pteronyssinus and to snails is an important consideration. To evaluate the responsibility of different snail components and of snail mites for inducing in vivo hypersensitivity in patients allergic to HDM, the in vivo reactivity of patients with clinical symptoms after ingestion of snails was assessed by skin prick tests with extracts and hemolymph from four different Helix species snails, and extracts from the snail parasitic mite, Riccardoella limacum. In addition, to obtain epidemiologic data on cosensitization to HDM and snails in allergic patients, the frequency of snail sensitization and its relationship to HDM sensitization were determined in a population of 169 allergic children. All patients allergic to snails had positive skin prick tests to the snail extracts and none to R. limacum extract. The number of positive skin reactions did not significantly differ whatever the species, snail part, or heating procedure used. The strongest reactions were obtained with Helix pomatia (Burgundy snail). Among the 169 prospectively tested children, 38 had a positive prick test to snail extracts; 79% of the snail-sensitized children had sensitization to HDM; and 31% of the children allergic to HDM were found to be sensitized to snails. These results show that snail components, and not the mite R. limacum, were responsible for the in vivo hypersensitivity. These snail components reacting in vivo are present in different parts of snails, including the hemolymph. One-third of the children allergic to HDM were sensitized to snails without any previous ingestion of snails: this observation suggests that HDM was the sensitizing agent and that the cross-reaction could be clinically relevant in countries where eating snails is common.

Primary structure of 21 novel monoantennary and diantennary N-linked carbohydrate chains from alphaD-hemocyanin of Helix pomatia

The primary structures of 21 novel monoantennary and diantennary N-glycans of the glycoprotein alphaD-hemocyanin (alphaD-Hc) of Helix pomatia have been determined. Outer oligosaccharide fragments (antennae) were released from the glycoprotein by Smith degradation of an alphaD-Hc pronase digest. The major antenna, obtained following HPLC fractionation on Lichrosorb-NH2, was characterized using 1H-NMR spectroscopy, fast-atom-bombardment mass spectrometry, and linkage analysis, and corresponds to a pentasaccharide fragment. The intact carbohydrate chains of alphaD-Hc were released with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase-F digestion, separated from the protein on Bio-Gel P-100, and subfractionated on Bio-Gel P-4. A portion of subfractions was reduced with sodium borodeuteride, and the non-reduced and reduced samples were further fractionated on CarboPac PA-1, Lichrosorb-NH2/Lichrosphere-NH2, and/or Lichrosphere-C18. Purified oligosaccharides and oligosaccharide-alditols were analyzed using 500/600-MHz 1H-NMR spectroscopy. In total, four novel types of antenna were identified, namely, [structures: see text] which are all attached to O-2 of alphaMan residues of the trimannosyl-N,N'-diacetylchitobiose core element, which is generally beta-1,2-xylosylated and alpha-1,6-fucosylated, Man(alpha1-6)[Man(alpha1-3)][+/-Xyl(beta1-2)]Man(beta1-4)GlcNAc(beta1-4) [+/-Fuc(alpha1-6)]GlcNAc.

The binding of azide to copper-containing and cobalt-containing forms of hemocyanin from the mediterranean crab Carcinus aestuarii

To establish the competence of the active site of hemocyanin to acquire diverse coordination geometries, the binding of azide to three forms of a crab hemocyanin, the dinuclear cupric or met-hemocyanin, the mononuclear cupric or met-apo-hemocyanin, and the mononuclear Co(II)-substituted derivative has been studied by near-ultraviolet circular dichroism and EPR spectroscopies. The near-ultraviolet circular dichroism spectra of the various derivatives present qualitatively similar features, namely a negative peak around 335 nm in the case of the two copper-containing derivatives and a three-component pattern with the Co(II) derivative. Upon decreasing the pH from 7.0 to 5.5 a decrease of optical activity is observed with all protein samples. The characteristic CD features, attributable to N(imidazole)-to-metal and to OH -to-metal charge-transfer transitions, are strongly affected by azide binding. In particular, the intensity of the negative band exhibited by the two copper-containing protein forms decreases with the onset of a new negative feature with maximum around 400 nm diagnostic for azide-to-Cu(II) charge-transfer transitions. The visible region is affected as well, indicating that changes in the coordination sphere of copper take place. The affinity for azide of the different protein forms is higher at low pH. EPR measurements on the paramagnetic met-apo-hemocyanin derivative as a function of pH demonstrate heterogeneity in the coordination environment at low pH. In the presence of azide an increase of rhombic distortion of the EPR spectra is observed and on the basis of the identified sets of copper hyperfine features in the course of azide titration experiments two different azide bound forms of met-apo-hemocyanin can be detected. The CD and EPR data at the different pH values are consistent with a reaction scheme in which azide replaces a fourth ligand in the metal-coordination sphere, identified as a water or hydroxide molecule.

Small-angle X-ray scattering reveals differences between the quaternary structures of oxygenated and deoxygenated tarantula hemocyanin

Small-angle X-ray scattering (SAXS) curves have been recorded for the oxygenated and deoxygenated states of the 4 x 6-meric hemocyanin from the tarantula Eurypelma californicum. A comparison of the curves shows that the quaternary structures of the two states are different by three criteria, which all indicate that the hemocyanin is less compact in the oxygenated compared to the deoxygenated form: (a) The radius of gyration is 8.65 +/- 0.05 nm for the deoxy- and 8.80 +/- 0.05 nm for the oxy-form. (b) The maximum particle dimension amounts to 25.0 +/- 0.5 nm for the deoxy- and to 27.0 +/- 0.5 nm for the oxy-form. (c) A dip in the intramolecular distance distribution function p(r) is more pronounced and shifted to larger distances in the oxy-form. The p(r) functions based on SAXS measurements were compared to p(r) functions deduced from published electron microscopical images of three different 4 x 6-meric hemocyanins from closely related species. The p(r) functions of SAXS and electron microscopy were similar in one case, whereas in the other two cases the distance between the two 12-meric half-molecules had to be changed by 1-1.5 nm to obtain good agreement. The differences between the p(r) functions of oxygenated and deoxygenated 4 x 6-meric tarantula hemocyanin are much larger than one would expect from a comparison of X-ray structures of the oxygenated and deoxygenated states of a closely related 6-meric hemocyanin. Thus, the conformational changes upon oxygenation occur at various levels of the quaternary structure, as postulated by hierarchical theories of allosteric interactions.

The dinuclear copper site structure of Agaricus bisporus tyrosinase in solution probed by X-ray absorption spectroscopy

We have measured the x-ray absorption near edge structure (XANES) spectra of the enzyme tyrosinase from the mushroom Agaricus bisporus in solution in the oxy and deoxy forms. The spectra, obtained under the same conditions as the analogous forms of mollusc hemocyanin (Hc), show that the oxidation state of copper changes from Cu(II) (oxy form) to Cu(I) (deoxy form), and the copper active site(s) of A. bisporus tyrosinase in solution undergoes the same main conformational changes as Hc. We have applied the multiple scattering theory to simulate the XANES spectra of various alternative geometries of the copper site, accounting for the residual differences between Hc and tyrosinase. While oxy-Hc is reasonably fitted only by the pseudo-square-pyramidal geometry reported by its crystallographic data, oxytyrosinase can be fitted, starting from the Hc coordinates, either by distortions toward a pseudo-tetrahedral geometry, with inequivalent copper sites, or by an apically distorted square-pyramidal geometry (with an elongation of the apical distance of no more than 0.2 A).

Keyhole limpet hemocyanin: structural and functional characterization of two different subunits and multimers

Keyhole limpet hemocyanin (KLH), the large respiratory glycoprotein from the primitive gastropod mollusc, Megathura crenulata, is a potent immunogen used classically as a carrier protein for haptens and more recently in human vaccines and for immunotherapy of bladder cancer. Two KLH isoforms were identified and isolated by high-performance anion exchange chromatography. Subsequent analyses disclosed that these isoforms--designated KLH-A and KLH-B--were composed of distinct subunits that differed in primary structure, molecular weight (KLH-A was 449,000 and KLH-B was 392,000), polymerization/reassociation characteristics, and O2-binding constants (KLH-A had a P50 of 7.32 and KLH-B had a P50 of 2.46). Both subunits appear to be composed of eight oxygen binding domains, and reassociate in solution only with like subunits. These results support the concept that structural and functional heterogeneity is a common feature of molluscan hemocyanins, and provide a rational basis for studying and optimizing the immunostimulatory properties of KLH.

A case of orthologous sequences of hemocyanin subunits for an evolutionary study of horseshoe crabs: amino acid sequence comparison of immunologically identical subunits of Carcinoscorpius rotundicauda and Tachypleus tridentatus

About 83% of the amino acid sequence of hemocyanin subunit HR6 from the Southeast Asian horseshoe crab, Carcinoscorpius rotundicauda, has been determined. There is a difference of about 43% between HR6 and complete sequences of chelicerate hemocyanin subunits from the American horseshoe crab, Limulus polyphemus, and a tarantula, Eurypelma californicum. However, the immunologically identical subunits HR6 and HT6 from Tachypleus tridentatus (Japanese horseshoe crab) show 2.7% sequence difference. Based on the amino acid sequences of HR6 and HT6, the divergence between C. rotundicauda and T. tridentatus occurred about 9.6 million years ago. In the case of horseshoe crab hemocyanin subunits, it seems that the orthologous homologues in many homologous subunits between species are immunologically detectable.

Characterization of N-linked carbohydrate chains of the crayfish, Astacus leptodactylus hemocyanin

The primary structure of the carbohydrate chains of hemocyanin from the crayfish Astacus leptodactylus were investigated. The carbohydrate content is 0.2% (w/w) as referred to total hemocyanin content, resp. 1.8% as referred only to the one subunit which is glycosylated. Mannose and N-acetylglucosamine are present in a molar ratio of 6:2. The carbohydrate chains are N-glycosidically linked as revealed by dot blot analysis using various lectins and enzymatic deglycosylation. Furthermore, they are part of only one hemocyanin subunit of A. leptodactylus. After enzymatic deglycosylation with PNGase F, the oligosaccharide pool was separated by FPLC on Mono Q and subsequent HPLC on Lichrosorb-NH2, the subfractions were characterized by 1H NMR spectroscopy. A total of six oligosaccharides, ranging from Man4GlcNAc2 to Man9GlcNAc2 is present, Man6GlcNAc2 representing the most abundant one with 57% of all oligosaccharides.

An oxygenation-sensitive dye binding to Carcinus maenas hemocyanin

The interaction of 4',6-diamidino-2-phenylindole (DAPI) with Carcinus maenas hemocyanin has been investigated by steady state fluorescence, dynamic fluorescence and circular dichroism measurements. The dye binds to apohemocyanin (without copper) as well as to oxygenated hemocyanin and to deoxygenated hemocyanin with very similar affinities (kd approximately equal to 1 microM ) and number of binding sites (one per subunit). In contrast, the fluorescence quantum yield enhancement of DAPI bound to oxygenated hemocyanin is nearly 60% lower than that observed for deoxygenated and apo forms. The decrease of fluorescence of the dye bound to deoxygenated hemocyanin is a sigmoidal function of the oxygen partial pressure, specular to that observed by following the absorbance of the copper-oxygen charge transfer band at 340 nm. This result provides preliminary evidence that DAPI may be used as a functional probe to monitor the cooperative binding of oxygen to the protein. The higher fluorescence quantum yield of DAPI bound to either apohemocyanin or deoxygenated protein is characterized by a single fluorescence decay with lifetime of about 3 ns, while with the oxygenated protein two components of about 1 ns and 3.0 ns are observed. This result is interpreted assuming the existence of two rotamers of DAPI in solution (Szabo et al. Photochem. Photobiol. 44 (1986) 143-150) both able to interact with oxygenated hemocyanin but only one to deoxygenated and apo forms. We conclude that the different fluorescence behaviour of the dye induced by the presence of oxygen bound to the protein is probably due to a structural change of hemocyanin in cooperative interaction with oxygen. Furthermore, the interaction is confirmed by the induced negative ellipticity of DAPI bound to apohemocyanin and deoxy-hemocyanin and by the increase of fluorescence anisotropy of DAPI bound to all forms of protein investigated.

Identification of two antibody-interaction sites on the surface of Panulirus interruptus hemocyanin

Negatively stained complexes of Panulirus interruptus (spiny lobster) hemocyanin with two different monoclonal antibodies, named E and J, were studied by electron microscopy and image processing. The attachment site of the antibodies to the hexameric hemocyanin molecule was deduced from two perpendicular views of hemocyanin/antibody complexes, in which either the threefold axis or one of the twofold axes was oriented perpendicular to the supporting film. Images of complexes in these orientations were searched with reference images simulated from the known X-ray structure of P. interruptus hemocyanin. The two sites were further characterized by combining our results from electron microscopy with structural data obtained by X-ray diffraction and other methods. These two antibodies recognize different non-overlapping epitopes. The epitope for clone E is located on domain 3 at the surface of the beta barrel and consists of certain loops, which form connections between beta-strand structures. The epitope for clone J is situated on domain 1 at the surface of an alpha-helical region and consists mainly of certain alpha-helices connecting loops. The orientation of the hemocyanin hexamers in the two complexes is very different, as is demonstrated most clearly when they form chains. Clone E forms complexes with the threefold axes perpendicular to the chain direction, while for clone J the threefold axes seem to be parallel to the main direction. The angle between the Fab part of an IgG molecule and the threefold axis of the hexamer is 60 +/- 5 degrees for clone E and 35 +/- 7 degrees for clone J. This observation is clearly related to the difference in orientation of the hexamers for the two complexes.

Tris: an allosteric effector of tarantula haemocyanin

The effect of the chemical buffering component Tris (hydroxy-methyl-amino-methane) and of chloride ions on the oxygen binding of tarantula hemocyanin was studied at constant pH. It revealed that Tris at micromolar concentrations decreases the oxygen pressure at half-saturation (p50) by a factor of more than two, whereas chloride does not influence oxygen affinity. A thermodynamic analysis in terms of the nested model of allostery [(1987) Proc. Natl. Acad. Sci. 84, 1891-1895] indicated that Tris acts a an allosteric activator of oxygen binding by influencing the interaction between the 12-meric half-molecules of the 24-meric tarantula haemocyanin.

An x-ray absorption near edge structure spectroscopy study of metal coordination in Co(II)-substituted Carcinus maenas hemocyanin

High-resolution x-ray absorption near edge structure spectroscopy was used to characterize the metal sites in three different cobalt-substituted derivatives of Carcinus maenas hemocyanin (Hc), including a mononuclear cobalt, a dinuclear cobalt and a copper-cobalt hybrid derivative. Co(II) model complexes with structures exemplifying octahedral, trigonal bipyramidal, pseudo-tetrahedral, and square planar geometries were also studied. The results provide structural information about the metal binding site(s) in the Co-Hcs that extend earlier results from EPR and optical spectroscopy (Bubacco et al. 1992. Biochemistry. 31: 9294-9303). Experimental spectra were compared to those calculated for atomic clusters of idealized geometry, generated using a multiple scattering approach. The energy of the dipole forbidden 1s-->3d transition and of the absorption edge in the spectra for all cobalt Hc derivatives confirmed the cobaltous oxidation state which rules out the presence of an oxygenated site. Comparisons between data and simulations showed that the mononuclear and dinuclear Co(II) derivatives, as well as the hybrid derivative, contain four-coordinate Co(II) in distorted tetrahedral sites. Although the spectra for Co(II) in dinuclear metal sites more closely resemble the simulated spectrum for a tetrahedral complex than do spectra for the mononuclear derivative, the Co(II) sites in all derivatives are very similar. The Cu K-edge high resolution x-ray absorption near edge structure spectrum of the hybrid Cu-Co-Hc resembles that of deoxy-Hc demonstrating the presence of three-coordinate Cu(I).

Purification and characterization of N-glycolyneuraminic-acid-specific lectin from Scylla serrata

A sialic-acid-binding lectin with specificity for N-glycolylneuraminic acid (NeuGc) was purified from the hemolymph of the marine crab Scylla serrata by affinity chromatography using thyroglobulin-coupled agarose. The binding specificity of Scylla lectin distinguishes it from other known sialic-acid-specific lectins found in Limulus polyphemus and Limax flavus, which show a broader range of specificity for sialic acids. The molecular mass of the purified lectin is about 55 kDa. Under reducing conditions (SDS/PAGE), it resolved into two subunits of 30 kDa and 25 kDa. NeuGc inhibited hemagglutination activity of the purified lectin at a concentration as low as 0.6 mM, whereas N-acetylneuraminic acid (NeuAc) even at a concentration of 100 mM, failed to inhibit hemagglutination. This finding was supported by potent inhibition of hemagglutination by bovine and porcine thyroglobulins, which contain a NeuGc alpha 2-6Gal as terminal component of oligosaccharide residues. Neither glycoproteins (glycophorin NN; porcine submaxillary mucin), which contain NeuAc alpha 2-3Gal/GalNAc and NeuAc alpha 2-6GalNAc, nor human acid glycoprotein, which contains NeuAc alpha 2-3/alpha 2-6 Gal, or colominic acid, a sialopolymer with NeuAc alpha 2-8NeuAc, inhibited the lectin activity. The specificity of the lectin for NeuGc appears to account for the fact that it agglutinates rabbit and mice erythrocytes, but not human A, O, AB, rat or chicken erythrocytes, which contain NeuAc. The inability of the lectin to agglutinate erythrocytes (horse) that prominently express NeuGc could be due to O-acetylation of NeuGc. In support of this, bovine submaxillary mucin, which contains O-acetylated NeuGc inhibited the hemagglutination of the lectin better after removal of O-acetyl groups by base treatment. The unique specificity of Scylla lectin is of diagnostic potential for human cancer tissues expressing NeuGc, since NeuGc is not found in normal human tissues.

Kinetics of the equilibration of O2 with Panulirus interruptus hemocyanin subunits a, b and c

The kinetics of the equilibration (25 degrees C) of O2 with the separated a, b and c subunits of Panulirus interruptus hemocyanin (Hc) as monomer and hexamer forms, as well as the native unfractionated abc mix, have been studied at pH values in the range 6.8-9.6, I = 0.100 M (NaCl). Rate constants kon and koff defined by deoxyHc + O2 <=> oxyHc have been determined by temperature-jump and stopped-flow techniques, respectively. The aggregation of monomer forms of different subunits to give hexamer is favoured by low pH and the availability of Ca2+, traces of which (or any other 2 + metals) are effectively removed by complexing with EDTA (5 mM). With or without EDTA, the hexamer form is present at the lower pH values. At the higher pH values with EDTA present a and b but not c give monomer forms. The hexamers are however retained at the higher pH values on addition of Ca2+ (10 mM). Cooperativity is observed for the hexamer forms at pH > 8, where the existence of relaxed (R) and tense (T) forms gives rise to sigmoidal kinetic plots in the determination of koff. Subunit c is different in that it retains its hexamer structure over the whole pH range, and does not display a Bohr effect. Native unfractionated protein is present as a hexamer mix of a, b and c in non-stoichiometric amounts, which has an enhanced Bohr effect as compared to the separated subunits.