The function of coelomic cell hemoglobin in the polychaete Glycera dibranchiata

Functional and structural characterization of the myoglobin from the polychaete Ophelia bicornis

The myoglobin of the polychaete annelid Ophelia bicornis was isolated, purified to homogeneity and characterized. The primary structure, obtained from cDNA and protein sequencing, consists of 139 amino acid residues. The alignment with other globin sequences showed that O. bicornis myoglobin misses the pre-A helix and the first six residues of the A helix. The presence of a PheB10-GlnE7 haem distal residue pair is in agreement with the measured oxygen affinity (P50=0.85 mmHg; 1 mmHg=0.133 kPa) and the only slightly higher autoxidation rate constant (0.28 h(-1)) with respect to that of the sperm whale myoglobin mutant E7 His-->Gln (0.21 h(-1)) and to elephant myoglobin (0.1 h(-1)). Oxygen-binding co-operativity was found to be absent under all the examined experimental conditions. The resistance of O. bicornis myoglobin towards autoxidation seems to confirm the important role of part of the A helix in the stability of the globin. The higher pKa of the acid-alkaline ferric transition of O. bicornis with respect to Asian elephant myoglobin, as well as the higher absorbance ratio of its ferric form to the oxy form measured in the Soret region (gammamet/gammaoxy) with respect to that of the African elephant myoglobin, suggested a stronger interaction between the distal glutamine and the water molecule at the sixth co-ordinate position.

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.

Functional differentiation in trematode hemoglobin isoforms

The Hbs and the major electrophoretic Hb components (isoHbs) were isolated from three species of the trematodes, Explanatum explanatum (Ee), Gastrothylax crumenifer (Gc) and Paramphistomum epiclitum (Pe), that parasitise the common Indian water buffalo Bubalus bubalis. The Hbs are monomeric and resemble the so-called nonfunctional mutant hemoglobins that have Tyr at B10 or E7 positions (replacing Leu and the His residues, respectively). However, they are capable of binding with O2 and CO. O2 equilibrium studies of trematode Hb isoforms reveal extremely high O2 affinities, with half-saturation O2 tension (P50) values up to 800 times lower than those of human hemoglobins. This correlates with Tyr residues at B10 and at the distal position (E7) that decrease the O2 dissociation rate by contributing hydrogen bonds (H-bonds) to the bound O2. These substitutions also increase the O2 association rates either due to orientation of E7-Tyr towards the solvent and/or by sterically hindering the entry of water molecules into the heme pocket. The latter may account for the low rate of autoxidation of trematode Hbs. The Hbs and their isoforms from different species exhibited pronounced variation in O2 affinity, which may relate to subtle differences in the structure of the heme pocket. The O2 affinities of the composite (unfractionated) Hbs were intermediate to those of the individual Hb isoform. The P50 values of Hbs here obtained by direct O2 equilibrium measurements differed from those calculated from kinetic data already published [Kiger, L., Rashid, A. K., Griffon, N., Haque, M., Moens, L.,Gibson, Q. H., Poyart, C., & Marden, M. C. (1998). Biophys. J. 75, 990-998.] Intermediate state(s) due to slow reorientation of E7-Tyr may account for this difference. Some Hb isoforms showed slight (either normal or reverse) Bohr effects. The hyperbolic O2 equilibrium curve, Hill coefficient (n) values near unity accord with a monomeric nature of trematode Hbs. In marked contrast to vertebrate Hbs, CO does not seem to compete effectively with O2 in trematode Hbs, as evident from partition coefficient values (M) below 1.

Structural features of Glycera dibranchiata monomer hemoglobins. Primary sequences of monomer hemoglobin components II and III

Primary sequences for the remaining two members (GMH2, GMH3) of the group of three major monomeric hemoglobins from the marine annelid Glycera dibranchiata have been obtained. Full sequences of each 147-amino acid globin were achieved with a high degree of confidence using standard Edman technology in combination with molecular mass determinations of the intact globins and of the cyanogen bromide cleavage fragments using electrospray ionization mass spectrometry. When minor assumptions concerning Q/E identities are made these new results indicate the likely correspondence of GMG2 with the protein represented by the first Glycera dibranchiata monomer hemoglobin complete sequence [Imamura et al., (1972), J. Biol. Chem. 247, 2785-2797]. When these new sequences are combined with the previously determined primary sequence for the third major monomer hemoglobin, GMH4 [Alam et al., J. Protein Chem. (1994), 13, 151-164], it becomes clear that these three (GMG2-4) are truly distinct proteins, contrary to previous suggestions. Surprisingly, our results show that none of these three primary sequences is identical to the published sequence of the refined monomer hemoglobin crystal structure protein; however, there is a strong correspondence to the GMG2 sequence. The present sequencing results, in combination with the published GMH4 sequence, confirm the presence of a distal Leu in place of the more commonly encountered distal His in all three of the major monomer hemoglobins isolated in this laboratory and indicate that the unusual B10 Phe occurs only in GMH4. Analysis of the sequences presented here, along with comparison of amino acid content for Glycera dibranchiata monomer hemoglobins isolated from three different laboratories, and comparison of NMR results from two laboratories suggest further correspondence which unify disparate published isolations.

The heterogeneity of the polymeric intracellular hemoglobin of Glycera dibranchiata and the cDNA-derived amino acid sequence of one component

The erythrocytes of the marine polychaete Glycera dibranchiata contain a number of different, single-chain hemoglobins, some of which self-associate into a 'polymeric' fraction. An oligodeoxynucleotide probe was synthesized based on partial amino acid sequences determined by chemical methods, and used to screen a cDNA library constructed from the poly(A+)mRNA of Glycera erythrocytes (Simons, P.C. and Satterlee, J.D. (1989) Biochemistry 28, 8525-8530). The longest positive inserts found were sequenced using the dideoxy nucleotide chain termination method. One complete clone was obtained: clone 5A, 816 bases long, contained 59 bases of 5'-untranslated RNA, an open reading frame of 441 bases coding for 147 amino acids and a 3'-untranslated region of 316 bases. The derived amino acid sequence of Glycera globin P1 was in agreement with the partial amino acid sequences obtained by chemical methods. Three additional inserts obtained in the screening were also sequenced: the inferred amino acid sequences proved to be partial globin sequences which were different from each other and from the sequence of P1. Thus, the 'polymeric' fraction of the intracellular hemoglobin of Glycera probably consists of at least four different globin chains much like the 'monomeric' fraction. Comparison of the 'polymeric' sequence with the two known 'monomeric' sequences, M-II and M-IV, shows that they share 54 identical residues. At 74 positions, the identical residues in M-II and M-IV differ from the corresponding residue in P1, including at E-7, where P1 has a distal His, in contrast to Leu in M-II and M-IV. The alignment of Bashford et al. ((1987) J. Mol. Biol. 196, 199-216) and their templates were used to examine the principal differences between the two types of Glycera globin sequences. They appear to consist of uncommon surface amino acid residues at positions C6 (Phe vs. Ala), E10 (Val vs. Lys), E17 (Lys vs. Val), G1 (Arg vs. Lys), G10 (Met vs. Ala) and H5 (Arg vs. Lys). One or more of these residues could be responsible for the self-association exhibited by the 'polymeric' Glycera globins.

Glycera dibranchiata hemoglobin. Structure and refinement at 1.5 A resolution

The coelomic cells of the common marine bloodworm Glycera dibranchiata contain several hemoglobin monomers and polydisperse polymers. We present the refined structure of one of the Glycera monomers at 1.5 A resolution. The molecular model for protein and ordered solvent for the deoxy form of the Glycera monomer has been refined to a crystallographic R-factor of 12.7% against an X-ray diffraction dataset at 1.5 A resolution. The positions of 1095 protein atoms have been determined with a maximum root-mean-square (r.m.s.) error of 0.13 A, and the r.m.s. deviation from ideal bond lengths is 0.015 A and from ideal bond angles is 1.0 degree. The r.m.s. deviation of planar groups from their least-squares planes is 0.007 A, and the r.m.s. deviation for torsion angles is 1.2 degrees for peptide groups and 16.8 degrees for side-chains. A total of 153 water molecules has been located, and they have been refined to a final average occupancy of 0.80. Multiple conformations have been found for five side-chains, and a change has been suggested for the sequence at five residues. The heme group is present in the "reverse" orientation that differs only in the positions of the vinyl beta-carbons from the "normal" orientation. The doming of the heme towards the proximal side, and the bond distances and angles of the heme and proximal histidine are typical of most deoxy globin structures. The substitution of leucine for the distal histidine residue (E7) creates an unusually hydrophobic heme pocket.

Conformational disorder of the distal leucine in monomeric Glycera hemoglobins and implications for oxygen binding

1H NMR studies of the carbon monoxide complexes of the major monomeric hemoglobins from Glycera dibranchiata show that distal leucine is conserved at position E7. The observed ring current shifts and nuclear Overhauser enhancements indicate conformational disorder of the leucine E7 side chain. The conformational substates interconvert rapidly on the NMR time scale. The rapid conformational fluctuations of leucine E7 may play a fundamental role in governing diffusion of ligands to the heme.

Evaluation of the extent of heterogeneity in the Glycera dibranchiata monomer haemoglobin fraction by the use of n.m.r. and ion-exchange chromatography

The coelomic haemoglobin of Glycera dibranchiata is known to be separable into monomeric and higher-Mr fractions. Although exhibiting homogeneity with respect to Mr, the extent of haemoglobin heterogeneity for the monomer fraction has never been adequately assayed. In the present paper we demonstrate that there exists in the monomer haemoglobin fraction reproducibly detectable heterogeneity regardless of the presence or absence of proteinase inhibitors during the isolations. These results show that, considered on the same time scale as previous preparations used for amino acid sequencing, crystallography and kinetics, the monomer haemoglobin fraction is highly heterogeneous. Application of ion-exchange chromatography and ion-filtration methods resulted in the isolation of four resolvable haem protein components from the Glycera monomer haemoglobin fraction. Three of these components were isolated in sufficient quantity to employ proton n.m.r. as a successful analytical tool for discriminating the individual haemoglobins. These results are not surprising. Several previous studies indicated less extensive heterogeneity in the monomer fraction. Moreover, the ability of the Glycera monomer haemoglobin to bind oxygen at even quite low partial pressures has been attributed to functional diversity originating in multiple haemoglobin components. The present work reveals the extent of the haemoglobin heterogeneity. The results show that it is more extensive than previously believed. Examination of this monomer fraction is particularly important, since crystallography indicates that one of the components of the monomer fraction lacks the E-7 (distal) histidine residue. As a consequence, the identification of such extensive heterogeneity is important to many previously published ligand-binding studies.

Anomalous pH dependence of the heme-bound carbon monoxide spectroscopic properties in the Glycera dibranchiata monomer hemoglobin fraction compared to vertebrate hemoglobins

The pH dependence of infrared and NMR spectroscopic parameters for carbon monoxide bound to human, equine, rabbit and Glycera dibranchiata monomer fraction hemoglobins has been examined. In all cases, the vertebrate hemoglobins exhibit CO vibrations and 13CO chemical shifts which are pH dependent, whereas the invertebrate hemoglobin does not. The Glycera dibranchiata monomer fraction exhibits the highest wavenumber CO vibration (1970 cm-1) and the most shielded chemical shift (206.2 ppm). The pH behavior of the vertebrate CO-hemoglobins is that the heme-coordinated carbon monoxide chemical shifts and principal infrared vibrations tend toward the values observed for the G. dibranchiata CO-hemoglobin fraction. These results are interpreted as originating in protonation of the distal histidine (E-7) in the vertebrate hemoglobins. The anomalous values for Glycera dibranchiata are concluded to be due to the absence of a distal histidine (E-7 His----Leu) in the heme pocket and not to gross structural dissimilarities between the proteins of the different species examined. Primary sequence similarity matrices have been constructed to compare the functional classes of amino acids at homologous positions for the CD and E helices and for the primary heme contacts in human, equine, sperm whale myoglobin, and the Glycera dibranchiata monomer hemoglobin to illustrate this point. They reveal a high correspondence for all globins and do not correlate with the spectroscopic parameters of heme-coordinated CO.

Electron addition to the (FeO2) unit of oxyhaemoglobin Glycera

Exposure of glassy solutions containing the monomeric fraction of the oxyhaemoglobin of the polychaete annelid Glycera dibranchiata to 60Co gamma-rays at 77 K resulted in electron addition to the (FeO2) moiety. The form of the g tensor components obtained from the ESR spectrum indicates that the spin-density on oxygen is much greater than that observed for similar paramagnetic centres formed in haemoglobin A or myoglobin. A major difference between these monomer haem units and normal haem units is that the distal histidine (E7 58) is replaced by leucine. We therefore postulate that the oxygen in the (FeO2)- units formed in haemoglobin A and myoglobin is hydrogen-bonded to the NH group of the distal histidine, whilst that of the (FeO2)- units in haemoglobin Glycera are not hydrogen-bonded. However, on annealing to approx. 160 K the spectrum changed irreversibly into one resembling those for (FeO2)- units in haemoglobin A and myoglobin. We postulate that this is caused by hydrogen-bonding to a water molecule in the haem pocket. Exposure of the polymeric fractions of haemoglobin Glycera to gamma-rays gave an (FeO2)- unit with an ESR spectrum remarkably similar to that obtained from oxymyoglobin. The X-ray structure of this protein is unknown but we suggest that our results could indicate the presence of a distal histidine in this material.

Solution studies on heme proteins. Circular dichroism and optical rotation of Glycera dibranchiata hemoglobins

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of several liganded derivatives of the monomer and polymer hemoglobin components of the marine annelid, Glycera dibranchiata were measured over the wavelength range 650--195 nm. The differences observed between the monomer and polymer components for the heme dichroic bands in the visible, Soret and ultraviolet wavelength regions seem to result from changes in the heme environment, geometry and coordination state of the central heme iron in these proteins. Within the Soret region, the liganded derivatives of the monomer hemoglobin exhibit predominantly negative circular dichroic bands. The heme band at 260 nm is also absent for the monomer hemoglobin. The ORD and CD spectra in the far-ultraviolet, peptide absorbing region suggest also differences in the alpha-helix content of the monomer and polymer hemoglobins. The values for the single-chain G. dibranchiata hemoglobin are in the expected range (about 70% alpha-helix) as predicted by the X-ray structure of this protein. The lower estimates of the alpha-helix content for the polymer hemoglobin (approx. 50%), may reflect the differences in amino acid composition, primary structure and polypeptide chain foldings. Changes in oxidation state and ligand binding appears to have no pronounced effect on the helicity of either the monomer or polymer hemoglobins. The removal of the heme moiety from the monomer hemoglobin did result in a major decrease in its helix content similar to the loss of heme from myoglobin.

The function of hemocyanin in respiration of the blue crab Callinectes sapidus

Blood PO2 in the blue crab Callinectes sapidus, a very active species of tropical origin, is lower at 22 degrees C than that of larger crabs in colder waters. These low oxygen levels permit its hemocyanin to be highly oxygenated at the gill, and to deliver almost half of its oxygen to the tissues in resting animals. Sustained muscular activity results in conspicuous decreases in blood PO2, pH and hemocyanin oxygenation. Although the venous reserve is fully utilized, hemocyanin oxygenation at the gill decreases so much that there is no change in its total quantitative function. The large Bohr shift becomes functional during activity, but its quantitative importance is not clear.

The influence of environmental salinity on hemocyanin function in the blue crab, Callinectes sapidus

The effects of inorganic ions and of the hydrogen ion on oxygen-binding properties of most respiratory pigments are opposite. The addition of salt to the medium increases oxygen affinities, and the addition of H+ decreases oxygen affinities of crustacean hemocyanins. These oxygenation properties, as observed in vitro, suggest that the oxygen-transport system must adapt to ionic changes in the blood. In fact, decreases in the salt concentration of the blood of estaurine blue crabs are accompanied by increases in pH, probably resulting from the input of ammonia produced in deamination of the intracellular pool of free amino acids as the cells conform to osmotic changes in body fluids. The result is a stability of hemocyanin function until the blood becomes very dilute. As the acclimation salinity is reduced from 35 to 15 o/ooo, the ionic effects on respiratory transport are balanced and there is no change in total oxygen uptake. At 5 o/ooo salinity, however, the higher blood pH is manifested in an elevation of the total oxygen concentration of prebranchial blood, probably because the Bohr shift is no longer opposed by a critical level of salt in the blood. Under these conditions the role of hemocyanin in aerobic respiration is reduced at high environmental oxygen levels, but it may be enhanced in hypoxic uaters.