The amino acid sequence of myoglobin from the mollusc Aplysia limacina

Heme protein identified from scaly-foot gastropod can synthesize pyrite (FeS2) nanoparticles

The scaly-foot gastropod (Chrysomallon squamiferum), which lives in the deep-sea zone of oceans around thermal vents, has a black shell and scales on the foot. Both the black shell and scales contain iron sulfide minerals such as greigite (Fe₃S₄) and pyrite (FeS₂). Although pyrite nanoparticles can be used as materials for solar panels, it is difficult to synthesize stable and spherical nanoparticles in vitro. In this study, we extracted organic molecules that interact with nano-pyrite from the shell of the scaly-foot gastropod to develop a low-cost, eco-friendly method for pyrite nanoparticles synthesis. Myoglobin (csMG), a heme protein, was identified in the iron sulfide layer of the shell. We purified recombinant csMG (r-csMG) and demonstrated that r-csMG helped in the conversion of ferric ions, sulfide ions and sulfur into spherical shaped pyrite nanoparticles at 80°C. To reduce the effort and cost of production, we showed that commercially available myoglobin from Equus caballus (ecMG) also induced the in vitro synthesis of pyrite nanoparticles. Using structure-function experiments with digested peptides, we highlighted that the amino acid sequence of r-csMG peptides controlled the spherical shape of the nanoparticle while the hemin molecules, which the peptides interacted with, maintained the size of nanoparticles. Synthesized pyrite nanoparticles exhibited strong photoluminescence in the visible wavelength region, suggesting its potential application as a photovoltaic solar cell material. These results suggest that materials for solar cells can be produced at low cost and energy under eco-friendly conditions. STATEMENT OF SIGNIFICANCE: Pyrite is a highly promising material for photovoltaic devices because of its excellent optical, electrical, magnetic, and transport properties and high optical absorption coefficient. Almost all current pyrite synthesis methods use organic solvents at high temperature and pressure under reducing conditions. Synthesized pyrite nanoparticles are unstable and are difficult to use in devices. The scaly-foot gastropod can synthesize pyrite nanoparticles in vivo, meaning that pyrite nanoparticles can be generated in an aqueous environment at low temperature. In this study, we demonstrated the synthesis of pyrite nanoparticles using a heme protein identified in the iron sulfide layer of the scaly-foot gastropod shell. These results exemplify how natural products in organisms can inspire the innovation of new technology.

Structure-Function Relationships in Unusual Nonvertebrate Globins

Based on the literature and our own results, this review summarizes the most recent state of nonvertebrate myoglobin (Mb) and hemoglobin (Hb) research, not as a general survey of the subject but as a case study. For this purpose, we have selected here four typical globins to discuss their unique structures and properties in detail. These include Aplysia myoglobin, which served as a prototype for the unusual globins lacking the distal histidine residue; midge larval hemoglobin showing a high degree of polymorphism; Tetrahymena hemoglobin evolved with a truncated structure; and yeast flavohemoglobin carrying an enigmatic two-domain structure. These proteins are not grouped by any common features other than the fact they have globin domains and heme groups. As a matter of course, various biochemical functions other than the conventional oxygen transport or storage have been proposed so far to these primitive or ancient hemoglobins or myoglobins, but the precise in vivo activity is still unclear. In this review, special emphasis is placed on the stability properties of the heme-bound O2. Whatever the possible roles of nonvertebrate myoglobins and hemoglobins may be (or might have been), the binding of molecular oxygen to iron(II) must be the primary event to manifest their physiological functions in vivo. However, the reversible and stable binding of O2 to iron(II) is not a simple process, since the oxygenated form of Mb or Hb is oxidized easily to its ferric met-form with the generation of superoxide anion. The metmyoglobin or methemoglobin thus produced cannot bind molecular oxygen and is therefore physiologically inactive. In this respect, protozoan ciliate myoglobin and yeast flavohemoglobin are of particular interest in their very unique structures. Indeed, both proteins have been found to have completely different strategies for overcoming many difficulties in the reversible and stable binding of molecular oxygen, as opposed to the irreversible oxidation of heme iron(II). Such comparative studies of the stability of MbO2 or HbO2 are of primary importance, not only for a full understanding of the globin evolution, but also for planning new molecular designs for synthetic oxygen carriers that may be able to function in aqueous solution and at physiological temperature.

1H-NMR study of dynamics and thermodynamics of Cl− binding to ferric hemoglobin of a midge larva (Tokunagayusurika akamusi)

The functional properties of the Arg residue at the E10 helical position in myoglobin and hemoglobin lacking the highly conserved His residue at the E7 position have received considerable interest as to the structure-function relationship of the oxygen-binding hemoproteins, because Arg E10 in such proteins has been shown to play similar roles to those His E7 plays in ordinary proteins. One of the components of hemoglobin from the larval hemolyph of Tokunagayusurika akamusi is also a naturally occurring E7 genetic variant with Ile E7 and Arg E10. This study demonstrated, for the first time, that the positively charged, elongated, and flexible side-chain of Arg E10 in T.akamusi hemoglobin contributes to stabilization of the coordination of biologically relevant Cl(-) to heme iron. Determination of the dynamics of the Cl(-) binding to T. akamusi ferric hemoglobin involving paramagnetic 1H-NMR indicated that the Cl(-) affinity increases with decreasing pH as a result of the fact that the binding rate increases with decreasing pH, whereas the dissociation rate is almost completely independent of pH. The pH-dependent character of the Cl(-) binding rate correlated well with the ionization state of heme peripheral side-chain propionate groups, which was clearly manifested in the pH-dependent shift changes of heme methyl proton signals, suggesting that negative charges of heme propionate groups constitute a kinetic barrier for Cl(-) entry into the heme pocket. These findings provide an insight into the pH-dependent ligand binding properties of T. akamusi hemoglobin.

Solution (1)H NMR study of the influence of distal hydrogen bonding and N terminus acetylation on the active site electronic and molecular structure of Aplysia limacina cyanomet myoglobin

The sea hare Aplysia limacina possesses a myoglobin in which a distal H-bond is provided by Arg E10 rather than the common His E7. Solution (1)H NMR studies of the cyanomet complexes of true wild-type (WT), recombinant wild-type (rWT), and the V(E7)H/R(E10)T and V(E7)H mutants of Aplysia Mb designed to mimic the mammalian Mb heme pocket reveal that the distal His in the mutants is rotated out of the heme pocket and is unable to provide a stabilizing H-bond to bound ligand and that WT and rWT differ both in the thermodynamics of heme orientational disorder and in heme contact shift pattern. The mean of the four heme methyl shifts is shown to serve as a sensitive indicator of variations in distal H-bonding among a set of mutant cyanomet globins. The heme pocket perturbations in rWT relative to WT were traced to the absence of the N-terminal acetyl group in rWT that participates in an H-bond to the EF corner in WT. Analysis of dipolar contacts between heme and axial His and between heme and the protein matrix reveal a small approximately 2 degrees rotation of the axial His in rWT relative to true WT and a approximately 3 degrees rotation of the heme in the double mutant relative to rWT Mb. It is demonstrated that both the direction and magnitude of the rotation of the axial His relative to the heme can be determined from the change in the pattern of the contact-dominated heme methyl shift and from the dipolar-dominated heme meso-H shift. However, only NOE data can determine whether it is the His or heme that actually rotates in the protein matrix.

Aplysia limacina myoglobin cDNA cloning: an alternative mechanism of oxygen stabilization as studied by active-site mutagenesis

The isolation and cloning of the cDNA coding for myoglobin (Mb) from the mollusc Aplysia limacina is reported here. Five amino acid differences from the previously published protein sequence have been found in positions 22, 26, 27, 77 and 80 by back transplanting the cDNA; some of these may be relevant for overall structure stabilization in this Mb. High-level expression of the holoprotein in Escherichia coli has been achieved in the presence of the haem precursor delta-aminolevulinic acid, underlying the importance of tuning haem and apoprotein biosynthesis to achieve high-level expression of haemproteins in bacteria. The recombinant protein is identical to the protein purified from the mollusc buccal muscle. Native A. limacina Mb has an oxygen dissociation rate constant of 70 s(-1) [as compared with the value of 15 s(-1) for sperm whale Mb, which displays His(E7) and Thr(E10)] (amino acid positions are referred to within the eight helices A-H of the globin fold). In order to understand the mechanism of oxygen stabilization in A. limacina Mb, we have prepared and investigated three active-site mutants: two single mutants in which Val(E7) and Arg(E10) have been replaced by His and Thr, respectively, and a double mutant carrying both mutations. When Arg(E10) is substituted with Thr, the oxygen dissociation rate constant is increased from 70 s(-1) to more than 700 s(-1), in complete agreement with the previously proposed role of the former residue in ligand stabilization. In the His(E7)-containing single and double mutants, both displaying high oxygen dissociation rates, the stabilization of bound oxygen by the distal His is insufficient to slow down the ligand dissociation rate constant to the value of sperm whale Mb. These results essentially prove the hypothesis that in A. limacina Mb a mechanism of oxygen stabilization involving Arg(E10), and thus different from that mediated by His(E7), has evolved.

1H-NMR and EPR studies on met-azido and met-imidazole Dolabella auricularia myoglobin

Met-azido and met-imidazole forms of the myoglobin from the mollusc Dolabella auricularia have been studied by 1H-NMR and EPR spectroscopy. In the mollusc myoglobin, in which His-E7 is replaced by Val, the guanidino group of Arg-E10 serves as an alternative hydrogen-bond donor to the bound ligand. Therefore, the guanidino group of Arg-E10 plays similar roles in ligand stabilization to that the His-E7 imidazole does in most vertebrate myoglobins. Differences in both the structural and electronic properties between Arg and His side chains largely affect the stability of met-azido and met-imidazole forms of the protein. Due to a weak stabilization by Arg-E10, the bound-N3- ligand is replaced by OH- at higher pH, although it is stable at neutral and acidic pH. In the absence of the hydrogen-bonding interaction, Fe-bound imidazole in met-imidazole Dolabella myoglobin is only stable at neutral pH and is removed at acidic pH and replaced by OH- at basic pH. The temperature study also revealed that the bound imidazole is replaced by OH- at higher temperature. These results confirm that the presence of steric hindrance between these bulky ligands and the long and bulky side chain of Arg-E10 in the distal pocket of the mollusc myoglobin. Thus steric effects contribute significantly to the stability of exogenous ligand in the distal pocket of myoglobin.

Polymorphic hemoglobin from a midge larva (Tokunagayusurika akamusi) can be divided into two different types

The hemoglobin from the 4th-instar larva of Tokunagayusurika akamusi, a common midge found in eutrophic lakes in Japan, was composed of as many as 11 separable components (IA, IB, II, III, IV, V, VIA, VIB, VII, VIII, IX) on a DEAE-cellulose column. However, we have found that these components can be divided into two groups on the basis of their spectroscopic properties, one being named as the normal type (N-type) and the other being referred to as the low type (L-type). Since the major difference between them seemed to be the presence or absence of the distal (E7) histidine residue, which plays an important role in the stability properties of the bound dioxygen, the complete amino acid sequence was then determined for each typical component, namely, VII (N-type) and V (L-type): the former hemoglobin contained the usual distal histidine residue at position 64, whereas the latter one replaced it by isoleucine at position 66. The homology test for 40 N-terminal amino acid residues of all components also demonstrates that T. akamusi hemoglobin is composed of two different clusters showing a very early separation in the phylogenetic tree.

Amino-acid sequence of the cooperative dimeric myoglobin from the radular muscles of the marine gastropod Nassa mutabilis

The complete amino-acid sequence of the dimeric and cooperative myoglobin from the radular muscles of Nassa mutabilis, a common edible gastropod mollusc on the Italian coast, has been determined. The molecule is a homodimer. The monomer is composed of 147 amino-acid residues, with a molecular mass of 15,760 Da. Its sequence is homologous with those of the dimeric myoglobins of the gastropod molluscs of the Prosobranchia subclass Busycon canaliculatum (63% conserved residues) and Cerithidea rhizophorarum (46% conserved residues). The rate of autoxidation to met-myoglobin of N. mutabilis oxymyoglobin at 25 degrees C is strongly pH-dependent with relative minimal rate values in the pH range 7 to 8.

Solution 1H nuclear magnetic resonance determination of hydrogen bonding of the E10 (66) Arg side-chain to the bound ligand in Aplysia cyano-met myoglobin

A combined one-dimensional nuclear Overhauser effect, paramagnetic-induced relaxation and two-dimensional sequence-specific 1H n.m.r. assignment of the spectrum of portions of the distal pocket of Aplysia cyano metMyoglobin (metMbCN) has been carried out in order to establish the presence and identity of distal residues in the heme pocket. In the absence of the usual distal E7 His in Aplysia Mb (E7 Val), the sequence-specific assignment of the E7 and E10 residues, together with their hyperfine shift patterns, relaxivities and dipolar connectivities to each other and the remainder of the E helix, reveal that the E10 Arg is turned into the pocket and hydrogen bonds to the bound cyanide group. We have previously found a similar rearrangement of the E10 Arg in Aplysia fluoro metMyoglobin, and the stabilizing effect of this residue was proposed to be responsible for the slow rate of cyanide dissociation from rapidly reduced ferrous Aplysia myoglobin. Based on the similar distal E7 His hydrogen-bonding interaction to the bound ligand in the crystal of sperm whale MbO2 and in solution of its cyano met complex, we propose that the E10 Arg similarly hydrogen bonds to the bound O2 in Aplysia MbO2 and accounts for its strong ligand binding and slow dissociation rate.

Isolation and sequencing of a cDNA for an unusual hemoglobin from the parasitic nematode Pseudoterranova decipiens

A cDNA clone encoding a 333-amino acid hemoglobin was isolated from the nematode Pseudoterranova decipiens. The protein contains an 18-amino acid hydrophobic signal sequence and has a calculated mass of 37.6 kDa in the mature form. The predicted protein reveals an internal duplication of a 154-amino acid domain (51% identity). Both domains have significant sequence homology to other primitive hemoglobins, in agreement with a duplication event. Hydrophobicity plots reveal identical strongly hydrophobic regions in each domain, which are potential heme binding sites. This confirms previous suggestions that nematode hemoglobins can have two heme groups per molecule. In addition, each domain contains several conserved histidine motifs that may serve as potential copper binding sites. This result provides further evidence that hemoglobins may have evolved from a primitive cytochrome-like molecule.

Binding mode of azide to ferric Aplysia limacina myoglobin. Crystallographic analysis at 1.9 A resolution

The binding mode of azide to the ferric form of Aplysia limacina myoglobin has been studied by X-ray crystallography. The three-dimensional structure of the complex has been refined at 1.9 A resolution to a crystallographic R-factor of 13.9%, including 126 ordered solvent molecules. Azide binds to the heme iron, at the sixth co-ordination position, and is oriented towards the outer part of the distal site crevice. This orientation is stabilized by an ionic interaction with the side-chain of Arg66 (E10) which, from an outer orientation in the 'aquo-met' ligand-free myoglobin, folds back towards the distal site in the presence of the anionic ligand. In the absence of a hydrogen bond donor residue at the distal E7 position in Aplysia limacina myoglobin, a different polar residue, Arg66 at the E10 topological position, has been selected by molecular evolution in order to grant ligand stabilization.

Proton nuclear magnetic resonance study of the molecular and electronic structure of the heme cavity in Aplysia cyanometmyoglobin

The 1H NMR spectrum of the low-spin, cyanide-ligated ferric complex of the myoglobin from the mollusc Aplysia limacina has been investigated. All of the resolved resonances from both the hemin and the proximal histidine have been assigned by a combination of isotope labeling, spin decoupling, analysis of differential paramagnetic relaxation, and nuclear Overhauser (NOE) experiments. The pattern of the heme contact shifts is unprecedented for low-spin ferric hemoproteins in exhibiting minimal rhombic asymmetry. This low in-plane asymmetry is correlated with the X-ray-determined orientation of the proximal histidyl imidazole plane relative to the heme and provides an important test case for the interpretation of hyperfine shifts of low-spin ferric hemoproteins. The bonding of the proximal histidine is shown to be similar to that in sperm whale myoglobin and is largely unperturbed by conformational transitions down to pH approximately 4. The two observed conformational transitions appear to be linked to the titration of the two heme propionate groups, which are suggested to exist in various orientations as a function of both pH and temperature. Heme orientational disorder in the ratio 5:1 was demonstrated by both isotope labeling and NOE experiments. The exchange rate with bulk water of the proximal histidyl labile ring proton is faster in Aplysia than in sperm whale myoglobin, consistent with a greater tendency for local unfolding of the heme pocket in the former protein. A similar increased heme pocket lability in Aplysia myoglobin has been noted in the rate of heme reorientation [Bellelli, A., Foon, R., Ascoli, F., & Brunori, M. (1987) Biochem. J. 246, 787-789].

Surface topography of histidine residues: a facile probe by immobilized metal ion affinity chromatography

Immobilized metal ion affinity chromatography (IMAC) has been explored as a probe into the topography of histidyl residues of a protein molecule. An evaluation of the chromatographic behavior of selected model proteins--thioredoxin, ubiquitin, calmodulin, lysozyme, cytochrome c, and myoglobin on immobilized transition metal ions (Co2+, Ni2+, Cu2+, and Zn2+)--allows establishment of the following facets of the histidyl side chain distribution: (i) either interior or surface; (ii) when localized on the surface, accessible or unaccessible for coordination; (iii) single or multiple; (iv) when multiple, either distant or vicinal. Moreover, proteins displaying single histidyl side chains on their surfaces may, in some instances, be resolved by IMAC; apparently, the microenvironments of histidyl residues are sufficiently diverse to result in different affinities for the immobilized metal ions. IMAC, previously introduced as an approach to the fractionation of proteins, has become also, upon closer examination, a facile probe into the topography of histidyl residues. This is possible because of the inherent versatility of IMAC; an appropriate metal ion (M2+) can be selected to suit the analytical purpose and a particular chromatographic protocol can be applied (isocratic pH, falling pH, and imidazole elution).

Aplysia limacina myoglobin. Crystallographic analysis at 1.6 A resolution

The crystal structure of the ferric form of myoglobin from the mollusc Aplysia limacina has been refined at 1.6 A resolution, by restrained crystallographic refinement methods. The crystallographic R-factor is 0.19. The tertiary structure of the molecule conforms to the common globin fold, consisting of eight alpha-helices. The N-terminal helix A and helix G deviate significantly from linearity. The distal residue is recognized as Val63 (E7), which, however, does not contact the heme directly. Moreover the sixth (distal) co-ordination position of heme iron is not occupied by a water molecule at neutrality, i.e. below the acid-alkaline transition point of A. limacina myoglobin. The heme group sits in its crevice in the conventional orientation and no signs of heme isomerism are evident. The iron atom is 0.26 A out of the porphyrin plane, with a mean Fe-N (porphyrin) distance of 2.01 A. The co-ordination bond to the proximal histidine has a length of 2.05 A, and forms an angle of 4 degrees with the heme normal. A plane containing the imidazole ring of the proximal His intersects the heme at an angle of 29 degrees with the (porphyrin) 4N-2N direction. Inspection of the structure of pH 9.0 indicates that a hydroxyl ion is bound to the Fe sixth co-ordination position.

Aplysia oxymyoglobin with an unusual stability property: involvement of two kinds of carboxyl groups

Unlike mammalian oxymyoglobins, Aplysia MbO2 is extremely susceptible to autoxidation, and its pH dependence is also unusual. Kinetic formulation has revealed that two kinds of dissociable group with pK1 = 4.3 and pK2 = 6.1, respectively, at 25 degrees C are involved in the stability property of Aplysia MbO2. In order to characterize thermodynamically these dissociation processes involved, the effect of temperature on K1 and K2 was studied by analyzing the pH dependence for the autoxidation rate of Aplysia MbO2 in 0.1 M buffer over the pH range of 4-11, and at 15, 25 and 35 degrees C. The resulting thermodynamic parameters for each group were both those to be expected for the ionization of a carboxyl group; the delta H degrees value being numerically much less than 1 kcal.mol-1, or zero in practice, but being associated with a large negative value of delta S degrees of the order of -20 cal.mol-1.K-1. Taking into account the fact that Aplysia myoglobin contains only a single histidine residue corresponding to the heme-binding proximal one, we can unequivocally conclude that the two kinds of the dissociable group involved in the unusual stability of Aplysia MbO2 must both be carboxyl groups, the protonation of these groups being responsible for an increase in its autoxidation rate in the acidic pH range.

A kinetic and equilibrium study of ligand binding to the monomeric and dimeric haem-containing globins of two chitons

The radular muscles of the amphineuran molluscs Amaurochiton glaucus and Sipharochiton pelliserpentis contain both a dimeric and a monomeric form of myoglobin. The dimeric form of the protein is composed of two polypeptide chains covalently linked to each other via one or more disulphide bonds. The dimeric protein shows co-operative O2-binding curves. Kinetic investigations indicate that CO binding is co-operative in the dimeric protein, subsequent to full photolysis, but mono-exponential following 10% photolysis. O2 recombination following part photolysis is mono-exponential in the dimeric form, whereas O2 dissociation kinetics indicates the presence of chain heterogeneity. The monomeric form of the protein exhibits mono-exponential time courses in all the experimental situations explored. Although the rate constants associated with the reactions of individual dimer and monomer molecular species are very different, the two species of chiton investigated show remarkably similar properties when compared with each other. All the reactions studied are pH-independent in the range pH 6-8. Amino acid analysis indicates that the monomeric units that combine to form the dimeric species are not identical with the naturally occurring monomeric form. A comparison is made between the chiton myoglobins and other similar O2-binding proteins.

An evolutionary tree for invertebrate globin sequences

A phylogenetic tree was constructed from 245 globin amino acid sequences. Of the six plant globins, five represented the Leguminosae and one the Ulmaceae. Among the invertebrate sequences, 7 represented the phylum Annelida, 13 represented Insecta and Crustacea of the phylum Arthropoda, and 6 represented the phylum Mollusca. Of the vertebrate globins, 4 represented the Agnatha and 209 represented the Gnathostomata. A common alignment was achieved for the 245 sequences using the parsimony principle, and a matrix of minimum mutational distances was constructed. The most parsimonious phylogenetic tree, i.e., the one having the lowest number of nucleotide substitutions that cause amino acid replacements, was obtained employing clustering and branch-swapping algorithms. Based on the available fossil record, the earliest split in the ancestral metazoan lineage was placed at 680 million years before present (Myr BP), the origin of vertebrates was placed at 510 Myr BP, and the separation of the Chondrichthyes and the Osteichthyes was placed at 425 Myr BP. Local "molecular clock" calculations were used to date the branch points on the descending branches of the various lineages within the plant and invertebrate portions of the tree. The tree divided the 245 sequences into five distinct clades that corresponded exactly to the five groups plants, annelids, arthropods, molluscs, and vertebrates. Furthermore, the maximum parsimony tree, in contrast to the unweighted pair group and distance Wagner trees, was consistent with the available fossil record and supported the hypotheses that the primitive hemoglobin of metazoans was monomeric and that the multisubunit extracellular hemoglobins found among the Annelida and the Arthropoda represent independently derived states.

Structural alignment and analysis of two distantly related proteins: Aplysia limacina myoglobin and sea lamprey globin

Two new globin structures have recently been determined at high resolution: the globin from the mollusc Aplysia limacina at 1.6 A resolution and a new refinement of the structure from sea lamprey. Two amino acid sequences of these homologous molecules have only 30% residue identity in an optimal alignment. We discuss some of the problems arising in the alignment of Aplysia globin with other globins of known structure, a challenging problem because of the distant relationship. Four independent approaches were applied to the alignment of the Aplysia and lamprey globins, including those based on individual sequence comparisons, structural analysis, and the relatively new method of templates or fingerprints derived for an entire family of proteins. We also compare these two new structures with what is already known about the globin family. A detailed description of the two structures shows that the two molecules contain the main structural features common to all the globins so far studied, with several minor but interesting hitherto unobserved variations.

Fluorescence and energy transfer of tryptophans in Aplysia myoglobin

The fluorescence decay of tryptophan residues in apo and met Aplysia limacina myoglobin and sperm whale myoglobin were measured in aqueous solution at 10 degrees-15 degrees C. In all species, multiexponential behavior was observed in which the individual components displayed unique frequency-dependent emission characteristics. The results suggest that the tryptophan fluorescence in all met samples are quenched by rapid Forster energy transfer to the heme as predicted from the crystal geometry. Fluorescence from the apo protein is similar to that in solutions of free tryptophans. In addition, the fluorescence properties of the reversible thermal denaturation of Aplysia limacina met myoglobin was investigated between 25 degrees and 75 degrees C.

Molecular models for the putative dimer of sea lamprey hemoglobin

The known structures for the tetramers of mammalian and clam hemoglobins provide a point of departure for the modeling of putative dimers of lamprey hemoglobin. The association of subunits is dissimilar for the clam and mammalian tetramers; the superposition of the molecular model for lamprey methemoglobin onto the mammalian and clam tetramers gives five distinct dimers. After energy minimization of the interface regions of the five models, three models afford promising interactions between side chains. One model is analogous to the alpha 1 beta 2 pairing of subunits of mammalian hemoglobins. The other two models are similar to the interfaces between the E and F helices and between the A and B helices of clam hemoglobin. Although the model based on the alpha 1 beta 2 mode of association provides the best explanation of biochemical properties of lamprey hemoglobin, such as the Bohr effect and the dependency of dimer formation on pH, interfaces between the E and F and the A and B helices could be important in the aggregation of monomers of lamprey hemoglobin beyond the level of the dimer.

Heme protein fluorescence versus pressure

Fluorescence spectra of several ferric heme proteins have been measured vs. pressure to 6,000 bars. Sperm whale myoglobin (SW Mb), Aplysia myoglobin, leghemoglobin (Lb), and cytochrome P450 all show excitation and emission spectra characteristic of tryptophan in proteins with peak emission at 330-340 nm. At one bar, the fluorescence is weak due to energy transfer to the heme group, which makes the yield a sensitive probe of protein unfolding at high pressure. After an initial decrease of a few percent per kbar, the protein shows a large increase in fluorescence at high pressure. The increase is pH dependent and the results indicate that several high pressure states occur. For SW Mb at 15 degrees C an increase of a factor of 20 occurs with midpoint at 2,000 bars at pH 5 and is only partially reversible, while the increase at pH 7 occurs at 4,000 bars and is only half as large and is completely reversible. Aplysia Mb and Lb show a similar effect, but unfold at a higher pressure than SW Mb. P450 also shows a transition to a state of higher fluorescence, but the transition in this case is irreversible as a stable form, P420, is formed. The fluorescence intensity measurements permit an estimation of the increase in the TRY-heme distance in the high pressure state.

Crystal structure of ferric Aplysia limacina myoglobin at 2 X 0 A resolution

The three-dimensional structure of ferric myoglobin from the mollusc Aplysia limacina has been refined at 2 X 0 A resolution. The crystallographic R factor, calculated at this stage, is 0 X 194. Despite its high content of apolar residues (both aromatic and aliphatic), Aplysia limacina myoglobin, which contains only one histidine residue (at the proximal position), has a structure that conforms to the common eight-helices globin fold observed in other phyla.

Thermodynamics of the reaction of ferric myoglobin from Aplysia limacina with azide and fluoride. Dependence of enthalpy changes on pH

The effect of pH on the enthalpy changes for binding of azide and fluoride to ferric myoglobin from Aplysia limacina, which lacks the distal histidine, has been investigated. Over the whole pH range explored (3.8 to 9.5), -delta H degrees values for the formation of the hemoprotein-ligand complexes are: (1) much greater than the variations in -delta G degrees; (2) always negative; and (3) show a dependence upon pH characterized by a maximum for azide and a minimum for fluoride binding, centered at pH 4.55 (identical to pHch). This value agrees well with that expected from the linear correlation between pHch and the simple function "Lys+Arg-Glu-Asp-2" proposed by Beetlestone and others. Data reported here greatly extend the pH range for which the linear correlation between the net charge of the macromolecule and pHch has been found to hold, and indicate unequivocally that the pH dependence of -delta H degree for the binding of anionic ligands does not uniquely require the presence of the histidyl residue at the distal position.

Aplysia myoglobins with an unusual amino acid sequence

The complete amino acid sequence of the myoglobin from Aplysia juliana, a species distributed world-wide, has been determined and compared with the sequence of the myoglobin of Aplysia limacina, a Mediterranean species, and of Aplysia kurodai, a Japanese and Asian species. Unlike mammalian myoglobins, Aplysia myoglobins contain only a single histidine residue, lacking the distal one, the homology being 76% between A. juliana and A. limacina, 74% between A. juliana and A. kurodai, and 83% between A. limacina and A. kurodai. The hydropathy profiles of the Aplysia myoglobins are very similar, but completely different from that of sperm whale myoglobin, taken as the reference.

Aplysia oxymyoglobin with an unusual stability property

Native oxymyoglobin was isolated directly from the radular muscle of Aplysia kurodai with complete separation from metmyoglobin on a DEAE-cellulose column. It was examined for its spectral and stability properties. The spectrum of Aplysia MbO2 , which lacks the distal histidine, is very similar to those of mammalian oxymyoglobins , the alpha-peak being higher than the beta-peak and the absorbance ratio being 1.03. Its stability, however, is quite different from those of the mammalian oxymyoglobins , and Aplysia MbO2 is found to be extremely susceptible to autoxidation. Its rate is one-hundred times higher at pH 9.0, and its pH dependence is unusual and much less steep, when compared with sperm whale MbO2 as reference.

Metal coordination centres of class II cytochromes c

The class II cytochromes Rhodospirillum molischianum cytochrome c', Rhodopseudomonas palustris cytochrome C556 and Agrobacterium tumefaciens (B2a) cytochrome c556 have been investigated with a variety of spectroscopic techniques. The cytochrome c' was found to be high-spin and the two cytochromes c556 were found to be mainly low-spin and sx-coordinate with the fifth and sixth ligands being histidine and methionine. The implications of the different types of iron coordination are discussed.

Hemoglobins, XLVIIII. The primary structure of a monomeric hemoglobin from the hagfish, Myxine glutinosa L.: evolutionary aspects and comparative studies of the function with special reference to the heme linkage

Hagfish hemoglobin has three main components, one of which is Hb III. It is monomeric and consists of 148 amino acid residues (M = 17 350). Its complete primary structure, previously published, is discussed here. The proximal amino acid (F8) of the heme linkage is histidine as always in the hemoglobins, but the regularly expected distal histidine E7 is substituted by glutamine. This substitution, leading to a new kind of heme linkage, has hitherto only been demonstrated in opossum hemoglobin. It is suggested that E7, Gln, is directed out of the heme pocket, and that the adjacent Ell, Ile, is directed toward the inside of the pocket, giving the distal heme contact instead of histidine. Myxine Hb III has an additional tail of 9 amino acid residues at its N-terminal end, as has the hemoglobin of Lampetra fluviatilis. The genetic codes of Myxine and Lampetra hemoglobins show 117 differences, in spite of many morphological resemblances between hagfish and lamprey. Their primary hemoglobin structures show differences substantial enough to be compatible with the divergence of the two families some 400-500 million years ago.

Evolutionary changes in protein composition -- evidence for an optimal strategy

The information contained in the composition of different proteins of the same family is analyzed. It is found that within each family the gain in information per amino acid replacement is constant. This finding is interpreted to imply that evolutionary changes in proteins follow an "optimal" path in the sense that they maximize the number of potentially functional sequences that can be generated by T accepted point mutations from a given protein, subject to restrictions due to biological function.

Isolation and characterization of the hemoglobin from the lanceolate fluke Dicrocoelium dendriticum

The hemoglobin of the flatworm Dicrocoelium dendriticum, a lanceolate fluke which infests the hepatic ducts of certain mammals, has been isolated by gel filtration and ion-exchange chromatography. The molecular weight of the denatured protein was found to be 15500, a value in the same range as hemoglobin subunits. The fact that the native hemoglobin has an apparent molecular weight of 22000 in 0.01 M phosphate buffer, pH 7.4, suggests limited aggregation. The protein contains, as all other myoglobins and hemoglobins, one molecule of non-covalently associated ferroprotoporphyrin IX per polypeptide chain. It forms the same ligand derivatives with very similar spectral properties as vertebrate hemoglobins. The high oxygen affinity (p50 is 0.07--0.1 mmHg or 9.3--13.3 Pa at 20 degrees C and pH 7.0) and the absence of heme-heme interaction of (Hill coefficient nH=1.0) are properties which this heme protein shares with other monomeric hemoglobins from invertebrate and lower vertebrate organisms. The native hemoglobin exists in two forms, having isoelectric points of 4.51 and 4.53, which do not differ in their amino-acid compositions. Dansylation indicated that the amino-terminal amino-acid residue is alanine. The carboxy-terminal sequence, determined by carboxypeptidase A digestion of the globin, is -His-Ala-Leu.

Oxygenation and EPR spectral properties of Aplysia myoglobins containing cobaltous porphyrins

Cobalt myoglobins (Aplysia) have been reconstituted from apo-myoglobin (Aplysia) and proto-, meso-, and deutero-cobalt porphyrins. Each of them showed the 30--60 times lower oxygen affinity than those of the corresponding cobalt myoglobins (Sperm whale). Kinetic investigation of their oxygenation by the temperature-junp relaxation technique showed that the low oxygen affinity of cobalt myoglobin (Aplysia) is due to a large dissociation rate constant. the electron paramagnetic resonance (EPR) spectrum of oxy cobalt myoglobin (Aplysia) is affected by the replacement of H2O with D2O, suggesting a possible interaction between the bound oxygen and the neighboring hydrogen atom. A low temperature photodissociation study showed that the product of photolysis of oxy cobalt myoglobin (Aplysia) gives an EPR spectrum different from that of the deoxy-cobalt myoglobin (Aplysia) and from that of the photolysed form of oxy-cobalt myogloin (Sperm whale). These observations suggest that in oxy-cobalt myoglobin (Aplysia) the bound oxygen might interact with amino acid adjacent to it, but the interaction is weaker than that in oxy cobalt myoglobin (Sperm whale).

Primitive haemoglobin

The sequences of Petromyzon and Aplysia globins are compared with the postulated vertebrate and mollusc-vertebrate ancestors to see if differences exist in the rates of evolution of different types of residue positions. Between the mollusc-vertebrate ancestor and Aplysia globin there is no very striking pattern of changes except that the interior positions are relatively conserved. In the evolution of Petromyzon haemoglobin, the alpha 1 beta 2 contact area is relatively conserved. The homopolymeric binding of lamprey Hb seems to be a primitive function.

The myoglobin of the fruit-bat (Rousettus aegyptiacus)

The myoglobin of the fruit-bat (Rousettus aegyptiacus) has been investigated. It has 153 amino acid residues. When the possible therian ancestral myoglobin is taken into consideration, the myoglobin of the fruit-bat resembles most closely that of the hedgehog.